Advanced Bash-Scripting Guide An in-depth exploration of the art of shell scripting Mendel Cooper 6.1 30 September 2009 Revision History Revision 5.6 26 Jan 2009 Revised by: mc 'WORCESTERBERRY' release: Minor Update. Revision 6.0 23 Mar 2009 Revised by: mc 'THIMBLEBERRY' release: Major Update. Revision 6.1 30 Sep 2009 Revised by: mc 'BUFFALOBERRY' release: Minor Update. This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX(R) wisdom and lore. It serves as a textbook, a manual for self-study, and a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts. This book is suitable for classroom use as a general introduction to programming concepts. ------------------------------------------------------------------------ ------------------------------------------------------------------------ Dedication For Anita, the source of all the magic Table of Contents Part 1. Introduction 1. Shell Programming! 2. Starting Off With a Sha-Bang Part 2. Basics 3. Special Characters 4. Introduction to Variables and Parameters 5. Quoting 6. Exit and Exit Status 7. Tests 8. Operations and Related Topics Part 3. Beyond the Basics 9. Variables Revisited 10. Loops and Branches 11. Command Substitution 12. Arithmetic Expansion 13. Recess Time Part 4. Commands 14. Internal Commands and Builtins 15. External Filters, Programs and Commands 16. System and Administrative Commands Part 5. Advanced Topics 17. Regular Expressions 18. Here Documents 19. I/O Redirection 20. Subshells 21. Restricted Shells 22. Process Substitution 23. Functions 24. Aliases 25. List Constructs 26. Arrays 27. /dev and /proc 28. Of Zeros and Nulls 29. Debugging 30. Options 31. Gotchas 32. Scripting With Style 33. Miscellany 34. Bash, versions 2, 3, and 4 35. Endnotes 35.1. Author's Note 35.2. About the Author 35.3. Where to Go For Help 35.4. Tools Used to Produce This Book 35.5. Credits 35.6. Disclaimer Bibliography A. Contributed Scripts B. Reference Cards C. A Sed and Awk Micro-Primer C.1. Sed C.2. Awk D. Exit Codes With Special Meanings E. A Detailed Introduction to I/O and I/O Redirection F. Command-Line Options F.1. Standard Command-Line Options F.2. Bash Command-Line Options G. Important Files H. Important System Directories I. An Introduction to Programmable Completion J. Localization K. History Commands L. A Sample .bashrc File M. Converting DOS Batch Files to Shell Scripts N. Exercises N.1. Analyzing Scripts N.2. Writing Scripts O. Revision History P. Download and Mirror Sites Q. To Do List R. Copyright S. ASCII Table Index List of Tables 14-1. Job identifiers 30-1. Bash options 33-1. Operator Precedence 33-2. Numbers representing colors in Escape Sequences B-1. Special Shell Variables B-2. TEST Operators: Binary Comparison B-3. TEST Operators: Files B-4. Parameter Substitution and Expansion B-5. String Operations B-6. Miscellaneous Constructs C-1. Basic sed operators C-2. Examples of sed operators D-1. Reserved Exit Codes M-1. Batch file keywords / variables / operators, and their shell equivalents M-2. DOS commands and their UNIX equivalents O-1. Revision History List of Examples 2-1. cleanup: A script to clean up the log files in /var/log 2-2. cleanup: An improved clean-up script 2-3. cleanup: An enhanced and generalized version of above scripts. 3-1. Code blocks and I/O redirection 3-2. Saving the output of a code block to a file 3-3. Running a loop in the background 3-4. Backup of all files changed in last day 4-1. Variable assignment and substitution 4-2. Plain Variable Assignment 4-3. Variable Assignment, plain and fancy 4-4. Integer or string? 4-5. Positional Parameters 4-6. wh, whois domain name lookup 4-7. Using shift 5-1. Echoing Weird Variables 5-2. Escaped Characters 6-1. exit / exit status 6-2. Negating a condition using ! 7-1. What is truth? 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[ 7-3. Arithmetic Tests using (( )) 7-4. Testing for broken links 7-5. Arithmetic and string comparisons 7-6. Testing whether a string is null 7-7. zmore 8-1. Greatest common divisor 8-2. Using Arithmetic Operations 8-3. Compound Condition Tests Using && and || 8-4. Representation of numerical constants 9-1. $IFS and whitespace 9-2. Timed Input 9-3. Once more, timed input 9-4. Timed read 9-5. Am I root? 9-6. arglist: Listing arguments with $* and $@ 9-7. Inconsistent $* and $@ behavior 9-8. $* and $@ when $IFS is empty 9-9. Underscore variable 9-10. Inserting a blank line between paragraphs in a text file 9-11. Generating an 8-character "random" string 9-12. Converting graphic file formats, with filename change 9-13. Converting streaming audio files to ogg 9-14. Emulating getopt 9-15. Alternate ways of extracting and locating substrings 9-16. Using parameter substitution and error messages 9-17. Parameter substitution and "usage" messages 9-18. Length of a variable 9-19. Pattern matching in parameter substitution 9-20. Renaming file extensions: 9-21. Using pattern matching to parse arbitrary strings 9-22. Matching patterns at prefix or suffix of string 9-23. Using declare to type variables 9-24. Indirect Variable References 9-25. Passing an indirect reference to awk 9-26. Generating random numbers 9-27. Picking a random card from a deck 9-28. Brownian Motion Simulation 9-29. Random between values 9-30. Rolling a single die with RANDOM 9-31. Reseeding RANDOM 9-32. Pseudorandom numbers, using awk 9-33. C-style manipulation of variables 10-1. Simple for loops 10-2. for loop with two parameters in each [list] element 10-3. Fileinfo: operating on a file list contained in a variable 10-4. Operating on files with a for loop 10-5. Missing in [list] in a for loop 10-6. Generating the [list] in a for loop with command substitution 10-7. A grep replacement for binary files 10-8. Listing all users on the system 10-9. Checking all the binaries in a directory for authorship 10-10. Listing the symbolic links in a directory 10-11. Symbolic links in a directory, saved to a file 10-12. A C-style for loop 10-13. Using efax in batch mode 10-14. Simple while loop 10-15. Another while loop 10-16. while loop with multiple conditions 10-17. C-style syntax in a while loop 10-18. until loop 10-19. Nested Loop 10-20. Effects of break and continue in a loop 10-21. Breaking out of multiple loop levels 10-22. Continuing at a higher loop level 10-23. Using continue N in an actual task 10-24. Using case 10-25. Creating menus using case 10-26. Using command substitution to generate the case variable 10-27. Simple string matching 10-28. Checking for alphabetic input 10-29. Creating menus using select 10-30. Creating menus using select in a function 11-1. Stupid script tricks 11-2. Generating a variable from a loop 11-3. Finding anagrams 14-1. A script that spawns multiple instances of itself 14-2. printf in action 14-3. Variable assignment, using read 14-4. What happens when read has no variable 14-5. Multi-line input to read 14-6. Detecting the arrow keys 14-7. Using read with file redirection 14-8. Problems reading from a pipe 14-9. Changing the current working directory 14-10. Letting let do arithmetic. 14-11. Showing the effect of eval 14-12. Using eval to select among variables 14-13. Echoing the command-line parameters 14-14. Forcing a log-off 14-15. A version of rot13 14-16. Using set with positional parameters 14-17. Reversing the positional parameters 14-18. Reassigning the positional parameters 14-19. "Unsetting" a variable 14-20. Using export to pass a variable to an embedded awk script 14-21. Using getopts to read the options/arguments passed to a script 14-22. "Including" a data file 14-23. A (useless) script that sources itself 14-24. Effects of exec 14-25. A script that exec's itself 14-26. Waiting for a process to finish before proceeding 14-27. A script that kills itself 15-1. Using ls to create a table of contents for burning a CDR disk 15-2. Hello or Good-bye 15-3. Badname, eliminate file names in current directory containing bad characters and whitespace. 15-4. Deleting a file by its inode number 15-5. Logfile: Using xargs to monitor system log 15-6. Copying files in current directory to another 15-7. Killing processes by name 15-8. Word frequency analysis using xargs 15-9. Using expr 15-10. Using date 15-11. Date calculations 15-12. Word Frequency Analysis 15-13. Which files are scripts? 15-14. Generating 10-digit random numbers 15-15. Using tail to monitor the system log 15-16. Printing out the From lines in stored e-mail messages 15-17. Emulating grep in a script 15-18. Crossword puzzle solver 15-19. Looking up definitions in Webster's 1913 Dictionary 15-20. Checking words in a list for validity 15-21. toupper: Transforms a file to all uppercase. 15-22. lowercase: Changes all filenames in working directory to lowercase. 15-23. du: DOS to UNIX text file conversion. 15-24. rot13: ultra-weak encryption. 15-25. Generating "Crypto-Quote" Puzzles 15-26. Formatted file listing. 15-27. Using column to format a directory listing 15-28. nl: A self-numbering script. 15-29. manview: Viewing formatted manpages 15-30. Using cpio to move a directory tree 15-31. Unpacking an rpm archive 15-32. Stripping comments from C program files 15-33. Exploring /usr/X11R6/bin 15-34. An "improved" strings command 15-35. Using cmp to compare two files within a script. 15-36. basename and dirname 15-37. A script that copies itself in sections 15-38. Checking file integrity 15-39. Uudecoding encoded files 15-40. Finding out where to report a spammer 15-41. Analyzing a spam domain 15-42. Getting a stock quote 15-43. Updating FC4 15-44. Using ssh 15-45. A script that mails itself 15-46. Generating prime numbers 15-47. Monthly Payment on a Mortgage 15-48. Base Conversion 15-49. Invoking bc using a here document 15-50. Calculating PI 15-51. Converting a decimal number to hexadecimal 15-52. Factoring 15-53. Calculating the hypotenuse of a triangle 15-54. Using seq to generate loop arguments 15-55. Letter Count" 15-56. Using getopt to parse command-line options 15-57. A script that copies itself 15-58. Exercising dd 15-59. Capturing Keystrokes 15-60. Securely deleting a file 15-61. Filename generator 15-62. Converting meters to miles 15-63. Using m4 16-1. Setting a new password 16-2. Setting an erase character 16-3. secret password: Turning off terminal echoing 16-4. Keypress detection 16-5. Checking a remote server for identd 16-6. pidof helps kill a process 16-7. Checking a CD image 16-8. Creating a filesystem in a file 16-9. Adding a new hard drive 16-10. Using umask to hide an output file from prying eyes 16-11. killall, from /etc/rc.d/init.d 18-1. broadcast: Sends message to everyone logged in 18-2. dummyfile: Creates a 2-line dummy file 18-3. Multi-line message using cat 18-4. Multi-line message, with tabs suppressed 18-5. Here document with replaceable parameters 18-6. Upload a file pair to Sunsite incoming directory 18-7. Parameter substitution turned off 18-8. A script that generates another script 18-9. Here documents and functions 18-10. "Anonymous" Here Document 18-11. Commenting out a block of code 18-12. A self-documenting script 18-13. Prepending a line to a file 18-14. Parsing a mailbox 19-1. Redirecting stdin using exec 19-2. Redirecting stdout using exec 19-3. Redirecting both stdin and stdout in the same script with exec 19-4. Avoiding a subshell 19-5. Redirected while loop 19-6. Alternate form of redirected while loop 19-7. Redirected until loop 19-8. Redirected for loop 19-9. Redirected for loop (both stdin and stdout redirected) 19-10. Redirected if/then test 19-11. Data file names.data for above examples 19-12. Logging events 20-1. Variable scope in a subshell 20-2. List User Profiles 20-3. Running parallel processes in subshells 21-1. Running a script in restricted mode 22-1. Code block redirection without forking 23-1. Simple functions 23-2. Function Taking Parameters 23-3. Functions and command-line args passed to the script 23-4. Passing an indirect reference to a function 23-5. Dereferencing a parameter passed to a function 23-6. Again, dereferencing a parameter passed to a function 23-7. Maximum of two numbers 23-8. Converting numbers to Roman numerals 23-9. Testing large return values in a function 23-10. Comparing two large integers 23-11. Real name from username 23-12. Local variable visibility 23-13. Demonstration of a simple recursive function 23-14. Another simple demonstration 23-15. Recursion, using a local variable 23-16. The Fibonacci Sequence 23-17. The Towers of Hanoi 24-1. Aliases within a script 24-2. unalias: Setting and unsetting an alias 25-1. Using an and list to test for command-line arguments 25-2. Another command-line arg test using an and list 25-3. Using or lists in combination with an and list 26-1. Simple array usage 26-2. Formatting a poem 26-3. Various array operations 26-4. String operations on arrays 26-5. Loading the contents of a script into an array 26-6. Some special properties of arrays 26-7. Of empty arrays and empty elements 26-8. Initializing arrays 26-9. Copying and concatenating arrays 26-10. More on concatenating arrays 26-11. The Bubble Sort 26-12. Embedded arrays and indirect references 26-13. The Sieve of Eratosthenes 26-14. The Sieve of Eratosthenes, Optimized 26-15. Emulating a push-down stack 26-16. Complex array application: Exploring a weird mathematical series 26-17. Simulating a two-dimensional array, then tilting it 27-1. Using /dev/tcp for troubleshooting 27-2. Playing music 27-3. Finding the process associated with a PID 27-4. On-line connect status 28-1. Hiding the cookie jar 28-2. Setting up a swapfile using /dev/zero 28-3. Creating a ramdisk 29-1. A buggy script 29-2. Missing keyword 29-3. test24: another buggy script 29-4. Testing a condition with an assert 29-5. Trapping at exit 29-6. Cleaning up after Control-C 29-7. Tracing a variable 29-8. Running multiple processes (on an SMP box) 31-1. Numerical and string comparison are not equivalent 31-2. Subshell Pitfalls 31-3. Piping the output of echo to a read 33-1. shell wrapper 33-2. A slightly more complex shell wrapper 33-3. A generic shell wrapper that writes to a logfile 33-4. A shell wrapper around an awk script 33-5. A shell wrapper around another awk script 33-6. Perl embedded in a Bash script 33-7. Bash and Perl scripts combined 33-8. A (useless) script that recursively calls itself 33-9. A (useful) script that recursively calls itself 33-10. Another (useful) script that recursively calls itself 33-11. A "colorized" address database 33-12. Drawing a box 33-13. Echoing colored text 33-14. A "horserace" game 33-15. A Progress Bar 33-16. Return value trickery 33-17. Even more return value trickery 33-18. Passing and returning arrays 33-19. Fun with anagrams 33-20. Widgets invoked from a shell script 33-21. Test Suite 34-1. String expansion 34-2. Indirect variable references - the new way 34-3. Simple database application, using indirect variable referencing 34-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards 34-5. A simple address database 34-6. A somewhat more elaborate address database 34-7. Testing characters A-1. mailformat: Formatting an e-mail message A-2. rn: A simple-minded file renaming utility A-3. blank-rename: Renames filenames containing blanks A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted password A-5. copy-cd: Copying a data CD A-6. Collatz series A-7. days-between: Days between two dates A-8. Making a dictionary A-9. Soundex conversion A-10. Game of Life A-11. Data file for Game of Life A-12. behead: Removing mail and news message headers A-13. password: Generating random 8-character passwords A-14. fifo: Making daily backups, using named pipes A-15. Generating prime numbers using the modulo operator A-16. tree: Displaying a directory tree A-17. tree2: Alternate directory tree script A-18. string functions: C-style string functions A-19. Directory information A-20. Library of hash functions A-21. Colorizing text using hash functions A-22. More on hash functions A-23. Mounting USB keychain storage devices A-24. Converting to HTML A-25. Preserving weblogs A-26. Protecting literal strings A-27. Unprotecting literal strings A-28. Spammer Identification A-29. Spammer Hunt A-30. Making wget easier to use A-31. A podcasting script A-32. Nightly backup to a firewire HD A-33. An expanded cd command A-34. A soundcard setup script A-35. Locating split paragraphs in a text file A-36. Insertion sort A-37. Standard Deviation A-38. A pad file generator for shareware authors A-39. A man page editor A-40. Petals Around the Rose A-41. Quacky: a Perquackey-type word game A-42. Nim A-43. A command-line stopwatch A-44. An all-purpose shell scripting homework assignment solution A-45. The Knight's Tour A-46. Magic Squares A-47. Fifteen Puzzle A-48. The Towers of Hanoi, graphic version A-49. The Towers of Hanoi, alternate graphic version A-50. An alternate version of the getopt-simple.sh script A-51. The version of the UseGetOpt.sh example used in the Tab Expansion appendix A-52. Cycling through all the possible color backgrounds A-53. Basics Reviewed C-1. Counting Letter Occurrences I-1. Completion script for UseGetOpt.sh L-1. Sample .bashrc file M-1. VIEWDATA.BAT: DOS Batch File M-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT Q-1. Print the server environment S-1. A script that generates an ASCII table Part 1. Introduction Script: A writing; a written document. [Obs.] --Webster's Dictionary, 1913 ed. The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by "gluing together" system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, lend additional power and flexibility to scripts. Shell scripts are especially well suited for administrative system tasks and other routine repetitive tasks not requiring the bells and whistles of a full-blown tightly structured programming language. Table of Contents 1. Shell Programming! 2. Starting Off With a Sha-Bang 2.1. Invoking the script 2.2. Preliminary Exercises ------------------------------------------------------------------------ Chapter 1. Shell Programming! No programming language is perfect. There is not even a single best language; there are only languages well suited or perhaps poorly suited for particular purposes. --Herbert Mayer A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it. The craft of scripting is not hard to master, since the scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options [1] to learn. The syntax is simple and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" governing their use. Most short scripts work right the first time, and debugging even the longer ones is straightforward. In the 1970s, the BASIC language enabled anyone reasonably computer proficient to write programs on an early generation of microcomputers. Decades later, the Bash scripting language enables anyone with a rudimentary knowledge of Linux or UNIX to do the same on much more powerful machines. A shell script is a quick-and-dirty method of prototyping a complex application. Getting even a limited subset of the functionality to work in a script is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, Perl, or Python. Shell scripting hearkens back to the classic UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool. According to Herbert Mayer, "a useful language needs arrays, pointers, and a generic mechanism for building data structures." By these criteria, shell scripting falls somewhat short of being "useful." Or, perhaps not. . . . +----------------------------------------------------------------------+ | When not to use shell scripts | | | | * Resource-intensive tasks, especially where speed is a factor | | (sorting, hashing, recursion [2] ...) | | | | * Procedures involving heavy-duty math operations, especially | | floating point arithmetic, arbitrary precision calculations, or | | complex numbers (use C++ or FORTRAN instead) | | | | * Cross-platform portability required (use C or Java instead) | | | | * Complex applications, where structured programming is a | | necessity (type-checking of variables, function prototypes, | | etc.) | | | | * Mission-critical applications upon which you are betting the | | future of the company | | | | * Situations where security is important, where you need to | | guarantee the integrity of your system and protect against | | intrusion, cracking, and vandalism | | | | * Project consists of subcomponents with interlocking dependencies | | | | * Extensive file operations required (Bash is limited to serial | | file access, and that only in a particularly clumsy and | | inefficient line-by-line fashion.) | | | | * Need native support for multi-dimensional arrays | | | | * Need data structures, such as linked lists or trees | | | | * Need to generate / manipulate graphics or GUIs | | | | * Need direct access to system hardware | | | | * Need port or socket I/O | | | | * Need to use libraries or interface with legacy code | | | | * Proprietary, closed-source applications (Shell scripts put the | | source code right out in the open for all the world to see.) | | | | If any of the above applies, consider a more powerful scripting | | language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a | | compiled language such as C, C++, or Java. Even then, prototyping | | the application as a shell script might still be a useful | | development step. | +----------------------------------------------------------------------+ We will be using Bash, an acronym for "Bourne-Again shell" and a pun on Stephen Bourne's now classic Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, [3] and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.) What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work -- they've been tested, insofar as was possible -- and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (scriptname.sh or scriptname.bash), [4] give them execute permission (chmod u+rx scriptname), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the [http://www.tldp.org/LDP/abs/abs-guide.html.tar.gz] HTML or [http://bash.webofcrafts.net/abs-guide.pdf] pdf rendered versions. Be aware that some of the scripts presented here introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment. Unless otherwise noted, [mailto:thegrendel.abs@gmail.com] the author of this book wrote the example scripts that follow. His countenance was bold and bashed not. --Edmund Spenser ------------------------------------------------------------------------ Chapter 2. Starting Off With a Sha-Bang Shell programming is a 1950s juke box . . . --Larry Wall In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked. Example 2-1. cleanup: A script to clean up the log files in /var/log # Cleanup # Run as root, of course. cd /var/log cat /dev/null > messages cat /dev/null > wtmp echo "Logs cleaned up." There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one from the command-line on the console or in a terminal window. The advantages of placing the commands in a script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it can easily be modified or customized for a particular application. Example 2-2. cleanup: An improved clean-up script #!/bin/bash # Proper header for a Bash script. # Cleanup, version 2 # Run as root, of course. # Insert code here to print error message and exit if not root. LOG_DIR=/var/log # Variables are better than hard-coded values. cd $LOG_DIR cat /dev/null > messages cat /dev/null > wtmp echo "Logs cleaned up." exit # The right and proper method of "exiting" from a script. Now that's beginning to look like a real script. But we can go even farther . . . Example 2-3. cleanup: An enhanced and generalized version of above scripts. #!/bin/bash # Cleanup, version 3 # Warning: # ------- # This script uses quite a number of features that will be explained #+ later on. # By the time you've finished the first half of the book, #+ there should be nothing mysterious about it. LOG_DIR=/var/log ROOT_UID=0 # Only users with $UID 0 have root privileges. LINES=50 # Default number of lines saved. E_XCD=86 # Can't change directory? E_NOTROOT=87 # Non-root exit error. # Run as root, of course. if [ "$UID" -ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi if [ -n "$1" ] # Test whether command-line argument is present (non-empty). then lines=$1 else lines=$LINES # Default, if not specified on command-line. fi # Stephane Chazelas suggests the following, #+ as a better way of checking command-line arguments, #+ but this is still a bit advanced for this stage of the tutorial. # # E_WRONGARGS=85 # Non-numerical argument (bad argument format). # # case "$1" in # "" ) lines=50;; # *[!0-9]*) echo "Usage: `basename $0` file-to-cleanup"; exit $E_WRONGARGS;; # * ) lines=$1;; # esac # #* Skip ahead to "Loops" chapter to decipher all this. cd $LOG_DIR if [ `pwd` != "$LOG_DIR" ] # or if [ "$PWD" != "$LOG_DIR" ] # Not in /var/log? then echo "Can't change to $LOG_DIR." exit $E_XCD fi # Doublecheck if in right directory before messing with log file. # Far more efficient is: # # cd /var/log || { # echo "Cannot change to necessary directory." >&2 # exit $E_XCD; # } tail -n $lines messages > mesg.temp # Save last section of message log file. mv mesg.temp messages # Becomes new log directory. # cat /dev/null > messages #* No longer needed, as the above method is safer. cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect. echo "Logs cleaned up." exit 0 # A zero return value from the script upon exit indicates success #+ to the shell. Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of the message log intact. You will constantly discover ways of fine-tuning previously written scripts for increased effectiveness. * * * The sha-bang ( #!) [5] at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte [6] magic number, a special marker that designates a file type, or in this case an executable shell script (type man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (the line following the sha-bang line), and ignoring comments. [7] #!/bin/sh #!/bin/bash #!/usr/bin/perl #!/usr/bin/tcl #!/bin/sed -f #!/usr/awk -f Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. [8] Using #!/bin/sh, the default Bourne shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific features. The script will, however, conform to the POSIX [9] sh standard. Note that the path given at the "sha-bang" must be correct, otherwise an error message -- usually "Command not found." -- will be the only result of running the script. [10] #! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct. [11] Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine. Tip This tutorial encourages a modular approach to constructing a script. Make note of and collect "boilerplate" code snippets that might be useful in future scripts. Eventually you will build quite an extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters. E_WRONG_ARGS=85 script_parameters="-a -h -m -z" # -a = all, -h = help, etc. if [ $# -ne $Number_of_expected_args ] then echo "Usage: `basename $0` $script_parameters" # `basename $0` is the script's filename. exit $E_WRONG_ARGS fi Many times, you will write a script that carries out one particular task. The first script in this chapter is an example. Later, it might occur to you to generalize the script to do other, similar tasks. Replacing the literal ("hard-wired") constants by variables is a step in that direction, as is replacing repetitive code blocks by functions. ------------------------------------------------------------------------ 2.1. Invoking the script Having written the script, you can invoke it by sh scriptname, [12] or alternatively bash scriptname. (Not recommended is using sh redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it. : > data.xxx # File "data.xxx" now empty. # Same effect as cat /dev/null >data.xxx # However, this does not fork a new process, since ":" is a builtin. See also Example 15-15. In combination with the >> redirection operator, has no effect on a pre-existing target file (: >> target_file). If the file did not previously exist, creates it. Note This applies to regular files, not pipes, symlinks, and certain special files. May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may appear in a comment. However, this is not the case with :. : This is a comment that generates an error, ( if [ $x -eq 3] ). The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable. +----------------------------------------------------------------------+ |bash$ echo $PATH | |/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games| +----------------------------------------------------------------------+ ! reverse (or negate) the sense of a test or exit status [bang]. The ! operator inverts the exit status of the command to which it is applied (see Example 6-2). It also inverts the meaning of a test operator. This can, for example, change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash keyword. In a different context, the ! also appears in indirect variable references. In yet another context, from the command line, the ! invokes the Bash history mechanism (see Appendix K). Note that within a script, the history mechanism is disabled. * wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By itself, it matches every filename in a given directory. +-------------------------------------------------------+ |bash$ echo * | |abs-book.sgml add-drive.sh agram.sh alias.sh | | | +-------------------------------------------------------+ The * also represents any number (or zero) characters in a regular expression. * arithmetic operator. In the context of arithmetic operations, the * denotes multiplication. ** A double asterisk can represent the exponentiation operator or extended file-match globbing. ? test operator. Within certain expressions, the ? indicates a test for a condition. In a double-parentheses construct, the ? can serve as an element of a C-style trinary operator, ?:. (( var0 = var1<98?9:21 )) # ^ ^ # if [ "$var1" -lt 98 ] # then # var0=9 # else # var0=21 # fi In a parameter substitution expression, the ? tests whether a variable has been set. ? wild card. The ? character serves as a single-character "wild card" for filename expansion in globbing, as well as representing one character in an extended regular expression. $ Variable substitution (contents of a variable). var1=5 var2=23skidoo echo $var1 # 5 echo $var2 # 23skidoo A $ prefixing a variable name indicates the value the variable holds. $ end-of-line. In a regular expression, a "$" addresses the end of a line of text. ${} Parameter substitution. $*, $@ positional parameters. $? exit status variable. The $? variable holds the exit status of a command, a function, or of the script itself. $$ process ID variable. The $$ variable holds the process ID [16] of the script in which it appears. () command group. (a=hello; echo $a) Important A listing of commands within parentheses starts a subshell. Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell. a=123 ( a=321; ) echo "a = $a" # a = 123 # "a" within parentheses acts like a local variable. array initialization. Array=(element1 element2 element3) {xxx,yyy,zzz,...} Brace expansion. echo \"{These,words,are,quoted}\" # " prefix and suffix # "These" "words" "are" "quoted" cat {file1,file2,file3} > combined_file # Concatenates the files file1, file2, and file3 into combined_file. cp file22.{txt,backup} # Copies "file22.txt" to "file22.backup" A command may act upon a comma-separated list of file specs within braces. [17] Filename expansion (globbing) applies to the file specs between the braces. Caution No spaces allowed within the braces unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C {a..z} Extended Brace expansion. echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z # Echoes characters between a and z. echo {0..3} # 0 1 2 3 # Echoes characters between 0 and 3. The {a..z} extended brace expansion construction is a feature introduced in version 3 of Bash. {} Block of code [curly brackets]. Also referred to as an inline group, this construct, in effect, creates an anonymous function (a function without a name). However, unlike in a "standard" function, the variables inside a code block remain visible to the remainder of the script. +-------------------------------------------------------+ |bash$ { local a; | | a=123; } | |bash: local: can only be used in a | |function | | | +-------------------------------------------------------+ a=123 { a=321; } echo "a = $a" # a = 321 (value inside code block) # Thanks, S.C. The code block enclosed in braces may have I/O redirected to and from it. Example 3-1. Code blocks and I/O redirection #!/bin/bash # Reading lines in /etc/fstab. File=/etc/fstab { read line1 read line2 } < $File echo "First line in $File is:" echo "$line1" echo echo "Second line in $File is:" echo "$line2" exit 0 # Now, how do you parse the separate fields of each line? # Hint: use awk, or . . . # . . . Hans-Joerg Diers suggests using the "set" Bash builtin. Example 3-2. Saving the output of a code block to a file #!/bin/bash # rpm-check.sh # Queries an rpm file for description, listing, #+ and whether it can be installed. # Saves output to a file. # # This script illustrates using a code block. SUCCESS=0 E_NOARGS=65 if [ -z "$1" ] then echo "Usage: `basename $0` rpm-file" exit $E_NOARGS fi { # Begin code block. echo echo "Archive Description:" rpm -qpi $1 # Query description. echo echo "Archive Listing:" rpm -qpl $1 # Query listing. echo rpm -i --test $1 # Query whether rpm file can be installed. if [ "$?" -eq $SUCCESS ] then echo "$1 can be installed." else echo "$1 cannot be installed." fi echo # End code block. } > "$1.test" # Redirects output of everything in block to file. echo "Results of rpm test in file $1.test" # See rpm man page for explanation of options. exit 0 Note Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell. [18] {} placeholder for text. Used after xargs -i (replace strings option). The {} double curly brackets are a placeholder for output text. ls . | xargs -i -t cp ./{} $1 # ^^ ^^ # From "ex42.sh" (copydir.sh) example. anchor id="semicolonesc"> {} \; pathname. Mostly used in find constructs. This is not a shell builtin. Note The ";" ends the -exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell. [ ] test. Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a link to the external command /usr/bin/test. [[ ]] test. Test expression between [[ ]]. More flexible than the single-bracket [ ] test, this is a shell keyword. See the discussion on the [[ ... ]] construct. [ ] array element. In the context of an array, brackets set off the numbering of each element of that array. Array[1]=slot_1 echo ${Array[1]} [ ] range of characters. As part of a regular expression, brackets delineate a range of characters to match. $[ ... ] integer expansion. Evaluate integer expression between $[ ]. a=3 b=7 echo $[$a+$b] # 10 echo $[$a*$b] # 21 Note that this usage is deprecated, and has been replaced by the (( ... )) construct. (( )) integer expansion. Expand and evaluate integer expression between (( )). See the discussion on the (( ... )) construct. > &> >& >> < <> redirection. scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists. command &>filename redirects both the stdout and the stderr of command to filename. Note This is useful for suppressing output when testing for a condition. For example, let us test whether a certain command exists. +--------------------------------------------+ |bash$ type bogus_command &>/dev/null | | | | | | | |bash$ echo $? | |1 | | | +--------------------------------------------+ Or in a script: command_test () { type "$1" &>/dev/null; } # ^ cmd=rmdir # Legitimate command. command_test $cmd; echo $? # 0 cmd=bogus_command # Illegitimate command command_test $cmd; echo $? # 1 command >&2 redirects stdout of command to stderr. scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it is created. [i]<>filename opens file filename for reading and writing, and assigns file descriptor i to it. If filename does not exist, it is created. process substitution. (command)> <(command) In a different context, the "<" and ">" characters act as string comparison operators. In yet another context, the "<" and ">" characters act as integer comparison operators. See also Example 15-9. << redirection used in a here document. <<< redirection used in a here string. <, > ASCII comparison. veg1=carrots veg2=tomatoes if [[ "$veg1" < "$veg2" ]] then echo "Although $veg1 precede $veg2 in the dictionary," echo -n "this does not necessarily imply anything " echo "about my culinary preferences." else echo "What kind of dictionary are you using, anyhow?" fi \<, \> word boundary in a regular expression. bash$ grep '\' textfile | pipe. Passes the output (stdout of a previous command to the input (stdin) of the next one, or to the shell. This is a method of chaining commands together. echo ls -l | sh # Passes the output of "echo ls -l" to the shell, #+ with the same result as a simple "ls -l". cat *.lst | sort | uniq # Merges and sorts all ".lst" files, then deletes duplicate lines. +--------------------------------------------------------------+ | A pipe, as a classic method of interprocess communication, | | sends the stdout of one process to the stdin of another. In | | a typical case, a command, such as cat or echo, pipes a | | stream of data to a filter, a command that transforms its | | input for processing. [19] | | | | cat $filename1 $filename2 | grep $search_word | | | | For an interesting note on the complexity of using UNIX | | pipes, see the UNIX FAQ, Part 3. | +--------------------------------------------------------------+ The output of a command or commands may be piped to a script. #!/bin/bash # uppercase.sh : Changes input to uppercase. tr 'a-z' 'A-Z' # Letter ranges must be quoted #+ to prevent filename generation from single-letter filenames. exit 0 Now, let us pipe the output of ls -l to this script. +------------------------------------------------------------+ |bash$ ls -l | ./uppercase.sh | |-RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT | | -RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT | | -RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE| | | +------------------------------------------------------------+ Note The stdout of each process in a pipe must be read as the stdin of the next. If this is not the case, the data stream will block, and the pipe will not behave as expected. cat file1 file2 | ls -l | sort # The output from "cat file1 file2" disappears. A pipe runs as a child process, and therefore cannot alter script variables. variable="initial_value" echo "new_value" | read variable echo "variable = $variable" # variable = initial_value If one of the commands in the pipe aborts, this prematurely terminates execution of the pipe. Called a broken pipe, this condition sends a SIGPIPE signal. >| force redirection (even if the noclobber option is set). This will forcibly overwrite an existing file. || OR logical operator. In a test construct, the || operator causes a return of 0 (success) if either of the linked test conditions is true. & Run job in background. A command followed by an & will run in the background. +-------------------------------------------------------+ |bash$ sleep 10 & | |[1] 850 | |[1]+ Done sleep 10 | | | +-------------------------------------------------------+ Within a script, commands and even loops may run in the background. Example 3-3. Running a loop in the background #!/bin/bash # background-loop.sh for i in 1 2 3 4 5 6 7 8 9 10 # First loop. do echo -n "$i " done & # Run this loop in background. # Will sometimes execute after second loop. echo # This 'echo' sometimes will not display. for i in 11 12 13 14 15 16 17 18 19 20 # Second loop. do echo -n "$i " done echo # This 'echo' sometimes will not display. # ====================================================== # The expected output from the script: # 1 2 3 4 5 6 7 8 9 10 # 11 12 13 14 15 16 17 18 19 20 # Sometimes, though, you get: # 11 12 13 14 15 16 17 18 19 20 # 1 2 3 4 5 6 7 8 9 10 bozo $ # (The second 'echo' doesn't execute. Why?) # Occasionally also: # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # (The first 'echo' doesn't execute. Why?) # Very rarely something like: # 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20 # The foreground loop preempts the background one. exit 0 # Nasimuddin Ansari suggests adding sleep 1 #+ after the echo -n "$i" in lines 6 and 14, #+ for some real fun. Caution A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this. && AND logical operator. In a test construct, the && operator causes a return of 0 (success) only if both the linked test conditions are true. - option, prefix. Option flag for a command or filter. Prefix for an operator. Prefix for a default parameter in parameter substitution. COMMAND -[Option1][Option2][...] ls -al sort -dfu $filename if [ $file1 -ot $file2 ] then # ^ echo "File $file1 is older than $file2." fi if [ "$a" -eq "$b" ] then ^ echo "$a is equal to $b." fi if [ "$c" -eq 24 -a "$d" -eq 47 ] then ^ ^ echo "$c equals 24 and $d equals 47." fi param2=${param1:-$DEFAULTVAL} # ^ -- The double-dash -- prefixes long (verbatim) options to commands. sort --ignore-leading-blanks Used with a Bash builtin, it means the end of options to that particular command. Tip This provides a handy means of removing files whose names begin with a dash. +----------------------------------------------+ |bash$ ls -l | |-rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname| | | | | |bash$ rm -- -badname | | | |bash$ ls -l | |total 0 | +----------------------------------------------+ The double-dash is also used in conjunction with set. set -- $variable (as in Example 14-18) - redirection from/to stdin or stdout [dash]. +-------------------------------------------------------+ |bash$ cat - | |abc | |abc | | | |... | | | |Ctl-D | +-------------------------------------------------------+ As expected, cat - echoes stdin, in this case keyboarded user input, to stdout. But, does I/O redirection using - have real-world applications? (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) # Move entire file tree from one directory to another # [courtesy Alan Cox , with a minor change] # 1) cd /source/directory # Source directory, where the files to be moved are. # 2) && # "And-list": if the 'cd' operation successful, # then execute the next command. # 3) tar cf - . # The 'c' option 'tar' archiving command creates a new archive, # the 'f' (file) option, followed by '-' designates the target file # as stdout, and do it in current directory tree ('.'). # 4) | # Piped to ... # 5) ( ... ) # a subshell # 6) cd /dest/directory # Change to the destination directory. # 7) && # "And-list", as above # 8) tar xpvf - # Unarchive ('x'), preserve ownership and file permissions ('p'), # and send verbose messages to stdout ('v'), # reading data from stdin ('f' followed by '-'). # # Note that 'x' is a command, and 'p', 'v', 'f' are options. # # Whew! # More elegant than, but equivalent to: # cd source/directory # tar cf - . | (cd ../dest/directory; tar xpvf -) # # Also having same effect: # cp -a /source/directory/* /dest/directory # Or: # cp -a /source/directory/* /source/directory/.[^.]* /dest/directory # If there are hidden files in /source/directory. bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf - # --uncompress tar file-- | --then pass it to "tar"-- # If "tar" has not been patched to handle "bunzip2", #+ this needs to be done in two discrete steps, using a pipe. # The purpose of the exercise is to unarchive "bzipped" kernel source. Note that in this context the "-" is not itself a Bash operator, but rather an option recognized by certain UNIX utilities that write to stdout, such as tar, cat, etc. +-------------------------------------------------------+ |bash$ echo "whatever" | cat - | |whatever | +-------------------------------------------------------+ Where a filename is expected, - redirects output to stdout (sometimes seen with tar cf), or accepts input from stdin, rather than from a file. This is a method of using a file-oriented utility as a filter in a pipe. +-------------------------------------------------------------+ |bash$ file | |Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...| | | +-------------------------------------------------------------+ By itself on the command-line, file fails with an error message. Add a "-" for a more useful result. This causes the shell to await user input. +----------------------------------------------------------------------+ |bash$ file - | |abc | |standard input: ASCII text | | | | | | | |bash$ file - | |#!/bin/bash | |standard input: Bourne-Again shell script text executable| | | +----------------------------------------------------------------------+ Now the command accepts input from stdin and analyzes it. The "-" can be used to pipe stdout to other commands. This permits such stunts as prepending lines to a file. Using diff to compare a file with a section of another: grep Linux file1 | diff file2 - Finally, a real-world example using - with tar. Example 3-4. Backup of all files changed in last day #!/bin/bash # Backs up all files in current directory modified within last 24 hours #+ in a "tarball" (tarred and gzipped file). BACKUPFILE=backup-$(date +%m-%d-%Y) # Embeds date in backup filename. # Thanks, Joshua Tschida, for the idea. archive=${1:-$BACKUPFILE} # If no backup-archive filename specified on command-line, #+ it will default to "backup-MM-DD-YYYY.tar.gz." tar cvf - `find . -mtime -1 -type f -print` > $archive.tar gzip $archive.tar echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"." # Stephane Chazelas points out that the above code will fail #+ if there are too many files found #+ or if any filenames contain blank characters. # He suggests the following alternatives: # ------------------------------------------------------------------- # find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar" # using the GNU version of "find". # find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' \; # portable to other UNIX flavors, but much slower. # ------------------------------------------------------------------- exit 0 Caution Filenames beginning with "-" may cause problems when coupled with the "-" redirection operator. A script should check for this and add an appropriate prefix to such filenames, for example ./-FILENAME, $PWD/-FILENAME, or $PATHNAME/-FILENAME. If the value of a variable begins with a -, this may likewise create problems. var="-n" echo $var # Has the effect of "echo -n", and outputs nothing. - previous working directory. A cd - command changes to the previous working directory. This uses the $OLDPWD environmental variable. Caution Do not confuse the "-" used in this sense with the "-" redirection operator just discussed. The interpretation of the "-" depends on the context in which it appears. - Minus. Minus sign in an arithmetic operation. = Equals. Assignment operator a=28 echo $a # 28 In a different context, the "=" is a string comparison operator. + Plus. Addition arithmetic operator. In a different context, the + is a Regular Expression operator. + Option. Option flag for a command or filter. Certain commands and builtins use the + to enable certain options and the - to disable them. In parameter substitution, the + prefixes an alternate value that a variable expands to. % modulo. Modulo (remainder of a division) arithmetic operation. let "z = 5 % 3" echo $z # 2 In a different context, the % is a pattern matching operator. ~ home directory [tilde]. This corresponds to the $HOME internal variable. ~bozo is bozo's home directory, and ls ~bozo lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the contents of it. +-------------------------------------------------------+ |bash$ echo ~bozo | |/home/bozo | | | |bash$ echo ~ | |/home/bozo | | | |bash$ echo ~/ | |/home/bozo/ | | | |bash$ echo ~: | |/home/bozo: | | | |bash$ echo ~nonexistent-user | |~nonexistent-user | | | +-------------------------------------------------------+ ~+ current working directory. This corresponds to the $PWD internal variable. ~- previous working directory. This corresponds to the $OLDPWD internal variable. =~ regular expression match. This operator was introduced with version 3 of Bash. ^ beginning-of-line. In a regular expression, a "^" addresses the beginning of a line of text. ^, ^^ Uppercase conversion in parameter substitution (added in version 4 of Bash). Control Characters change the behavior of the terminal or text display. A control character is a CONTROL + key combination (pressed simultaneously). A control character may also be written in octal or hexadecimal notation, following an escape. Control characters are not normally useful inside a script. * Ctl-A Moves cursor to beginning of line of text (on the command-line). * Ctl-B Backspace (nondestructive). * Ctl-C Break. Terminate a foreground job. * Ctl-D Log out from a shell (similar to exit). EOF (end-of-file). This also terminates input from stdin. When typing text on the console or in an xterm window, Ctl-D erases the character under the cursor. When there are no characters present, Ctl-D logs out of the session, as expected. In an xterm window, this has the effect of closing the window. * Ctl-E Moves cursor to end of line of text (on the command-line). * Ctl-F Moves cursor forward one character position (on the command-line). * Ctl-G BEL. On some old-time teletype terminals, this would actually ring a bell. In an xterm it might beep. * Ctl-H Rubout (destructive backspace). Erases characters the cursor backs over while backspacing. #!/bin/bash # Embedding Ctl-H in a string. a="^H^H" # Two Ctl-H's -- backspaces # ctl-V ctl-H, using vi/vim echo "abcdef" # abcdef echo echo -n "abcdef$a " # abcd f # Space at end ^ ^ Backspaces twice. echo echo -n "abcdef$a" # abcdef # No space at end ^ Doesn't backspace (why?). # Results may not be quite as expected. echo; echo # Constantin Hagemeier suggests trying: # a=$'\010\010' # a=$'\b\b' # a=$'\x08\x08' # But, this does not change the results. * Ctl-I Horizontal tab. * Ctl-J Newline (line feed). In a script, may also be expressed in octal notation -- '\012' or in hexadecimal -- '\x0a'. * Ctl-K Vertical tab. When typing text on the console or in an xterm window, Ctl-K erases from the character under the cursor to end of line. Within a script, Ctl-K may behave differently, as in Lee Lee Maschmeyer's example, below. * Ctl-L Formfeed (clear the terminal screen). In a terminal, this has the same effect as the clear command. When sent to a printer, a Ctl-L causes an advance to end of the paper sheet. * Ctl-M Carriage return. #!/bin/bash # Thank you, Lee Maschmeyer, for this example. read -n 1 -s -p \ $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d' # Of course, '0d' is the hex equivalent of Control-M. echo >&2 # The '-s' makes anything typed silent, #+ so it is necessary to go to new line explicitly. read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a' # '0a' is the hex equivalent of Control-J, linefeed. echo >&2 ### read -n 1 -s -p $'And Control-K\x0bgoes straight down.' echo >&2 # Control-K is vertical tab. # A better example of the effect of a vertical tab is: var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a' echo "$var" # This works the same way as the above example. However: echo "$var" | col # This causes the right end of the line to be higher than the left end. # It also explains why we started and ended with a line feed -- #+ to avoid a garbled screen. # As Lee Maschmeyer explains: # -------------------------- # In the [first vertical tab example] . . . the vertical tab #+ makes the printing go straight down without a carriage return. # This is true only on devices, such as the Linux console, #+ that can't go "backward." # The real purpose of VT is to go straight UP, not down. # It can be used to print superscripts on a printer. # The col utility can be used to emulate the proper behavior of VT. exit 0 * Ctl-N Erases a line of text recalled from history buffer [20] (on the command-line). * Ctl-O Issues a newline (on the command-line). * Ctl-P Recalls last command from history buffer (on the command-line). * Ctl-Q Resume (XON). This resumes stdin in a terminal. * Ctl-R Backwards search for text in history buffer (on the command-line). * Ctl-S Suspend (XOFF). This freezes stdin in a terminal. (Use Ctl-Q to restore input.) * Ctl-T Reverses the position of the character the cursor is on with the previous character (on the command-line). * Ctl-U Erase a line of input, from the cursor backward to beginning of line. In some settings, Ctl-U erases the entire line of input, regardless of cursor position. * Ctl-V When inputting text, Ctl-V permits inserting control characters. For example, the following two are equivalent: echo -e '\x0a' echo Ctl-V is primarily useful from within a text editor. * Ctl-W When typing text on the console or in an xterm window, Ctl-W erases from the character under the cursor backwards to the first instance of whitespace. In some settings, Ctl-W erases backwards to first non-alphanumeric character. * Ctl-X In certain word processing programs, Cuts highlighted text and copies to clipboard. * Ctl-Y Pastes back text previously erased (with Ctl-K or Ctl-U). * Ctl-Z Pauses a foreground job. Substitute operation in certain word processing applications. EOF (end-of-file) character in the MSDOS filesystem. Whitespace functions as a separator between commands and/or variables. Whitespace consists of either spaces, tabs, blank lines, or any combination thereof. [21] In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error. Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections. $IFS, the special variable separating fields of input to certain commands. It defaults to whitespace. +--------------------------------------------------------------+ | Definition: A field is a discrete chunk of data expressed as | | a string of consecutive characters. Separating each field | | from adjacent fields is either whitespace or some other | | designated character (often determined by the $IFS). In some | | contexts, a field may be called a record. | +--------------------------------------------------------------+ To preserve whitespace within a string or in a variable, use quoting. UNIX filters can target and operate on whitespace using the POSIX character class [:space:]. ------------------------------------------------------------------------ Chapter 4. Introduction to Variables and Parameters Variables are how programming and scripting languages represent data. A variable is nothing more than a label, a name assigned to a location or set of locations in computer memory holding an item of data. Variables appear in arithmetic operations and manipulation of quantities, and in string parsing. ------------------------------------------------------------------------ 4.1. Variable Substitution The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is called variable substitution. $ Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. [22] +-------------------------------------------------------+ |bash$ variable1=23 | | | | | |bash$ echo variable1 | |variable1 | | | |bash$ echo $variable1 | |23 | +-------------------------------------------------------+ The only time a variable appears "naked" -- without the $ prefix -- is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see Example 29-5). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3). Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as "weak quoting." Using single quotes (' ... ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as 'strong quoting.' See Chapter 5 for a detailed discussion. Note that $variable is actually a simplified form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see Section 9.3, below). Example 4-1. Variable assignment and substitution #!/bin/bash # ex9.sh # Variables: assignment and substitution a=375 hello=$a #------------------------------------------------------------------------- # No space permitted on either side of = sign when initializing variables. # What happens if there is a space? # "VARIABLE =value" # ^ #% Script tries to run "VARIABLE" command with one argument, "=value". # "VARIABLE= value" # ^ #% Script tries to run "value" command with #+ the environmental variable "VARIABLE" set to "". #------------------------------------------------------------------------- echo hello # hello # Not a variable reference, just the string "hello" . . . echo $hello # 375 # ^ This *is* a variable reference. echo ${hello} # 375 # Also a variable reference, as above. # Quoting . . . echo "$hello" # 375 echo "${hello}" # 375 echo hello="A B C D" echo $hello # A B C D echo "$hello" # A B C D # As you see, echo $hello and echo "$hello" give different results. # Why? # ======================================= # Quoting a variable preserves whitespace. # ======================================= echo echo '$hello' # $hello # ^ ^ # Variable referencing disabled (escaped) by single quotes, #+ which causes the "$" to be interpreted literally. # Notice the effect of different types of quoting. hello= # Setting it to a null value. echo "\$hello (null value) = $hello" # Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below). # -------------------------------------------------------------- # It is permissible to set multiple variables on the same line, #+ if separated by white space. # Caution, this may reduce legibility, and may not be portable. var1=21 var2=22 var3=$V3 echo echo "var1=$var1 var2=$var2 var3=$var3" # May cause problems with older versions of "sh" . . . # -------------------------------------------------------------- echo; echo numbers="one two three" # ^ ^ other_numbers="1 2 3" # ^ ^ # If there is whitespace embedded within a variable, #+ then quotes are necessary. # other_numbers=1 2 3 # Gives an error message. echo "numbers = $numbers" echo "other_numbers = $other_numbers" # other_numbers = 1 2 3 # Escaping the whitespace also works. mixed_bag=2\ ---\ Whatever # ^ ^ Space after escape (\). echo "$mixed_bag" # 2 --- Whatever echo; echo echo "uninitialized_variable = $uninitialized_variable" # Uninitialized variable has null value (no value at all!). uninitialized_variable= # Declaring, but not initializing it -- #+ same as setting it to a null value, as above. echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. uninitialized_variable=23 # Set it. unset uninitialized_variable # Unset it. echo "uninitialized_variable = $uninitialized_variable" # It still has a null value. echo exit 0 Caution An uninitialized variable has a "null" value -- no assigned value at all (not zero!). if [ -z "$unassigned" ] then echo "\$unassigned is NULL." fi # $unassigned is NULL. Using a variable before assigning a value to it may cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable. echo "$uninitialized" # (blank line) let "uninitialized += 5" # Add 5 to it. echo "$uninitialized" # 5 # Conclusion: # An uninitialized variable has no value, #+ however it acts as if it were 0 in an arithmetic operation. # This is undocumented (and probably non-portable) behavior, #+ and should not be used in a script. See also Example 14-23. ------------------------------------------------------------------------ 4.2. Variable Assignment = the assignment operator (no space before and after) Caution Do not confuse this with = and -eq, which test, rather than assign! Note that = can be either an assignment or a test operator, depending on context. Example 4-2. Plain Variable Assignment #!/bin/bash # Naked variables echo # When is a variable "naked", i.e., lacking the '$' in front? # When it is being assigned, rather than referenced. # Assignment a=879 echo "The value of \"a\" is $a." # Assignment using 'let' let a=16+5 echo "The value of \"a\" is now $a." echo # In a 'for' loop (really, a type of disguised assignment): echo -n "Values of \"a\" in the loop are: " for a in 7 8 9 11 do echo -n "$a " done echo echo # In a 'read' statement (also a type of assignment): echo -n "Enter \"a\" " read a echo "The value of \"a\" is now $a." echo exit 0 Example 4-3. Variable Assignment, plain and fancy #!/bin/bash a=23 # Simple case echo $a b=$a echo $b # Now, getting a little bit fancier (command substitution). a=`echo Hello!` # Assigns result of 'echo' command to 'a' ... echo $a # Note that including an exclamation mark (!) within a #+ command substitution construct will not work from the command-line, #+ since this triggers the Bash "history mechanism." # Inside a script, however, the history functions are disabled. a=`ls -l` # Assigns result of 'ls -l' command to 'a' echo $a # Unquoted, however, it removes tabs and newlines. echo echo "$a" # The quoted variable preserves whitespace. # (See the chapter on "Quoting.") exit 0 Variable assignment using the $(...) mechanism (a newer method than backquotes). This is actually a form of command substitution. # From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m) ------------------------------------------------------------------------ 4.3. Bash Variables Are Untyped Unlike many other programming languages, Bash does not segregate its variables by "type." Essentially, Bash variables are character strings, but, depending on context, Bash permits arithmetic operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits. Example 4-4. Integer or string? #!/bin/bash # int-or-string.sh a=2334 # Integer. let "a += 1" echo "a = $a " # a = 2335 echo # Integer, still. b=${a/23/BB} # Substitute "BB" for "23". # This transforms $b into a string. echo "b = $b" # b = BB35 declare -i b # Declaring it an integer doesn't help. echo "b = $b" # b = BB35 let "b += 1" # BB35 + 1 echo "b = $b" # b = 1 echo # Bash sets the "integer value" of a string to 0. c=BB34 echo "c = $c" # c = BB34 d=${c/BB/23} # Substitute "23" for "BB". # This makes $d an integer. echo "d = $d" # d = 2334 let "d += 1" # 2334 + 1 echo "d = $d" # d = 2335 echo # What about null variables? e='' # ... Or e="" ... Or e= echo "e = $e" # e = let "e += 1" # Arithmetic operations allowed on a null variable? echo "e = $e" # e = 1 echo # Null variable transformed into an integer. # What about undeclared variables? echo "f = $f" # f = let "f += 1" # Arithmetic operations allowed? echo "f = $f" # f = 1 echo # Undeclared variable transformed into an integer. # # However ... let "f /= $undecl_var" # Divide by zero? # let: f /= : syntax error: operand expected (error token is " ") # Syntax error! Variable $undecl_var is not set to zero here! # # But still ... let "f /= 0" # let: f /= 0: division by 0 (error token is "0") # Expected behavior. # Bash (usually) sets the "integer value" of null to zero #+ when performing an arithmetic operation. # But, don't try this at home, folks! # It's undocumented and probably non-portable behavior. # Conclusion: Variables in Bash are untyped, #+ with all attendant consequences. exit $? Untyped variables are both a blessing and a curse. They permit more flexibility in scripting and make it easier to grind out lines of code (and give you enough rope to hang yourself!). However, they likewise permit subtle errors to creep in and encourage sloppy programming habits. To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables. ------------------------------------------------------------------------ 4.4. Special Variable Types Local variables Variables visible only within a code block or function (see also local variables in functions) Environmental variables Variables that affect the behavior of the shell and user interface Note In a more general context, each process has an "environment", that is, a group of variables that the process may reference. In this sense, the shell behaves like any other process. Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment. Caution The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems. +--------------------------------------------------------------------+ |bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"| | | |bash$ du | |bash: /usr/bin/du: Argument list too long | | | +--------------------------------------------------------------------+ Note: this "error" has been fixed, as of kernel version 2.6.23. (Thank you, Stéphane Chazelas for the clarification, and for providing the above example.) If a script sets environmental variables, they need to be "exported," that is, reported to the environment local to the script. This is the function of the export command. Note A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command-line cannot export variables back to the command-line environment. Child processes cannot export variables back to the parent processes that spawned them. Definition: A child process is a subprocess launched by another process, its parent. Positional parameters Arguments passed to the script from the command line [23] : $0, $1, $2, $3 . . . $0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. [24] After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}. The special variables $* and $@ denote all the positional parameters. Example 4-5. Positional Parameters #!/bin/bash # Call this script with at least 10 parameters, for example # ./scriptname 1 2 3 4 5 6 7 8 9 10 MINPARAMS=10 echo echo "The name of this script is \"$0\"." # Adds ./ for current directory echo "The name of this script is \"`basename $0`\"." # Strips out path name info (see 'basename') echo if [ -n "$1" ] # Tested variable is quoted. then echo "Parameter #1 is $1" # Need quotes to escape # fi if [ -n "$2" ] then echo "Parameter #2 is $2" fi if [ -n "$3" ] then echo "Parameter #3 is $3" fi # ... if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}. then echo "Parameter #10 is ${10}" fi echo "-----------------------------------" echo "All the command-line parameters are: "$*"" if [ $# -lt "$MINPARAMS" ] then echo echo "This script needs at least $MINPARAMS command-line arguments!" fi echo exit 0 Bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command-line. This also requires indirect referencing. args=$# # Number of args passed. lastarg=${!args} # Note: This is an *indirect reference* to $args ... # Or: lastarg=${!#} (Thanks, Chris Monson.) # This is an *indirect reference* to the $# variable. # Note that lastarg=${!$#} doesn't work. Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There must also exist symbolic links to all the alternate names of the script. See Example 15-2. Tip If a script expects a command-line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter. variable1_=$1_ # Rather than variable1=$1 # This will prevent an error, even if positional parameter is absent. critical_argument01=$variable1_ # The extra character can be stripped off later, like so. variable1=${variable1_/_/} # Side effects only if $variable1_ begins with an underscore. # This uses one of the parameter substitution templates discussed later. # (Leaving out the replacement pattern results in a deletion.) # A more straightforward way of dealing with this is #+ to simply test whether expected positional parameters have been passed. if [ -z $1 ] then exit $E_MISSING_POS_PARAM fi # However, as Fabian Kreutz points out, #+ the above method may have unexpected side-effects. # A better method is parameter substitution: # ${1:-$DefaultVal} # See the "Parameter Substition" section #+ in the "Variables Revisited" chapter. --- Example 4-6. wh, whois domain name lookup #!/bin/bash # ex18.sh # Does a 'whois domain-name' lookup on any of 3 alternate servers: # ripe.net, cw.net, radb.net # Place this script -- renamed 'wh' -- in /usr/local/bin # Requires symbolic links: # ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe # ln -s /usr/local/bin/wh /usr/local/bin/wh-cw # ln -s /usr/local/bin/wh /usr/local/bin/wh-radb E_NOARGS=65 if [ -z "$1" ] then echo "Usage: `basename $0` [domain-name]" exit $E_NOARGS fi # Check script name and call proper server. case `basename $0` in # Or: case ${0##*/} in "wh" ) whois $1@whois.ripe.net;; "wh-ripe") whois $1@whois.ripe.net;; "wh-radb") whois $1@whois.radb.net;; "wh-cw" ) whois $1@whois.cw.net;; * ) echo "Usage: `basename $0` [domain-name]";; esac exit $? --- The shift command reassigns the positional parameters, in effect shifting them to the left one notch. $1 <--- $2, $2 <--- $3, $3 <--- $4, etc. The old $1 disappears, but $0 (the script name) does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10, although {bracket} notation also permits this. Example 4-7. Using shift #!/bin/bash # shft.sh: Using 'shift' to step through all the positional parameters. # Name this script something like shft.sh, #+ and invoke it with some parameters. #+ For example: # sh shft.sh a b c def 23 Skidoo until [ -z "$1" ] # Until all parameters used up . . . do echo -n "$1 " shift done echo # Extra linefeed. # But, what happens to the "used-up" parameters? echo "$2" # Nothing echoes! # When $2 shifts into $1 (and there is no $3 to shift into $2) #+ then $2 remains empty. # So, it is not a parameter *copy*, but a *move*. exit # See also the echo-params.sh script for a "shiftless" #+ alternative method of stepping through the positional params. The shift command can take a numerical parameter indicating how many positions to shift. #!/bin/bash # shift-past.sh shift 3 # Shift 3 positions. # n=3; shift $n # Has the same effect. echo "$1" exit 0 # ======================== # $ sh shift-past.sh 1 2 3 4 5 4 # However, as Eleni Fragkiadaki, points out, #+ attempting a 'shift' past the number of #+ positional parameters ($#) returns an exit status of 1, #+ and the positional parameters themselves do not change. # This means possibly getting stuck in an endless loop. . . . # For example: # until [ -z "$1" ] # do # echo -n "$1 " # shift 20 # If less than 20 pos params, # done #+ then loop never ends! # # When in doubt, add a sanity check. . . . # shift 20 || break # ^^^^^^^^ Note The shift command works in a similar fashion on parameters passed to a function. See Example 33-16. ------------------------------------------------------------------------ Chapter 5. Quoting Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an interpretation other than its literal meaning. For example, the asterisk * represents a wild card character in globbing and Regular Expressions). +----------------------------------------------------------------------+ |bash$ ls -l [Vv]* | |-rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT | | -rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh | | -rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh | | | |bash$ ls -l '[Vv]*' | |ls: [Vv]*: No such file or directory | +----------------------------------------------------------------------+ +----------------------------------------------------------------------+ | In everyday speech or writing, when we "quote" a phrase, we set it | | apart and give it special meaning. In a Bash script, when we quote a | | string, we set it apart and protect its literal meaning. | +----------------------------------------------------------------------+ Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it. +----------------------------------------------------------------------+ |bash$ grep '[Ff]irst' *.txt | |file1.txt:This is the first line of file1.txt. | | file2.txt:This is the First line of file2.txt. | +----------------------------------------------------------------------+ Note that the unquoted grep [Ff]irst *.txt works under the Bash shell. [25] Quoting can also suppress echo's "appetite" for newlines. +---------------------------------------------------------------------------------------+ |bash$ echo $(ls -l) | |total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh| | | | | |bash$ echo "$(ls -l)" | |total 8 | | -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh | | -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh | +---------------------------------------------------------------------------------------+ ------------------------------------------------------------------------ 5.1. Quoting Variables When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). [26] Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see Example 4-1, above). Use double quotes to prevent word splitting. [27] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators. List="one two three" for a in $List # Splits the variable in parts at whitespace. do echo "$a" done # one # two # three echo "---" for a in "$List" # Preserves whitespace in a single variable. do # ^ ^ echo "$a" done # one two three A more elaborate example: variable1="a variable containing five words" COMMAND This is $variable1 # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words" COMMAND "This is $variable1" # Executes COMMAND with 1 argument: # "This is a variable containing five words" variable2="" # Empty. COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments. COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments. COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces). # Thanks, Stéphane Chazelas. Tip Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue. Example 5-1. Echoing Weird Variables #!/bin/bash # weirdvars.sh: Echoing weird variables. echo var="'(]\\{}\$\"" echo $var # '(]\{}$" echo "$var" # '(]\{}$" Doesn't make a difference. echo IFS='\' echo $var # '(] {}$" \ converted to space. Why? echo "$var" # '(]\{}$" # Examples above supplied by Stephane Chazelas. echo var2="\\\\\"" echo $var2 # " echo "$var2" # \\" echo # But ... var2="\\\\"" is illegal. Why? var3='\\\\' echo "$var3" # \\\\ # Strong quoting works, though. exit Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting"). Note Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result. echo "Why can't I write 's between single quotes" echo # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' # |-------| |----------| |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between. # This example courtesy of Stéphane Chazelas. ------------------------------------------------------------------------ 5.2. Escaping Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally. Caution With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character. Special meanings of certain escaped characters used with echo and sed \n means newline \r means return \t means tab \v means vertical tab \b means backspace \a means alert (beep or flash) \0xx translates to the octal ASCII equivalent of 0nn, where nn is a string of digits Example 5-2. Escaped Characters #!/bin/bash # escaped.sh: escaped characters echo; echo # Escaping a newline. # ------------------ echo "" echo "This will print as two lines." # This will print # as two lines. echo "This will print \ as one line." # This will print as one line. echo; echo echo "=============" echo "\v\v\v\v" # Prints \v\v\v\v literally. # Use the -e option with 'echo' to print escaped characters. echo "=============" echo "VERTICAL TABS" echo -e "\v\v\v\v" # Prints 4 vertical tabs. echo "==============" echo "QUOTATION MARK" echo -e "\042" # Prints " (quote, octal ASCII character 42). echo "==============" # The $'\X' construct makes the -e option unnecessary. echo; echo "NEWLINE AND BEEP" echo $'\n' # Newline. echo $'\a' # Alert (beep). echo "===============" echo "QUOTATION MARKS" # Version 2 and later of Bash permits using the $'\nnn' construct. # Note that in this case, '\nnn' is an octal value. echo $'\t \042 \t' # Quote (") framed by tabs. # It also works with hexadecimal values, in an $'\xhhh' construct. echo $'\t \x22 \t' # Quote (") framed by tabs. # Thank you, Greg Keraunen, for pointing this out. # Earlier Bash versions allowed '\x022'. echo "===============" echo # Assigning ASCII characters to a variable. # ---------------------------------------- quote=$'\042' # " assigned to a variable. echo "$quote This is a quoted string, $quote and this lies outside the quotes." echo # Concatenating ASCII chars in a variable. triple_underline=$'\137\137\137' # 137 is octal ASCII code for '_'. echo "$triple_underline UNDERLINE $triple_underline" echo ABC=$'\101\102\103\010' # 101, 102, 103 are octal A, B, C. echo $ABC echo; echo escape=$'\033' # 033 is octal for escape. echo "\"escape\" echoes as $escape" # no visible output. echo; echo exit 0 See Example 34-1 for another example of the $' ... ' string-expansion construct. \" gives the quote its literal meaning echo "Hello" # Hello echo "\"Hello\" ... he said." # "Hello" ... he said. \$ gives the dollar sign its literal meaning (variable name following \$ will not be referenced) echo "\$variable01" # $variable01 echo "The book cost \$7.98." # The book cost $7.98. \\ gives the backslash its literal meaning echo "\\" # Results in \ # Whereas . . . echo "\" # Invokes secondary prompt from the command-line. # In a script, gives an error message. # However . . . echo '\' # Results in \ Note The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within command substitution or a here document. # Simple escaping and quoting echo \z # z echo \\z # \z echo '\z' # \z echo '\\z' # \\z echo "\z" # \z echo "\\z" # \z # Command substitution echo `echo \z` # z echo `echo \\z` # z echo `echo \\\z` # \z echo `echo \\\\z` # \z echo `echo \\\\\\z` # \z echo `echo \\\\\\\z` # \\z echo `echo "\z"` # \z echo `echo "\\z"` # \z # Here document cat < /dev/null # Suppress output. then echo "Files a and b are identical." else echo "Files a and b differ." fi # The very useful "if-grep" construct: # ----------------------------------- if grep -q Bash file then echo "File contains at least one occurrence of Bash." fi word=Linux letter_sequence=inu if echo "$word" | grep -q "$letter_sequence" # The "-q" option to grep suppresses output. then echo "$letter_sequence found in $word" else echo "$letter_sequence not found in $word" fi if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED then echo "Command succeeded." else echo "Command failed." fi * These last two examples courtesy of Stéphane Chazelas. Example 7-1. What is truth? #!/bin/bash # Tip: # If you're unsure of how a certain condition would evaluate, #+ test it in an if-test. echo echo "Testing \"0\"" if [ 0 ] # zero then echo "0 is true." else # Or else ... echo "0 is false." fi # 0 is true. echo echo "Testing \"1\"" if [ 1 ] # one then echo "1 is true." else echo "1 is false." fi # 1 is true. echo echo "Testing \"-1\"" if [ -1 ] # minus one then echo "-1 is true." else echo "-1 is false." fi # -1 is true. echo echo "Testing \"NULL\"" if [ ] # NULL (empty condition) then echo "NULL is true." else echo "NULL is false." fi # NULL is false. echo echo "Testing \"xyz\"" if [ xyz ] # string then echo "Random string is true." else echo "Random string is false." fi # Random string is true. echo echo "Testing \"\$xyz\"" if [ $xyz ] # Tests if $xyz is null, but... # it's only an uninitialized variable. then echo "Uninitialized variable is true." else echo "Uninitialized variable is false." fi # Uninitialized variable is false. echo echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ] # More pedantically correct. then echo "Uninitialized variable is true." else echo "Uninitialized variable is false." fi # Uninitialized variable is false. echo xyz= # Initialized, but set to null value. echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ] then echo "Null variable is true." else echo "Null variable is false." fi # Null variable is false. echo # When is "false" true? echo "Testing \"false\"" if [ "false" ] # It seems that "false" is just a string. then echo "\"false\" is true." #+ and it tests true. else echo "\"false\" is false." fi # "false" is true. echo echo "Testing \"\$false\"" # Again, uninitialized variable. if [ "$false" ] then echo "\"\$false\" is true." else echo "\"\$false\" is false." fi # "$false" is false. # Now, we get the expected result. # What would happen if we tested the uninitialized variable "$true"? echo exit 0 Exercise. Explain the behavior of Example 7-1, above. if [ condition-true ] then command 1 command 2 ... else # Or else ... # Adds default code block executing if original condition tests false. command 3 command 4 ... fi Note When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate. if [ -x "$filename" ]; then Else if and elif elif elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one. if [ condition1 ] then command1 command2 command3 elif [ condition2 ] # Same as else if then command4 command5 else default-command fi The if test condition-true construct is the exact equivalent of if [ condition-true ]. As it happens, the left bracket, [ , is a token [29] which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it. Note The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test. +-----------------------------------------------------------------+ |bash$ type test | |test is a shell builtin | |bash$ type '[' | |[ is a shell builtin | |bash$ type '[[' | |[[ is a shell keyword | |bash$ type ']]' | |]] is a shell keyword | |bash$ type ']' | |bash: type: ]: not found | | | +-----------------------------------------------------------------+ If, for some reason, you wish to use /usr/bin/test in a Bash script, then specify it by full pathname. Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[ #!/bin/bash echo if test -z "$1" then echo "No command-line arguments." else echo "First command-line argument is $1." fi echo if /usr/bin/test -z "$1" # Equivalent to "test" builtin. # ^^^^^^^^^^^^^ # Specifying full pathname. then echo "No command-line arguments." else echo "First command-line argument is $1." fi echo if [ -z "$1" ] # Functionally identical to above code blocks. # if [ -z "$1" should work, but... #+ Bash responds to a missing close-bracket with an error message. then echo "No command-line arguments." else echo "First command-line argument is $1." fi echo if /usr/bin/[ -z "$1" ] # Again, functionally identical to above. # if /usr/bin/[ -z "$1" # Works, but gives an error message. # # Note: # This has been fixed in Bash, version 3.x. then echo "No command-line arguments." else echo "First command-line argument is $1." fi echo exit 0 +-------------------------------------------------------------------------+ |The [[ ]] construct is the more versatile Bash version of [ ]. This is | |the extended test command, adopted from ksh88. | | | |* * * | | | |No filename expansion or word splitting takes place between [[ and ]], | |but there is parameter expansion and command substitution. | | | |file=/etc/passwd | | | |if [[ -e $file ]] | |then | | echo "Password file exists." | |fi | | | |Using the [[ ... ]] test construct, rather than [ ... ] can prevent many | |logic errors in scripts. For example, the &&, ||, <, and > operators work| |within a [[ ]] test, despite giving an error within a [ ] construct. | | | |Arithmetic evaluation of octal / hexadecimal constants takes place | |automatically within a [[ ... ]] construct. | | | |# [[ Octal and hexadecimal evaluation ]] | |# Thank you, Moritz Gronbach, for pointing this out. | | | | | |decimal=15 | |octal=017 # = 15 (decimal) | |hex=0x0f # = 15 (decimal) | | | |if [ "$decimal" -eq "$octal" ] | |then | | echo "$decimal equals $octal" | |else | | echo "$decimal is not equal to $octal" # 15 is not equal to 017 | |fi # Doesn't evaluate within [ single brackets ]! | | | | | |if [[ "$decimal" -eq "$octal" ]] | |then | | echo "$decimal equals $octal" # 15 equals 017 | |else | | echo "$decimal is not equal to $octal" | |fi # Evaluates within [[ double brackets ]]! | | | |if [[ "$decimal" -eq "$hex" ]] | |then | | echo "$decimal equals $hex" # 15 equals 0x0f | |else | | echo "$decimal is not equal to $hex" | |fi # [[ $hexadecimal ]] also evaluates! | +-------------------------------------------------------------------------+ Note Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary. dir=/home/bozo if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message. echo "Now in $dir." else echo "Can't change to $dir." fi The "if COMMAND" construct returns the exit status of COMMAND. Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct. var1=20 var2=22 [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2" home=/home/bozo [ -d "$home" ] || echo "$home directory does not exist." The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or "false". A non-zero expression returns an exit status of 0, or "true". This is in marked contrast to using the test and [ ] constructs previously discussed. Example 7-3. Arithmetic Tests using (( )) #!/bin/bash # Arithmetic tests. # The (( ... )) construct evaluates and tests numerical expressions. # Exit status opposite from [ ... ] construct! (( 0 )) echo "Exit status of \"(( 0 ))\" is $?." # 1 (( 1 )) echo "Exit status of \"(( 1 ))\" is $?." # 0 (( 5 > 4 )) # true echo "Exit status of \"(( 5 > 4 ))\" is $?." # 0 (( 5 > 9 )) # false echo "Exit status of \"(( 5 > 9 ))\" is $?." # 1 (( 5 - 5 )) # 0 echo "Exit status of \"(( 5 - 5 ))\" is $?." # 1 (( 5 / 4 )) # Division o.k. echo "Exit status of \"(( 5 / 4 ))\" is $?." # 0 (( 1 / 2 )) # Division result < 1. echo "Exit status of \"(( 1 / 2 ))\" is $?." # Rounded off to 0. # 1 (( 1 / 0 )) 2>/dev/null # Illegal division by 0. # ^^^^^^^^^^^ echo "Exit status of \"(( 1 / 0 ))\" is $?." # 1 # What effect does the "2>/dev/null" have? # What would happen if it were removed? # Try removing it, then rerunning the script. # ======================================= # # (( ... )) also useful in an if-then test. var1=5 var2=4 if (( var1 > var2 )) then #^ ^ Note: Not $var1, $var2. Why? echo "$var1 is greater than $var2" fi # 5 is greater than 4 exit 0 ------------------------------------------------------------------------ 7.2. File test operators Returns true if... -e file exists -a file exists This is identical in effect to -e. It has been "deprecated," [30] and its use is discouraged. -f file is a regular file (not a directory or device file) -s file is not zero size -d file is a directory -b file is a block device -c file is a character device device0="/dev/sda2" # / (root directory) if [ -b "$device0" ] then echo "$device0 is a block device." fi # /dev/sda2 is a block device. device1="/dev/ttyS1" # PCMCIA modem card. if [ -c "$device1" ] then echo "$device1 is a character device." fi # /dev/ttyS1 is a character device. -p file is a pipe -h file is a symbolic link -L file is a symbolic link -S file is a socket -t file (descriptor) is associated with a terminal device This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal. -r file has read permission (for the user running the test) -w file has write permission (for the user running the test) -x file has execute permission (for the user running the test) -g set-group-id (sgid) flag set on file or directory If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup. -u set-user-id (suid) flag set on file A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. [31] This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user. +---------------------------------------------------------------------------+ | -rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd| | | +---------------------------------------------------------------------------+ A file with the suid flag set shows an s in its permissions. -k sticky bit set Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. [32] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. +-----------------------------------------------------------------+ | drwxrwxrwt 7 root 1024 May 19 21:26 tmp/| | | +-----------------------------------------------------------------+ If a user does not own a directory that has the sticky bit set, but has write permission in that directory, she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. (The owner of the directory or root can, of course, delete or rename files there.) -O you are owner of file -G group-id of file same as yours -N file modified since it was last read f1 -nt f2 file f1 is newer than f2 f1 -ot f2 file f1 is older than f2 f1 -ef f2 files f1 and f2 are hard links to the same file ! "not" -- reverses the sense of the tests above (returns true if condition absent). Example 7-4. Testing for broken links #!/bin/bash # broken-link.sh # Written by Lee bigelow # Used in ABS Guide with permission. # A pure shell script to find dead symlinks and output them quoted #+ so they can be fed to xargs and dealt with :) #+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm # # This, however, is a better method: # # find "somedir" -type l -print0|\ # xargs -r0 file|\ # grep "broken symbolic"| # sed -e 's/^\|: *broken symbolic.*$/"/g' # #+ but that wouldn't be pure Bash, now would it. # Caution: beware the /proc file system and any circular links! ################################################################ # If no args are passed to the script set directories-to-search #+ to current directory. Otherwise set the directories-to-search #+ to the args passed. ###################### [ $# -eq 0 ] && directorys=`pwd` || directorys=$@ # Setup the function linkchk to check the directory it is passed #+ for files that are links and don't exist, then print them quoted. # If one of the elements in the directory is a subdirectory then #+ send that subdirectory to the linkcheck function. ########## linkchk () { for element in $1/*; do [ -h "$element" -a ! -e "$element" ] && echo \"$element\" [ -d "$element" ] && linkchk $element # Of course, '-h' tests for symbolic link, '-d' for directory. done } # Send each arg that was passed to the script to the linkchk() function #+ if it is a valid directoy. If not, then print the error message #+ and usage info. ################## for directory in $directorys; do if [ -d $directory ] then linkchk $directory else echo "$directory is not a directory" echo "Usage: $0 dir1 dir2 ..." fi done exit $? Example 28-1, Example 10-7, Example 10-3, Example 28-3, and Example A-1 also illustrate uses of the file test operators. ------------------------------------------------------------------------ 7.3. Other Comparison Operators A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators. integer comparison -eq is equal to if [ "$a" -eq "$b" ] -ne is not equal to if [ "$a" -ne "$b" ] -gt is greater than if [ "$a" -gt "$b" ] -ge is greater than or equal to if [ "$a" -ge "$b" ] -lt is less than if [ "$a" -lt "$b" ] -le is less than or equal to if [ "$a" -le "$b" ] < is less than (within double parentheses) (("$a" < "$b")) <= is less than or equal to (within double parentheses) (("$a" <= "$b")) > is greater than (within double parentheses) (("$a" > "$b")) >= is greater than or equal to (within double parentheses) (("$a" >= "$b")) string comparison = is equal to if [ "$a" = "$b" ] == is equal to if [ "$a" == "$b" ] This is a synonym for =. Note The == comparison operator behaves differently within a double-brackets test than within single brackets. [[ $a == z* ]] # True if $a starts with an "z" (regex pattern matching). [[ $a == "z*" ]] # True if $a is equal to z* (literal matching). [ $a == z* ] # File globbing and word splitting take place. [ "$a" == "z*" ] # True if $a is equal to z* (literal matching). # Thanks, Stéphane Chazelas != is not equal to if [ "$a" != "$b" ] This operator uses pattern matching within a [[ ... ]] construct. < is less than, in ASCII alphabetical order if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ] Note that the "<" needs to be escaped within a [ ] construct. > is greater than, in ASCII alphabetical order if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ] Note that the ">" needs to be escaped within a [ ] construct. See Example 26-11 for an application of this comparison operator. -z string is null, that is, has zero length String='' # Zero-length ("null") string variable. if [ -z "$String" ] then echo "\$String is null." else echo "\$String is NOT null." fi # $String is null. -n string is not null. Caution The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [33] Example 7-5. Arithmetic and string comparisons #!/bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed. # Bash permits integer operations and comparisons on variables #+ whose value consists of all-integer characters. # Caution advised, however. echo if [ "$a" -ne "$b" ] then echo "$a is not equal to $b" echo "(arithmetic comparison)" fi echo if [ "$a" != "$b" ] then echo "$a is not equal to $b." echo "(string comparison)" # "4" != "5" # ASCII 52 != ASCII 53 fi # In this particular instance, both "-ne" and "!=" work. echo exit 0 Example 7-6. Testing whether a string is null #!/bin/bash # str-test.sh: Testing null strings and unquoted strings, #+ but not strings and sealing wax, not to mention cabbages and kings . . . # Using if [ ... ] # If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero!). if [ -n $string1 ] # string1 has not been declared or initialized. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Wrong result. # Shows $string1 as not null, although it was not initialized. echo # Let's try it again. if [ -n "$string1" ] # This time, $string1 is quoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Quote strings within test brackets! echo if [ $string1 ] # This time, $string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # This works fine. # The [ ... ] test operator alone detects whether the string is null. # However it is good practice to quote it (if [ "$string1" ]). # # As Stephane Chazelas points out, # if [ $string1 ] has one argument, "]" # if [ "$string1" ] has two arguments, the empty "$string1" and "]" echo string1=initialized if [ $string1 ] # Again, $string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Again, gives correct result. # Still, it is better to quote it ("$string1"), because . . . string1="a = b" if [ $string1 ] # Again, $string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Not quoting "$string1" now gives wrong result! exit 0 # Thank you, also, Florian Wisser, for the "heads-up". Example 7-7. zmore #!/bin/bash # zmore # View gzipped files with 'more' filter. E_NOARGS=65 E_NOTFOUND=66 E_NOTGZIP=67 if [ $# -eq 0 ] # same effect as: if [ -z "$1" ] # $1 can exist, but be empty: zmore "" arg2 arg3 then echo "Usage: `basename $0` filename" >&2 # Error message to stderr. exit $E_NOARGS # Returns 65 as exit status of script (error code). fi filename=$1 if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces. then echo "File $filename not found!" >&2 # Error message to stderr. exit $E_NOTFOUND fi if [ ${filename##*.} != "gz" ] # Using bracket in variable substitution. then echo "File $1 is not a gzipped file!" exit $E_NOTGZIP fi zcat $1 | more # Uses the 'more' filter. # May substitute 'less' if desired. exit $? # Script returns exit status of pipe. # Actually "exit $?" is unnecessary, as the script will, in any case, #+ return the exit status of the last command executed. compound comparison -a logical and exp1 -a exp2 returns true if both exp1 and exp2 are true. -o logical or exp1 -o exp2 returns true if either exp1 or exp2 is true. These are similar to the Bash comparison operators && and ||, used within double brackets. [[ condition1 && condition2 ]] The -o and -a operators work with the test command or occur within single test brackets. if [ "$expr1" -a "$expr2" ] then echo "Both expr1 and expr2 are true." else echo "Either expr1 or expr2 is false." fi Caution But, as rihad points out: [ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output) # ^^^^^^^ False condition. So far, everything as expected. # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true # ^^^^^^^ False condition. So, why "true" output? # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output) # No, that's not it. # Apparently && and || "short-circuit" while -a and -o do not. Refer to Example 8-3, Example 26-17, and Example A-29 to see compound comparison operators in action. ------------------------------------------------------------------------ 7.4. Nested if/then Condition Tests Condition tests using the if/then construct may be nested. The net result is equivalent to using the && compound comparison operator. a=3 if [ "$a" -gt 0 ] then if [ "$a" -lt 5 ] then echo "The value of \"a\" lies somewhere between 0 and 5." fi fi # Same result as: if [ "$a" -gt 0 ] && [ "$a" -lt 5 ] then echo "The value of \"a\" lies somewhere between 0 and 5." fi Example 34-4 demonstrates a nested if/then condition test. ------------------------------------------------------------------------ 7.5. Testing Your Knowledge of Tests The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests. The following is excerpted from an "ancient" version of xinitrc (Red Hat 7.1, or thereabouts). if [ -f $HOME/.Xclients ]; then exec $HOME/.Xclients elif [ -f /etc/X11/xinit/Xclients ]; then exec /etc/X11/xinit/Xclients else # failsafe settings. Although we should never get here # (we provide fallbacks in Xclients as well) it can't hurt. xclock -geometry 100x100-5+5 & xterm -geometry 80x50-50+150 & if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then netscape /usr/share/doc/HTML/index.html & fi fi Explain the test constructs in the above snippet, then examine an updated version of the file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions. ------------------------------------------------------------------------ Chapter 8. Operations and Related Topics 8.1. Operators assignment variable assignment Initializing or changing the value of a variable = All-purpose assignment operator, which works for both arithmetic and string assignments. var=27 category=minerals # No spaces allowed after the "=". Caution Do not confuse the "=" assignment operator with the = test operator. # = as a test operator if [ "$string1" = "$string2" ] then command fi # if [ "X$string1" = "X$string2" ] is safer, #+ to prevent an error message should one of the variables be empty. # (The prepended "X" characters cancel out.) arithmetic operators + plus - minus * multiplication / division ** exponentiation # Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" # 5 * 5 * 5 echo "z = $z" # z = 125 % modulo, or mod (returns the remainder of an integer division operation) +-------------------------------------------------------+ |bash$ expr 5 % 3 | |2 | | | +-------------------------------------------------------+ 5/3 = 1, with remainder 2 This operator finds use in, among other things, generating numbers within a specific range (see Example 9-26 and Example 9-30) and formatting program output (see Example 26-16 and Example A-6). It can even be used to generate prime numbers, (see Example A-15). Modulo turns up surprisingly often in numerical recipes. Example 8-1. Greatest common divisor #!/bin/bash # gcd.sh: greatest common divisor # Uses Euclid's algorithm # The "greatest common divisor" (gcd) of two integers #+ is the largest integer that will divide both, leaving no remainder. # Euclid's algorithm uses successive division. # In each pass, #+ dividend <--- divisor #+ divisor <--- remainder #+ until remainder = 0. # The gcd = dividend, on the final pass. # # For an excellent discussion of Euclid's algorithm, see #+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm. # ------------------------------------------------------ # Argument check ARGS=2 E_BADARGS=85 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` first-number second-number" exit $E_BADARGS fi # ------------------------------------------------------ gcd () { dividend=$1 # Arbitrary assignment. divisor=$2 #! It doesn't matter which of the two is larger. # Why not? remainder=1 # If an uninitialized variable is used inside #+ test brackets, an error message results. until [ "$remainder" -eq 0 ] do # ^^^^^^^^^^ Must be previously initialized! let "remainder = $dividend % $divisor" dividend=$divisor # Now repeat with 2 smallest numbers. divisor=$remainder done # Euclid's algorithm } # Last $dividend is the gcd. gcd $1 $2 echo; echo "GCD of $1 and $2 = $dividend"; echo # Exercises : # --------- # 1) Check command-line arguments to make sure they are integers, #+ and exit the script with an appropriate error message if not. # 2) Rewrite the gcd () function to use local variables. exit 0 += plus-equal (increment variable by a constant) let "var += 5" results in var being incremented by 5. -= minus-equal (decrement variable by a constant) *= times-equal (multiply variable by a constant) let "var *= 4" results in var being multiplied by 4. /= slash-equal (divide variable by a constant) %= mod-equal (remainder of dividing variable by a constant) Arithmetic operators often occur in an expr or let expression. Example 8-2. Using Arithmetic Operations #!/bin/bash # Counting to 11 in 10 different ways. n=1; echo -n "$n " let "n = $n + 1" # let "n = n + 1" also works. echo -n "$n " : $((n = $n + 1)) # ":" necessary because otherwise Bash attempts #+ to interpret "$((n = $n + 1))" as a command. echo -n "$n " (( n = n + 1 )) # A simpler alternative to the method above. # Thanks, David Lombard, for pointing this out. echo -n "$n " n=$(($n + 1)) echo -n "$n " : $[ n = $n + 1 ] # ":" necessary because otherwise Bash attempts #+ to interpret "$[ n = $n + 1 ]" as a command. # Works even if "n" was initialized as a string. echo -n "$n " n=$[ $n + 1 ] # Works even if "n" was initialized as a string. #* Avoid this type of construct, since it is obsolete and nonportable. # Thanks, Stephane Chazelas. echo -n "$n " # Now for C-style increment operators. # Thanks, Frank Wang, for pointing this out. let "n++" # let "++n" also works. echo -n "$n " (( n++ )) # (( ++n ) also works. echo -n "$n " : $(( n++ )) # : $(( ++n )) also works. echo -n "$n " : $[ n++ ] # : $[ ++n ]] also works echo -n "$n " echo exit 0 Note Integer variables in older versions of Bash were signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that took a variable outside these limits gave an erroneous result. echo $BASH_VERSION # 1.14 a=2147483646 echo "a = $a" # a = 2147483646 let "a+=1" # Increment "a". echo "a = $a" # a = 2147483647 let "a+=1" # increment "a" again, past the limit. echo "a = $a" # a = -2147483648 # ERROR: out of range, # + and the leftmost bit, the sign bit, # + has been set, making the result negative. As of version >= 2.05b, Bash supports 64-bit integers. Caution Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings. a=1.5 let "b = $a + 1.3" # Error. # t2.sh: let: b = 1.5 + 1.3: syntax error in expression # (error token is ".5 + 1.3") echo "b = $b" # b=1 Use bc in scripts that that need floating point calculations or math library functions. bitwise operators. The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to be manipulating and testing values read from ports or sockets. "Bit flipping" is more relevant to compiled languages, such as C and C++, which provide direct access to system hardware. bitwise operators << bitwise left shift (multiplies by 2 for each shift position) <<= left-shift-equal let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4) >> bitwise right shift (divides by 2 for each shift position) >>= right-shift-equal (inverse of <<=) & bitwise AND &= bitwise AND-equal | bitwise OR |= bitwise OR-equal ~ bitwise NOT ^ bitwise XOR ^= bitwise XOR-equal logical (boolean) operators ! NOT if [ ! -f $FILENAME ] then ... && AND if [ $condition1 ] && [ $condition2 ] # Same as: if [ $condition1 -a $condition2 ] # Returns true if both condition1 and condition2 hold true... if [[ $condition1 && $condition2 ]] # Also works. # Note that && operator not permitted inside brackets #+ of [ ... ] construct. Note && may also be used, depending on context, in an and list to concatenate commands. || OR if [ $condition1 ] || [ $condition2 ] # Same as: if [ $condition1 -o $condition2 ] # Returns true if either condition1 or condition2 holds true... if [[ $condition1 || $condition2 ]] # Also works. # Note that || operator not permitted inside brackets #+ of a [ ... ] construct. Note Bash tests the exit status of each statement linked with a logical operator. Example 8-3. Compound Condition Tests Using && and || #!/bin/bash a=24 b=47 if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] then echo "Test #1 succeeds." else echo "Test #1 fails." fi # ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ] #+ attempts to execute ' [ "$a" -eq 24 ' #+ and fails to finding matching ']'. # # Note: if [[ $a -eq 24 && $b -eq 24 ]] works. # The double-bracket if-test is more flexible #+ than the single-bracket version. # (The "&&" has a different meaning in line 17 than in line 6.) # Thanks, Stephane Chazelas, for pointing this out. if [ "$a" -eq 98 ] || [ "$b" -eq 47 ] then echo "Test #2 succeeds." else echo "Test #2 fails." fi # The -a and -o options provide #+ an alternative compound condition test. # Thanks to Patrick Callahan for pointing this out. if [ "$a" -eq 24 -a "$b" -eq 47 ] then echo "Test #3 succeeds." else echo "Test #3 fails." fi if [ "$a" -eq 98 -o "$b" -eq 47 ] then echo "Test #4 succeeds." else echo "Test #4 fails." fi a=rhino b=crocodile if [ "$a" = rhino ] && [ "$b" = crocodile ] then echo "Test #5 succeeds." else echo "Test #5 fails." fi exit 0 The && and || operators also find use in an arithmetic context. +------------------------------------------------------------+ |bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))| |1 0 1 0 | | | +------------------------------------------------------------+ miscellaneous operators , Comma operator The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects. [34] let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1" ^^^^^^ # t1 = 11 # Here t1 is set to the result of the last operation. Why? let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". echo "t2 = $t2 a = $a" # t2 = 5 a = 9 The comma operator finds use mainly in for loops. See Example 10-12. ------------------------------------------------------------------------ 8.2. Numerical Constants A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions). Example 8-4. Representation of numerical constants #!/bin/bash # numbers.sh: Representation of numbers in different bases. # Decimal: the default let "dec = 32" echo "decimal number = $dec" # 32 # Nothing out of the ordinary here. # Octal: numbers preceded by '0' (zero) let "oct = 032" echo "octal number = $oct" # 26 # Expresses result in decimal. # --------- ------ -- ------- # Hexadecimal: numbers preceded by '0x' or '0X' let "hex = 0x32" echo "hexadecimal number = $hex" # 50 echo $((0x9abc)) # 39612 # ^^ ^^ double-parentheses arithmetic expansion/evaluation # Expresses result in decimal. # Other bases: BASE#NUMBER # BASE between 2 and 64. # NUMBER must use symbols within the BASE range, see below. let "bin = 2#111100111001101" echo "binary number = $bin" # 31181 let "b32 = 32#77" echo "base-32 number = $b32" # 231 let "b64 = 64#@_" echo "base-64 number = $b64" # 4031 # This notation only works for a limited range (2 - 64) of ASCII characters. # 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _ echo echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA)) # 1295 170 44822 3375 # Important note: # -------------- # Using a digit out of range of the specified base notation #+ gives an error message. let "bad_oct = 081" # (Partial) error message output: # bad_oct = 081: value too great for base (error token is "081") # Octal numbers use only digits in the range 0 - 7. exit $? # Thanks, Rich Bartell and Stephane Chazelas, for clarification. $ sh numbers.sh $ echo $? $ 1 Part 3. Beyond the Basics Table of Contents 9. Variables Revisited 9.1. Internal Variables 9.2. Manipulating Strings 9.3. Parameter Substitution 9.4. Typing variables: declare or typeset 9.5. Indirect References 9.6. $RANDOM: generate random integer 9.7. The Double-Parentheses Construct 10. Loops and Branches 10.1. Loops 10.2. Nested Loops 10.3. Loop Control 10.4. Testing and Branching 11. Command Substitution 12. Arithmetic Expansion 13. Recess Time ------------------------------------------------------------------------ Chapter 9. Variables Revisited Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and nuances. ------------------------------------------------------------------------ 9.1. Internal Variables Builtin variables: variables affecting bash script behavior $BASH The path to the Bash binary itself +-------------------------------------------------------+ |bash$ echo $BASH | |/bin/bash | +-------------------------------------------------------+ $BASH_ENV An environmental variable pointing to a Bash startup file to be read when a script is invoked $BASH_SUBSHELL A variable indicating the subshell level. This is a new addition to Bash, version 3. See Example 20-1 for usage. $BASHPID Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same result. +-------------------------------------------------------+ |bash4$ echo $$ | |11015 | | | | | |bash4$ echo $BASHPID | |11015 | | | | | |bash4$ ps ax | grep bash4 | |11015 pts/2 R 0:00 bash4 | | | +-------------------------------------------------------+ But ... #!/bin/bash4 echo "\$\$ outside of subshell = $$" # 9602 echo "\$BASH_SUBSHELL outside of subshell = $BASH_SUBSHELL" # 0 echo "\$BASHPID outside of subshell = $BASHPID" # 9602 echo ( echo "\$\$ inside of subshell = $$" # 9602 echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL" # 1 echo "\$BASHPID inside of subshell = $BASHPID" ) # 9603 # Note that $$ returns PID of parent process. $BASH_VERSINFO[n] A 6-element array containing version information about the installed release of Bash. This is similar to $BASH_VERSION, below, but a bit more detailed. # Bash version info: for n in 0 1 2 3 4 5 do echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}" done # BASH_VERSINFO[0] = 3 # Major version no. # BASH_VERSINFO[1] = 00 # Minor version no. # BASH_VERSINFO[2] = 14 # Patch level. # BASH_VERSINFO[3] = 1 # Build version. # BASH_VERSINFO[4] = release # Release status. # BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture # (same as $MACHTYPE). $BASH_VERSION The version of Bash installed on the system +-------------------------------------------------------+ |bash$ echo $BASH_VERSION | |3.2.25(1)-release | | | +-------------------------------------------------------+ +-------------------------------------------------------+ |tcsh% echo $BASH_VERSION | |BASH_VERSION: Undefined variable. | | | +-------------------------------------------------------+ Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer. $CDPATH A colon-separated list of search paths available to the cd command, similar in function to the $PATH variable for binaries. The $CDPATH variable may be set in the local ~/.bashrc file. +-------------------------------------------------------+ |bash$ cd bash-doc | |bash: cd: bash-doc: No such file or directory | | | | | |bash$ CDPATH=/usr/share/doc | |bash$ cd bash-doc | |/usr/share/doc/bash-doc | | | | | |bash$ echo $PWD | |/usr/share/doc/bash-doc | | | +-------------------------------------------------------+ $DIRSTACK The top value in the directory stack [35] (affected by pushd and popd) This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the directory stack. $EDITOR The default editor invoked by a script, usually vi or emacs. $EUID "effective" user ID number Identification number of whatever identity the current user has assumed, perhaps by means of su. Caution The $EUID is not necessarily the same as the $UID. $FUNCNAME Name of the current function xyz23 () { echo "$FUNCNAME now executing." # xyz23 now executing. } xyz23 echo "FUNCNAME = $FUNCNAME" # FUNCNAME = # Null value outside a function. See also Example A-50. $GLOBIGNORE A list of filename patterns to be excluded from matching in globbing. $GROUPS Groups current user belongs to This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd and /etc/group. +-------------------------------------------------------+ |root# echo $GROUPS | |0 | | | | | |root# echo ${GROUPS[1]} | |1 | | | | | |root# echo ${GROUPS[5]} | |6 | | | +-------------------------------------------------------+ $HOME Home directory of the user, usually /home/username (see Example 9-16) $HOSTNAME The hostname command assigns the system host name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 9-16. $HOSTTYPE host type Like $MACHTYPE, identifies the system hardware. +-------------------------------------------------------+ |bash$ echo $HOSTTYPE | |i686 | +-------------------------------------------------------+ $IFS internal field separator This variable determines how Bash recognizes fields, or word boundaries, when it interprets character strings. $IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS. See Example 5-1. +-------------------------------------------------------------------+ |bash$ echo "$IFS" | | | |(With $IFS set to default, a blank line displays.) | | | | | | | |bash$ echo "$IFS" | cat -vte | | ^I$ | | $ | |(Show whitespace: here a single space, ^I [horizontal tab], | | and newline, and display "$" at end-of-line.) | | | | | | | |bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"' | |w:x:y:z | |(Read commands from string and assign any arguments to pos params.)| | | +-------------------------------------------------------------------+ Caution $IFS does not handle whitespace the same as it does other characters. Example 9-1. $IFS and whitespace #!/bin/bash # ifs.sh # $IFS treats whitespace differently than other characters. output_args_one_per_line() { for arg do echo "[$arg]" done # ^ ^ Embed within brackets, for your viewing pleasure. } echo; echo "IFS=\" \"" echo "-------" IFS=" " var=" a b c " # ^ ^^ ^^^ output_args_one_per_line $var # output_args_one_per_line `echo " a b c "` # [a] # [b] # [c] echo; echo "IFS=:" echo "-----" IFS=: var=":a::b:c:::" # Same pattern as above, # ^ ^^ ^^^ #+ but substituting ":" for " " ... output_args_one_per_line $var # [] # [a] # [] # [b] # [c] # [] # [] # Note "empty" brackets. # The same thing happens with the "FS" field separator in awk. echo exit (Many thanks, Stéphane Chazelas, for clarification and above examples.) See also Example 15-41, Example 10-7, and Example 18-14 for instructive examples of using $IFS. $IGNOREEOF Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out. $LC_COLLATE Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in filename globbing. Note As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, ls [A-M]* would match both File1.txt and file1.txt. To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc. $LC_CTYPE This internal variable controls character interpretation in globbing and pattern matching. $LINENO This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes. # *** BEGIN DEBUG BLOCK *** last_cmd_arg=$_ # Save it. echo "At line number $LINENO, variable \"v1\" = $v1" echo "Last command argument processed = $last_cmd_arg" # *** END DEBUG BLOCK *** $MACHTYPE machine type Identifies the system hardware. +-------------------------------------------------------+ |bash$ echo $MACHTYPE | |i686 | +-------------------------------------------------------+ $OLDPWD Old working directory ("OLD-Print-Working-Directory", previous directory you were in). $OSTYPE operating system type +-------------------------------------------------------+ |bash$ echo $OSTYPE | |linux | +-------------------------------------------------------+ $PATH Path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc. When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, $PATH, a list of directories, separated by colons. Normally, the system stores the $PATH definition in /etc/profile and/or ~/.bashrc (see Appendix G). +-----------------------------------------------------------+ |bash$ echo $PATH | |/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin| +-----------------------------------------------------------+ PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original $PATH (a child process, such as a script, may not change the environment of the parent process, the shell). Note The current "working directory", ./, is usually omitted from the $PATH as a security measure. $PIPESTATUS Array variable holding exit status(es) of last executed foreground pipe. +-------------------------------------------------------+ |bash$ echo $PIPESTATUS | |0 | | | |bash$ ls -al | bogus_command | |bash: bogus_command: command not found | |bash$ echo ${PIPESTATUS[1]} | |127 | | | |bash$ ls -al | bogus_command | |bash: bogus_command: command not found | |bash$ echo $? | |127 | | | +-------------------------------------------------------+ The members of the $PIPESTATUS array hold the exit status of each respective command executed in a pipe. $PIPESTATUS[0] holds the exit status of the first command in the pipe, $PIPESTATUS[1] the exit status of the second command, and so on. Caution The $PIPESTATUS variable may contain an erroneous 0 value in a login shell (in releases prior to 3.0 of Bash). +------------------------------------------+ |tcsh% bash | | | |bash$ who | grep nobody | sort | |bash$ echo ${PIPESTATUS[*]} | |0 | | | +------------------------------------------+ The above lines contained in a script would produce the expected 0 1 0 output. Thank you, Wayne Pollock for pointing this out and supplying the above example. Note The $PIPESTATUS variable gives unexpected results in some contexts. +--------------------------------------------+ |bash$ echo $BASH_VERSION | |3.00.14(1)-release | | | |bash$ $ ls | bogus_command | wc | |bash: bogus_command: command not found | | 0 0 0 | | | |bash$ echo ${PIPESTATUS[@]} | |141 127 0 | | | +--------------------------------------------+ Chet Ramey attributes the above output to the behavior of ls. If ls writes to a pipe whose output is not read, then SIGPIPE kills it, and its exit status is 141. Otherwise its exit status is 0, as expected. This likewise is the case for tr. Note $PIPESTATUS is a "volatile" variable. It needs to be captured immediately after the pipe in question, before any other command intervenes. +--------------------------------------------+ |bash$ $ ls | bogus_command | wc | |bash: bogus_command: command not found | | 0 0 0 | | | |bash$ echo ${PIPESTATUS[@]} | |0 127 0 | | | |bash$ echo ${PIPESTATUS[@]} | |0 | | | +--------------------------------------------+ Note The pipefail option may be useful in cases where $PIPESTATUS does not give the desired information. $PPID The $PPID of a process is the process ID (pid) of its parent process. [36] Compare this with the pidof command. $PROMPT_COMMAND A variable holding a command to be executed just before the primary prompt, $PS1 is to be displayed. $PS1 This is the main prompt, seen at the command-line. $PS2 The secondary prompt, seen when additional input is expected. It displays as ">". $PS3 The tertiary prompt, displayed in a select loop (see Example 10-29). $PS4 The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x option. It displays as "+". $PWD Working directory (directory you are in at the time) This is the analog to the pwd builtin command. #!/bin/bash E_WRONG_DIRECTORY=83 clear # Clear screen. TargetDirectory=/home/bozo/projects/GreatAmericanNovel cd $TargetDirectory echo "Deleting stale files in $TargetDirectory." if [ "$PWD" != "$TargetDirectory" ] then # Keep from wiping out wrong directory by accident. echo "Wrong directory!" echo "In $PWD, rather than $TargetDirectory!" echo "Bailing out!" exit $E_WRONG_DIRECTORY fi rm -rf * rm .[A-Za-z0-9]* # Delete dotfiles. # rm -f .[^.]* ..?* to remove filenames beginning with multiple dots. # (shopt -s dotglob; rm -f *) will also work. # Thanks, S.C. for pointing this out. # A filename (`basename`) may contain all characters in the 0 - 255 range, #+ except "/". # Deleting files beginning with weird characters, such as - #+ is left as an exercise. echo echo "Done." echo "Old files deleted in $TargetDirectory." echo # Various other operations here, as necessary. exit $? $REPLY The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself. #!/bin/bash # reply.sh # REPLY is the default value for a 'read' command. echo echo -n "What is your favorite vegetable? " read echo "Your favorite vegetable is $REPLY." # REPLY holds the value of last "read" if and only if #+ no variable supplied. echo echo -n "What is your favorite fruit? " read fruit echo "Your favorite fruit is $fruit." echo "but..." echo "Value of \$REPLY is still $REPLY." # $REPLY is still set to its previous value because #+ the variable $fruit absorbed the new "read" value. echo exit 0 $SECONDS The number of seconds the script has been running. #!/bin/bash TIME_LIMIT=10 INTERVAL=1 echo echo "Hit Control-C to exit before $TIME_LIMIT seconds." echo while [ "$SECONDS" -le "$TIME_LIMIT" ] do if [ "$SECONDS" -eq 1 ] then units=second else units=seconds fi echo "This script has been running $SECONDS $units." # On a slow or overburdened machine, the script may skip a count #+ every once in a while. sleep $INTERVAL done echo -e "\a" # Beep! exit 0 $SHELLOPTS The list of enabled shell options, a readonly variable. +-------------------------------------------------------------------------+ |bash$ echo $SHELLOPTS | |braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs| | | +-------------------------------------------------------------------------+ $SHLVL Shell level, how deeply Bash is nested. [37] If, at the command-line, $SHLVL is 1, then in a script it will increment to 2. Note This variable is not affected by subshells. Use $BASH_SUBSHELL when you need an indication of subshell nesting. $TMOUT If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after $time seconds. This will cause a logout. As of version 2.05b of Bash, it is now possible to use $TMOUT in a script in combination with read. # Works in scripts for Bash, versions 2.05b and later. TMOUT=3 # Prompt times out at three seconds. echo "What is your favorite song?" echo "Quickly now, you only have $TMOUT seconds to answer!" read song if [ -z "$song" ] then song="(no answer)" # Default response. fi echo "Your favorite song is $song." There are other, more complex, ways of implementing timed input in a script. One alternative is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see Example 29-5) the interrupt generated by the timing loop (whew!). Example 9-2. Timed Input #!/bin/bash # timed-input.sh # TMOUT=3 Also works, as of newer versions of Bash. TIMER_INTERRUPT=14 TIMELIMIT=3 # Three seconds in this instance. # May be set to different value. PrintAnswer() { if [ "$answer" = TIMEOUT ] then echo $answer else # Don't want to mix up the two instances. echo "Your favorite veggie is $answer" kill $! # Kills no-longer-needed TimerOn function #+ running in background. # $! is PID of last job running in background. fi } TimerOn() { sleep $TIMELIMIT && kill -s 14 $$ & # Waits 3 seconds, then sends sigalarm to script. } Int14Vector() { answer="TIMEOUT" PrintAnswer exit $TIMER_INTERRUPT } trap Int14Vector $TIMER_INTERRUPT # Timer interrupt (14) subverted for our purposes. echo "What is your favorite vegetable " TimerOn read answer PrintAnswer # Admittedly, this is a kludgy implementation of timed input. # However, the "-t" option to "read" simplifies this task. # See the "t-out.sh" script. # However, what about timing not just single user input, #+ but an entire script? # If you need something really elegant ... #+ consider writing the application in C or C++, #+ using appropriate library functions, such as 'alarm' and 'setitimer.' exit 0 An alternative is using stty. Example 9-3. Once more, timed input #!/bin/bash # timeout.sh # Written by Stephane Chazelas, #+ and modified by the document author. INTERVAL=5 # timeout interval timedout_read() { timeout=$1 varname=$2 old_tty_settings=`stty -g` stty -icanon min 0 time ${timeout}0 eval read $varname # or just read $varname stty "$old_tty_settings" # See man page for "stty." } echo; echo -n "What's your name? Quick! " timedout_read $INTERVAL your_name # This may not work on every terminal type. # The maximum timeout depends on the terminal. #+ (it is often 25.5 seconds). echo if [ ! -z "$your_name" ] # If name input before timeout ... then echo "Your name is $your_name." else echo "Timed out." fi echo # The behavior of this script differs somewhat from "timed-input.sh." # At each keystroke, the counter resets. exit 0 Perhaps the simplest method is using the -t option to read. Example 9-4. Timed read #!/bin/bash # t-out.sh # Inspired by a suggestion from "syngin seven" (thanks). TIMELIMIT=4 # 4 seconds read -t $TIMELIMIT variable <&1 # ^^^ # In this instance, "<&1" is needed for Bash 1.x and 2.x, # but unnecessary for Bash 3.x. echo if [ -z "$variable" ] # Is null? then echo "Timed out, variable still unset." else echo "variable = $variable" fi exit 0 $UID User ID number Current user's user identification number, as recorded in /etc/passwd This is the current user's real id, even if she has temporarily assumed another identity through su. $UID is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin. Example 9-5. Am I root? #!/bin/bash # am-i-root.sh: Am I root or not? ROOT_UID=0 # Root has $UID 0. if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up? then echo "You are root." else echo "You are just an ordinary user (but mom loves you just the same)." fi exit 0 # ============================================================= # # Code below will not execute, because the script already exited. # An alternate method of getting to the root of matters: ROOTUSER_NAME=root username=`id -nu` # Or... username=`whoami` if [ "$username" = "$ROOTUSER_NAME" ] then echo "Rooty, toot, toot. You are root." else echo "You are just a regular fella." fi See also Example 2-3. Note The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are not Bash builtins. These are, however, often set as environmental variables in one of the Bash startup files. $SHELL, the name of the user's login shell, may be set from /etc/passwd or in an "init" script, and it is likewise not a Bash builtin. +--------------------------------------------+ |tcsh% echo $LOGNAME | |bozo | |tcsh% echo $SHELL | |/bin/tcsh | |tcsh% echo $TERM | |rxvt | | | |bash$ echo $LOGNAME | |bozo | |bash$ echo $SHELL | |/bin/tcsh | |bash$ echo $TERM | |rxvt | | | +--------------------------------------------+ Positional Parameters $0, $1, $2, etc. Positional parameters, passed from command line to script, passed to a function, or set to a variable (see Example 4-5 and Example 14-16) $# Number of command-line arguments [38] or positional parameters (see Example 33-2) $* All of the positional parameters, seen as a single word Note "$*" must be quoted. $@ Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word. Note Of course, "$@" should be quoted. Example 9-6. arglist: Listing arguments with $* and $@ #!/bin/bash # arglist.sh # Invoke this script with several arguments, such as "one two three". E_BADARGS=65 if [ ! -n "$1" ] then echo "Usage: `basename $0` argument1 argument2 etc." exit $E_BADARGS fi echo index=1 # Initialize count. echo "Listing args with \"\$*\":" for arg in "$*" # Doesn't work properly if "$*" isn't quoted. do echo "Arg #$index = $arg" let "index+=1" done # $* sees all arguments as single word. echo "Entire arg list seen as single word." echo index=1 # Reset count. # What happens if you forget to do this? echo "Listing args with \"\$@\":" for arg in "$@" do echo "Arg #$index = $arg" let "index+=1" done # $@ sees arguments as separate words. echo "Arg list seen as separate words." echo index=1 # Reset count. echo "Listing args with \$* (unquoted):" for arg in $* do echo "Arg #$index = $arg" let "index+=1" done # Unquoted $* sees arguments as separate words. echo "Arg list seen as separate words." exit 0 Following a shift, the $@ holds the remaining command-line parameters, lacking the previous $1, which was lost. #!/bin/bash # Invoke with ./scriptname 1 2 3 4 5 echo "$@" # 1 2 3 4 5 shift echo "$@" # 2 3 4 5 shift echo "$@" # 3 4 5 # Each "shift" loses parameter $1. # "$@" then contains the remaining parameters. The $@ special parameter finds use as a tool for filtering input into shell scripts. The cat "$@" construction accepts input to a script either from stdin or from files given as parameters to the script. See Example 15-24 and Example 15-25. Caution The $* and $@ parameters sometimes display inconsistent and puzzling behavior, depending on the setting of $IFS. Example 9-7. Inconsistent $* and $@ behavior #!/bin/bash # Erratic behavior of the "$*" and "$@" internal Bash variables, #+ depending on whether they are quoted or not. # Inconsistent handling of word splitting and linefeeds. set -- "First one" "second" "third:one" "" "Fifth: :one" # Setting the script arguments, $1, $2, etc. echo echo 'IFS unchanged, using "$*"' c=0 for i in "$*" # quoted do echo "$((c+=1)): [$i]" # This line remains the same in every instance. # Echo args. done echo --- echo 'IFS unchanged, using $*' c=0 for i in $* # unquoted do echo "$((c+=1)): [$i]" done echo --- echo 'IFS unchanged, using "$@"' c=0 for i in "$@" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS unchanged, using $@' c=0 for i in $@ do echo "$((c+=1)): [$i]" done echo --- IFS=: echo 'IFS=":", using "$*"' c=0 for i in "$*" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $*' c=0 for i in $* do echo "$((c+=1)): [$i]" done echo --- var=$* echo 'IFS=":", using "$var" (var=$*)' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $var (var=$*)' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- var="$*" echo 'IFS=":", using $var (var="$*")' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$var" (var="$*")' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$@"' c=0 for i in "$@" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $@' c=0 for i in $@ do echo "$((c+=1)): [$i]" done echo --- var=$@ echo 'IFS=":", using $var (var=$@)' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using "$var" (var=$@)' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- var="$@" echo 'IFS=":", using "$var" (var="$@")' c=0 for i in "$var" do echo "$((c+=1)): [$i]" done echo --- echo 'IFS=":", using $var (var="$@")' c=0 for i in $var do echo "$((c+=1)): [$i]" done echo # Try this script with ksh or zsh -y. exit 0 # This example script by Stephane Chazelas, # and slightly modified by the document author. Note The $@ and $* parameters differ only when between double quotes. Example 9-8. $* and $@ when $IFS is empty #!/bin/bash # If $IFS set, but empty, #+ then "$*" and "$@" do not echo positional params as expected. mecho () # Echo positional parameters. { echo "$1,$2,$3"; } IFS="" # Set, but empty. set a b c # Positional parameters. mecho "$*" # abc,, # ^^ mecho $* # a,b,c mecho $@ # a,b,c mecho "$@" # a,b,c # The behavior of $* and $@ when $IFS is empty depends #+ on which Bash or sh version being run. # It is therefore inadvisable to depend on this "feature" in a script. # Thanks, Stephane Chazelas. exit Other Special Parameters $- Flags passed to script (using set). See Example 14-16. Caution This was originally a ksh construct adopted into Bash, and unfortunately it does not seem to work reliably in Bash scripts. One possible use for it is to have a script self-test whether it is interactive. $! PID (process ID) of last job run in background LOG=$0.log COMMAND1="sleep 100" echo "Logging PIDs background commands for script: $0" >> "$LOG" # So they can be monitored, and killed as necessary. echo >> "$LOG" # Logging commands. echo -n "PID of \"$COMMAND1\": " >> "$LOG" ${COMMAND1} & echo $! >> "$LOG" # PID of "sleep 100": 1506 # Thank you, Jacques Lederer, for suggesting this. Using $! for job control: possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; } # Forces completion of an ill-behaved program. # Useful, for example, in init scripts. # Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable. Or, alternately: # This example by Matthew Sage. # Used with permission. TIMEOUT=30 # Timeout value in seconds count=0 possibly_hanging_job & { while ((count < TIMEOUT )); do eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))' # /proc is where information about running processes is found. # "-d" tests whether it exists (whether directory exists). # So, we're waiting for the job in question to show up. ((count++)) sleep 1 done eval '[ -d "/proc/$!" ] && kill -15 $!' # If the hanging job is running, kill it. } $_ Special variable set to final argument of previous command executed. Example 9-9. Underscore variable #!/bin/bash echo $_ # /bin/bash # Just called /bin/bash to run the script. # Note that this will vary according to #+ how the script is invoked. du >/dev/null # So no output from command. echo $_ # du ls -al >/dev/null # So no output from command. echo $_ # -al (last argument) : echo $_ # : $? Exit status of a command, function, or the script itself (see Example 23-7) $$ Process ID (PID) of the script itself. [39] The $$ variable often finds use in scripts to construct "unique" temp file names (see Example 29-6, Example 15-31, and Example 14-27). This is usually simpler than invoking mktemp. ------------------------------------------------------------------------ 9.2. Manipulating Strings Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion. String Length ${#string} expr length $string These are the equivalent of strlen() in C. expr "$string" : '.*' stringZ=abcABC123ABCabc echo ${#stringZ} # 15 echo `expr length $stringZ` # 15 echo `expr "$stringZ" : '.*'` # 15 Example 9-10. Inserting a blank line between paragraphs in a text file #!/bin/bash # paragraph-space.sh # Ver. 2.0, Reldate 05Aug08 # Inserts a blank line between paragraphs of a single-spaced text file. # Usage: $0 "$filename.$SUFFIX" # Redirect conversion to new filename. rm -f $file # Delete original files after converting. echo "$filename.$SUFFIX" # Log what is happening to stdout. done exit 0 # Exercise: # -------- # As it stands, this script converts *all* the files in the current #+ working directory. # Modify it to work *only* on files with a ".mac" suffix. Example 9-13. Converting streaming audio files to ogg #!/bin/bash # ra2ogg.sh: Convert streaming audio files (*.ra) to ogg. # Uses the "mplayer" media player program: # http://www.mplayerhq.hu/homepage # Uses the "ogg" library and "oggenc": # http://www.xiph.org/ # # This script may need appropriate codecs installed, such as sipr.so ... # Possibly also the compat-libstdc++ package. OFILEPREF=${1%%ra} # Strip off the "ra" suffix. OFILESUFF=wav # Suffix for wav file. OUTFILE="$OFILEPREF""$OFILESUFF" E_NOARGS=85 if [ -z "$1" ] # Must specify a filename to convert. then echo "Usage: `basename $0` [filename]" exit $E_NOARGS fi ########################################################################## mplayer "$1" -ao pcm:file=$OUTFILE oggenc "$OUTFILE" # Correct file extension automatically added by oggenc. ########################################################################## rm "$OUTFILE" # Delete intermediate *.wav file. # If you want to keep it, comment out above line. exit $? # Note: # ---- # On a Website, simply clicking on a *.ram streaming audio file #+ usually only downloads the URL of the actual *.ra audio file. # You can then use "wget" or something similar #+ to download the *.ra file itself. # Exercises: # --------- # As is, this script converts only *.ra filenames. # Add flexibility by permitting use of *.ram and other filenames. # # If you're really ambitious, expand the script #+ to do automatic downloads and conversions of streaming audio files. # Given a URL, batch download streaming audio files (using "wget") #+ and convert them on the fly. A simple emulation of getopt using substring-extraction constructs. Example 9-14. Emulating getopt #!/bin/bash # getopt-simple.sh # Author: Chris Morgan # Used in the ABS Guide with permission. getopt_simple() { echo "getopt_simple()" echo "Parameters are '$*'" until [ -z "$1" ] do echo "Processing parameter of: '$1'" if [ ${1:0:1} = '/' ] then tmp=${1:1} # Strip off leading '/' . . . parameter=${tmp%%=*} # Extract name. value=${tmp##*=} # Extract value. echo "Parameter: '$parameter', value: '$value'" eval $parameter=$value fi shift done } # Pass all options to getopt_simple(). getopt_simple $* echo "test is '$test'" echo "test2 is '$test2'" exit 0 # See also, UseGetOpt.sh, a modified versio of this script. --- sh getopt_example.sh /test=value1 /test2=value2 Parameters are '/test=value1 /test2=value2' Processing parameter of: '/test=value1' Parameter: 'test', value: 'value1' Processing parameter of: '/test2=value2' Parameter: 'test2', value: 'value2' test is 'value1' test2 is 'value2' Substring Replacement ${string/substring/replacement} Replace first match of $substring with $replacement. [41] ${string//substring/replacement} Replace all matches of $substring with $replacement. stringZ=abcABC123ABCabc echo ${stringZ/abc/xyz} # xyzABC123ABCabc # Replaces first match of 'abc' with 'xyz'. echo ${stringZ//abc/xyz} # xyzABC123ABCxyz # Replaces all matches of 'abc' with # 'xyz'. echo --------------- echo "$stringZ" # abcABC123ABCabc echo --------------- # The string itself is not altered! # Can the match and replacement strings be parameterized? match=abc repl=000 echo ${stringZ/$match/$repl} # 000ABC123ABCabc # ^ ^ ^^^ echo ${stringZ//$match/$repl} # 000ABC123ABC000 # Yes! ^ ^ ^^^ ^^^ echo # What happens if no $replacement string is supplied? echo ${stringZ/abc} # ABC123ABCabc echo ${stringZ//abc} # ABC123ABC # A simple deletion takes place. ${string/#substring/replacement} If $substring matches front end of $string, substitute $replacement for $substring. ${string/%substring/replacement} If $substring matches back end of $string, substitute $replacement for $substring. stringZ=abcABC123ABCabc echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc # Replaces front-end match of 'abc' with 'XYZ'. echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ # Replaces back-end match of 'abc' with 'XYZ'. ------------------------------------------------------------------------ 9.2.1. Manipulating strings using awk A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in operations. Example 9-15. Alternate ways of extracting and locating substrings #!/bin/bash # substring-extraction.sh String=23skidoo1 # 012345678 Bash # 123456789 awk # Note different string indexing system: # Bash numbers first character of string as 0. # Awk numbers first character of string as 1. echo ${String:2:4} # position 3 (0-1-2), 4 characters long # skid # The awk equivalent of ${string:pos:length} is substr(string,pos,length). echo | awk ' { print substr("'"${String}"'",3,4) # skid } ' # Piping an empty "echo" to awk gives it dummy input, #+ and thus makes it unnecessary to supply a filename. echo "----" # And likewise: echo | awk ' { print index("'"${String}"'", "skid") # 3 } # (skid starts at position 3) ' # The awk equivalent of "expr index" ... exit 0 ------------------------------------------------------------------------ 9.2.2. Further Reference For more on string manipulation in scripts, refer to Section 9.3 and the relevant section of the expr command listing. Script examples: 1. Example 15-9 2. Example 9-18 3. Example 9-19 4. Example 9-20 5. Example 9-22 6. Example A-36 7. Example A-41 ------------------------------------------------------------------------ 9.3. Parameter Substitution Manipulating and/or expanding variables ${parameter} Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works. May be used for concatenating variables with strings. your_id=${USER}-on-${HOSTNAME} echo "$your_id" # echo "Old \$PATH = $PATH" PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH" ${parameter-default}, ${parameter:-default} If parameter not set, use default. echo ${username-`whoami`} # Echoes the result of `whoami`, if variable $username is still unset. Note ${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null. #!/bin/bash # param-sub.sh # Whether a variable has been declared #+ affects triggering of the default option #+ even if the variable is null. username0= echo "username0 has been declared, but is set to null." echo "username0 = ${username0-`whoami`}" # Will not echo. echo echo username1 has not been declared. echo "username1 = ${username1-`whoami`}" # Will echo. username2= echo "username2 has been declared, but is set to null." echo "username2 = ${username2:-`whoami`}" # ^ # Will echo because of :- rather than just - in condition test. # Compare to first instance, above. # # Once again: variable= # variable has been declared, but is set to null. echo "${variable-0}" # (no output) echo "${variable:-1}" # 1 # ^ unset variable echo "${variable-2}" # 2 echo "${variable:-3}" # 3 exit 0 The default parameter construct finds use in providing "missing" command-line arguments in scripts. DEFAULT_FILENAME=generic.data filename=${1:-$DEFAULT_FILENAME} # If not otherwise specified, the following command block operates #+ on the file "generic.data". # Begin-Command-Block # ... # ... # ... # End-Command-Block # From "hanoi2.bash" example: DISKS=${1:-E_NOPARAM} # Must specify how many disks. # Set $DISKS to $1 command-line-parameter, #+ or to $E_NOPARAM if that is unset. See also Example 3-4, Example 28-2, and Example A-6. Compare this method with using an and list to supply a default command-line argument. ${parameter=default}, ${parameter:=default} If parameter not set, set it to default. Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, [42] as above. echo ${username=`whoami`} # Variable "username" is now set to `whoami`. ${parameter+alt_value}, ${parameter:+alt_value} If parameter set, use alt_value, else use null string. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below. echo "###### \${parameter+alt_value} ########" echo a=${param1+xyz} echo "a = $a" # a = param2= a=${param2+xyz} echo "a = $a" # a = xyz param3=123 a=${param3+xyz} echo "a = $a" # a = xyz echo echo "###### \${parameter:+alt_value} ########" echo a=${param4:+xyz} echo "a = $a" # a = param5= a=${param5:+xyz} echo "a = $a" # a = # Different result from a=${param5+xyz} param6=123 a=${param6:+xyz} echo "a = $a" # a = xyz ${parameter?err_msg}, ${parameter:?err_msg} If parameter set, use it, else print err_msg. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above. Example 9-16. Using parameter substitution and error messages #!/bin/bash # Check some of the system's environmental variables. # This is good preventative maintenance. # If, for example, $USER, the name of the person at the console, is not set, #+ the machine will not recognize you. : ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?} echo echo "Name of the machine is $HOSTNAME." echo "You are $USER." echo "Your home directory is $HOME." echo "Your mail INBOX is located in $MAIL." echo echo "If you are reading this message," echo "critical environmental variables have been set." echo echo # ------------------------------------------------------ # The ${variablename?} construction can also check #+ for variables set within the script. ThisVariable=Value-of-ThisVariable # Note, by the way, that string variables may be set #+ to characters disallowed in their names. : ${ThisVariable?} echo "Value of ThisVariable is $ThisVariable". echo; echo : ${ZZXy23AB?"ZZXy23AB has not been set."} # Since ZZXy23AB has not been set, #+ then the script terminates with an error message. # You can specify the error message. # : ${variablename?"ERROR MESSAGE"} # Same result with: dummy_variable=${ZZXy23AB?} # dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."} # # echo ${ZZXy23AB?} >/dev/null # Compare these methods of checking whether a variable has been set #+ with "set -u" . . . echo "You will not see this message, because script already terminated." HERE=0 exit $HERE # Will NOT exit here. # In fact, this script will return an exit status (echo $?) of 1. Example 9-17. Parameter substitution and "usage" messages #!/bin/bash # usage-message.sh : ${1?"Usage: $0 ARGUMENT"} # Script exits here if command-line parameter absent, #+ with following error message. # usage-message.sh: 1: Usage: usage-message.sh ARGUMENT echo "These two lines echo only if command-line parameter given." echo "command-line parameter = \"$1\"" exit 0 # Will exit here only if command-line parameter present. # Check the exit status, both with and without command-line parameter. # If command-line parameter present, then "$?" is 0. # If not, then "$?" is 1. Parameter substitution and/or expansion. The following expressions are the complement to the match in expr string operations (see Example 15-9). These particular ones are used mostly in parsing file path names. Variable length / Substring removal ${#var} String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array. Note Exceptions: * ${#*} and ${#@} give the number of positional parameters. * For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array. Example 9-18. Length of a variable #!/bin/bash # length.sh E_NO_ARGS=65 if [ $# -eq 0 ] # Must have command-line args to demo script. then echo "Please invoke this script with one or more command-line arguments." exit $E_NO_ARGS fi var01=abcdEFGH28ij echo "var01 = ${var01}" echo "Length of var01 = ${#var01}" # Now, let's try embedding a space. var02="abcd EFGH28ij" echo "var02 = ${var02}" echo "Length of var02 = ${#var02}" echo "Number of command-line arguments passed to script = ${#@}" echo "Number of command-line arguments passed to script = ${#*}" exit 0 ${var#Pattern}, ${var##Pattern} ${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var. ${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end of $var. A usage illustration from Example A-7: # Function from "days-between.sh" example. # Strips leading zero(s) from argument passed. strip_leading_zero () # Strip possible leading zero(s) { #+ from argument passed. return=${1#0} # The "1" refers to "$1" -- passed arg. } # The "0" is what to remove from "$1" -- strips zeros. Manfred Schwarb's more elaborate variation of the above: strip_leading_zero2 () # Strip possible leading zero(s), since otherwise { # Bash will interpret such numbers as octal values. shopt -s extglob # Turn on extended globbing. local val=${1##+(0)} # Use local variable, longest matching series of 0's. shopt -u extglob # Turn off extended globbing. _strip_leading_zero2=${val:-0} # If input was 0, return 0 instead of "". } Another usage illustration: echo `basename $PWD` # Basename of current working directory. echo "${PWD##*/}" # Basename of current working directory. echo echo `basename $0` # Name of script. echo $0 # Name of script. echo "${0##*/}" # Name of script. echo filename=test.data echo "${filename##*.}" # data # Extension of filename. ${var%Pattern}, ${var%%Pattern} ${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end of $var. ${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end of $var. Version 2 of Bash added additional options. Example 9-19. Pattern matching in parameter substitution #!/bin/bash # patt-matching.sh # Pattern matching using the # ## % %% parameter substitution operators. var1=abcd12345abc6789 pattern1=a*c # * (wild card) matches everything between a - c. echo echo "var1 = $var1" # abcd12345abc6789 echo "var1 = ${var1}" # abcd12345abc6789 # (alternate form) echo "Number of characters in ${var1} = ${#var1}" echo echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c') echo "--------------" echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789 # Shortest possible match, strips out first 3 characters abcd12345abc6789 # ^^^^^ |-| echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789 # Longest possible match, strips out first 12 characters abcd12345abc6789 # ^^^^^ |----------| echo; echo; echo pattern2=b*9 # everything between 'b' and '9' echo "var1 = $var1" # Still abcd12345abc6789 echo echo "pattern2 = $pattern2" echo "--------------" echo '${var1%pattern2} =' "${var1%$pattern2}" # abcd12345a # Shortest possible match, strips out last 6 characters abcd12345abc6789 # ^^^^ |----| echo '${var1%%pattern2} =' "${var1%%$pattern2}" # a # Longest possible match, strips out last 12 characters abcd12345abc6789 # ^^^^ |-------------| # Remember, # and ## work from the left end (beginning) of string, # % and %% work from the right end. echo exit 0 Example 9-20. Renaming file extensions: #!/bin/bash # rfe.sh: Renaming file extensions. # # rfe old_extension new_extension # # Example: # To rename all *.gif files in working directory to *.jpg, # rfe gif jpg E_BADARGS=65 case $# in 0|1) # The vertical bar means "or" in this context. echo "Usage: `basename $0` old_file_suffix new_file_suffix" exit $E_BADARGS # If 0 or 1 arg, then bail out. ;; esac for filename in *.$1 # Traverse list of files ending with 1st argument. do mv $filename ${filename%$1}$2 # Strip off part of filename matching 1st argument, #+ then append 2nd argument. done exit 0 Variable expansion / Substring replacement These constructs have been adopted from ksh. ${var:pos} Variable var expanded, starting from offset pos. ${var:pos:len} Expansion to a max of len characters of variable var, from offset pos. See Example A-13 for an example of the creative use of this operator. ${var/Pattern/Replacement} First match of Pattern, within var replaced with Replacement. If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted. ${var//Pattern/Replacement} Global replacement. All matches of Pattern, within var replaced with Replacement. As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted. Example 9-21. Using pattern matching to parse arbitrary strings #!/bin/bash var1=abcd-1234-defg echo "var1 = $var1" t=${var1#*-*} echo "var1 (with everything, up to and including first - stripped out) = $t" # t=${var1#*-} works just the same, #+ since # matches the shortest string, #+ and * matches everything preceding, including an empty string. # (Thanks, Stephane Chazelas, for pointing this out.) t=${var1##*-*} echo "If var1 contains a \"-\", returns empty string... var1 = $t" t=${var1%*-*} echo "var1 (with everything from the last - on stripped out) = $t" echo # ------------------------------------------- path_name=/home/bozo/ideas/thoughts.for.today # ------------------------------------------- echo "path_name = $path_name" t=${path_name##/*/} echo "path_name, stripped of prefixes = $t" # Same effect as t=`basename $path_name` in this particular case. # t=${path_name%/}; t=${t##*/} is a more general solution, #+ but still fails sometimes. # If $path_name ends with a newline, then `basename $path_name` will not work, #+ but the above expression will. # (Thanks, S.C.) t=${path_name%/*.*} # Same effect as t=`dirname $path_name` echo "path_name, stripped of suffixes = $t" # These will fail in some cases, such as "../", "/foo////", # "foo/", "/". # Removing suffixes, especially when the basename has no suffix, #+ but the dirname does, also complicates matters. # (Thanks, S.C.) echo t=${path_name:11} echo "$path_name, with first 11 chars stripped off = $t" t=${path_name:11:5} echo "$path_name, with first 11 chars stripped off, length 5 = $t" echo t=${path_name/bozo/clown} echo "$path_name with \"bozo\" replaced by \"clown\" = $t" t=${path_name/today/} echo "$path_name with \"today\" deleted = $t" t=${path_name//o/O} echo "$path_name with all o's capitalized = $t" t=${path_name//o/} echo "$path_name with all o's deleted = $t" exit 0 ${var/#Pattern/Replacement} If prefix of var matches Pattern, then substitute Replacement for Pattern. ${var/%Pattern/Replacement} If suffix of var matches Pattern, then substitute Replacement for Pattern. Example 9-22. Matching patterns at prefix or suffix of string #!/bin/bash # var-match.sh: # Demo of pattern replacement at prefix / suffix of string. v0=abc1234zip1234abc # Original variable. echo "v0 = $v0" # abc1234zip1234abc echo # Match at prefix (beginning) of string. v1=${v0/#abc/ABCDEF} # abc1234zip1234abc # |-| echo "v1 = $v1" # ABCDEF1234zip1234abc # |----| # Match at suffix (end) of string. v2=${v0/%abc/ABCDEF} # abc1234zip123abc # |-| echo "v2 = $v2" # abc1234zip1234ABCDEF # |----| echo # ---------------------------------------------------- # Must match at beginning / end of string, #+ otherwise no replacement results. # ---------------------------------------------------- v3=${v0/#123/000} # Matches, but not at beginning. echo "v3 = $v3" # abc1234zip1234abc # NO REPLACEMENT. v4=${v0/%123/000} # Matches, but not at end. echo "v4 = $v4" # abc1234zip1234abc # NO REPLACEMENT. exit 0 ${!varprefix*}, ${!varprefix@} Matches names of all previously declared variables beginning with varprefix. # This is a variation on indirect reference, but with a * or @. # Bash, version 2.04, adds this feature. xyz23=whatever xyz24= a=${!xyz*} # Expands to *names* of declared variables # ^ ^ ^ + beginning with "xyz". echo "a = $a" # a = xyz23 xyz24 a=${!xyz@} # Same as above. echo "a = $a" # a = xyz23 xyz24 echo "---" abc23=something_else b=${!abc*} echo "b = $b" # b = abc23 c=${!b} # Now, the more familiar type of indirect reference. echo $c # something_else ------------------------------------------------------------------------ 9.4. Typing variables: declare or typeset The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing [43] available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts. declare/typeset options -r readonly (declare -r var1 works the same as readonly var1) This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message. declare -r var1=1 echo "var1 = $var1" # var1 = 1 (( var1++ )) # x.sh: line 4: var1: readonly variable -i integer declare -i number # The script will treat subsequent occurrences of "number" as an integer. number=3 echo "Number = $number" # Number = 3 number=three echo "Number = $number" # Number = 0 # Tries to evaluate the string "three" as an integer. Certain arithmetic operations are permitted for declared integer variables without the need for expr or let. n=6/3 echo "n = $n" # n = 6/3 declare -i n n=6/3 echo "n = $n" # n = 2 -a array declare -a indices The variable indices will be treated as an array. -f function(s) declare -f A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script. declare -f function_name A declare -f function_name in a script lists just the function named. -x export declare -x var3 This declares a variable as available for exporting outside the environment of the script itself. -x var=$value declare -x var3=373 The declare command permits assigning a value to a variable in the same statement as setting its properties. Example 9-23. Using declare to type variables #!/bin/bash func1 () { echo This is a function. } declare -f # Lists the function above. echo declare -i var1 # var1 is an integer. var1=2367 echo "var1 declared as $var1" var1=var1+1 # Integer declaration eliminates the need for 'let'. echo "var1 incremented by 1 is $var1." # Attempt to change variable declared as integer. echo "Attempting to change var1 to floating point value, 2367.1." var1=2367.1 # Results in error message, with no change to variable. echo "var1 is still $var1" echo declare -r var2=13.36 # 'declare' permits setting a variable property #+ and simultaneously assigning it a value. echo "var2 declared as $var2" # Attempt to change readonly variable. var2=13.37 # Generates error message, and exit from script. echo "var2 is still $var2" # This line will not execute. exit 0 # Script will not exit here. Caution Using the declare builtin restricts the scope of a variable. foo () { FOO="bar" } bar () { foo echo $FOO } bar # Prints bar. However . . . foo (){ declare FOO="bar" } bar () { foo echo $FOO } bar # Prints nothing. # Thank you, Michael Iatrou, for pointing this out. ------------------------------------------------------------------------ 9.4.1. Another use for declare The declare command can be helpful in identifying variables, environmental or otherwise. This can be especially useful with arrays. +----------------------------------------------------------------------+ |bash$ declare | grep HOME | |/home/bozo | | | | | |bash$ zzy=68 | |bash$ declare | grep zzy | |zzy=68 | | | | | |bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green") | |bash$ echo ${Colors[@]} | |purple reddish-orange light green | |bash$ declare | grep Colors | |Colors=([0]="purple" [1]="reddish-orange" [2]="light green") | | | +----------------------------------------------------------------------+ ------------------------------------------------------------------------ 9.5. Indirect References We have seen that referencing a variable, $var, fetches its value. But, what about the value of a value? What about $$var? The actual notation is \$$var, usually preceded by an eval (and sometimes an echo). This is called an indirect reference. Example 9-24. Indirect Variable References #!/bin/bash # ind-ref.sh: Indirect variable referencing. # Accessing the contents of the contents of a variable. # First, let's fool around a little. var=23 echo "\$var = $var" # $var = 23 # So far, everything as expected. But ... echo "\$\$var = $$var" # $$var = 4570var # Not useful ... # \$\$ expanded to PID of the script # -- refer to the entry on the $$ variable -- #+ and "var" is echoed as plain text. # (Thank you, Jakob Bohm, for pointing this out.) echo "\\\$\$var = \$$var" # \$$var = $23 # As expected. The first $ is escaped and pasted on to #+ the value of var ($var = 23 ). # Meaningful, but still not useful. # Now, let's start over and do it the right way. # ============================================== # a=letter_of_alphabet # Variable "a" holds the name of another variable. letter_of_alphabet=z echo # Direct reference. echo "a = $a" # a = letter_of_alphabet # Indirect reference. eval a=\$$a # ^^^ Forcing an eval(uation), and ... # ^ Escaping the first $ ... # ------------------------------------------------------------------------ # The 'eval' forces an update of $a, sets it to the updated value of \$$a. # So, we see why 'eval' so often shows up in indirect reference notation. # ------------------------------------------------------------------------ echo "Now a = $a" # Now a = z echo # Now, let's try changing the second-order reference. t=table_cell_3 table_cell_3=24 echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24 echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24 # In this simple case, the following also works (why?). # eval t=\$$t; echo "\"t\" = $t" echo t=table_cell_3 NEW_VAL=387 table_cell_3=$NEW_VAL echo "Changing value of \"table_cell_3\" to $NEW_VAL." echo "\"table_cell_3\" now $table_cell_3" echo -n "dereferenced \"t\" now "; eval echo \$$t # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3) echo # (Thanks, Stephane Chazelas, for clearing up the above behavior.) # A more straightforward method is the ${!t} notation, discussed in the #+ "Bash, version 2" section. # See also ex78.sh. exit 0 +----------------------------------------------------------------------+ | Indirect referencing in Bash is a multi-step process. First, take | | the name of a variable: varname. Then, reference it: $varname. Then, | | reference the reference: $$varname. Then, escape the first $: | | \$$varname. Finally, force a reevaluation of the expression and | | assign it: eval newvar=\$$varname. | +----------------------------------------------------------------------+ Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers in C, for instance, in table lookup. And, it also has some other very interesting applications. . . . Nils Radtke shows how to build "dynamic" variable names and evaluate their contents. This can be useful when sourcing configuration files. #!/bin/bash # --------------------------------------------- # This could be "sourced" from a separate file. isdnMyProviderRemoteNet=172.16.0.100 isdnYourProviderRemoteNet=10.0.0.10 isdnOnlineService="MyProvider" # --------------------------------------------- remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)") remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)") remoteNet=$(eval "echo \$isdnMyProviderRemoteNet") remoteNet=$(eval "echo $isdnMyProviderRemoteNet") echo "$remoteNet" # 172.16.0.100 # ================================================================ # And, it gets even better. # Consider the following snippet given a variable named getSparc, #+ but no such variable getIa64: chkMirrorArchs () { arch="$1"; if [ "$(eval "echo \${$(echo get$(echo -ne $arch | sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch | sed 's/^.\(.*\)/\1/g')):-false}")" = true ] then return 0; else return 1; fi; } getSparc="true" unset getIa64 chkMirrorArchs sparc echo $? # 0 # True chkMirrorArchs Ia64 echo $? # 1 # False # Notes: # ----- # Even the to-be-substituted variable name part is built explicitly. # The parameters to the chkMirrorArchs calls are all lower case. # The variable name is composed of two parts: "get" and "Sparc" . . . Example 9-25. Passing an indirect reference to awk #!/bin/bash # Another version of the "column totaler" script #+ that adds up a specified column (of numbers) in the target file. # This one uses indirect references. ARGS=2 E_WRONGARGS=85 if [ $# -ne "$ARGS" ] # Check for proper number of command-line args. then echo "Usage: `basename $0` filename column-number" exit $E_WRONGARGS fi filename=$1 # Name of file to operate on. column_number=$2 # Which column to total up. #===== Same as original script, up to this point =====# # A multi-line awk script is invoked by # awk " # ... # ... # ... # " # Begin awk script. # ------------------------------------------------- awk " { total += \$${column_number} # Indirect reference } END { print total } " "$filename" # Note that awk doesn't need an eval preceding \$$. # ------------------------------------------------- # End awk script. # Indirect variable reference avoids the hassles #+ of referencing a shell variable within the embedded awk script. # Thanks, Stephane Chazelas. exit $? Caution This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example 34-2 and Example A-22) makes indirect referencing more intuitive. +----------------------------------------------------------------------+ | Bash does not support pointer arithmetic, and this severely limits | | the usefulness of indirect referencing. In fact, indirect | | referencing in a scripting language is, at best, something of an | | afterthought. | +----------------------------------------------------------------------+ ------------------------------------------------------------------------ 9.6. $RANDOM: generate random integer $RANDOM is an internal Bash function (not a constant) that returns a pseudorandom [44] integer in the range 0 - 32767. It should not be used to generate an encryption key. Example 9-26. Generating random numbers #!/bin/bash # $RANDOM returns a different random integer at each invocation. # Nominal range: 0 - 32767 (signed 16-bit integer). MAXCOUNT=10 count=1 echo echo "$MAXCOUNT random numbers:" echo "-----------------" while [ "$count" -le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integers. do number=$RANDOM echo $number let "count += 1" # Increment count. done echo "-----------------" # If you need a random int within a certain range, use the 'modulo' operator. # This returns the remainder of a division operation. RANGE=500 echo number=$RANDOM let "number %= $RANGE" # ^^ echo "Random number less than $RANGE --- $number" echo # If you need a random integer greater than a lower bound, #+ then set up a test to discard all numbers below that. FLOOR=200 number=0 #initialize while [ "$number" -le $FLOOR ] do number=$RANDOM done echo "Random number greater than $FLOOR --- $number" echo # Let's examine a simple alternative to the above loop, namely # let "number = $RANDOM + $FLOOR" # That would eliminate the while-loop and run faster. # But, there might be a problem with that. What is it? # Combine above two techniques to retrieve random number between two limits. number=0 #initialize while [ "$number" -le $FLOOR ] do number=$RANDOM let "number %= $RANGE" # Scales $number down within $RANGE. done echo "Random number between $FLOOR and $RANGE --- $number" echo # Generate binary choice, that is, "true" or "false" value. BINARY=2 T=1 number=$RANDOM let "number %= $BINARY" # Note that let "number >>= 14" gives a better random distribution #+ (right shifts out everything except last binary digit). if [ "$number" -eq $T ] then echo "TRUE" else echo "FALSE" fi echo # Generate a toss of the dice. SPOTS=6 # Modulo 6 gives range 0 - 5. # Incrementing by 1 gives desired range of 1 - 6. # Thanks, Paulo Marcel Coelho Aragao, for the simplification. die1=0 die2=0 # Would it be better to just set SPOTS=7 and not add 1? Why or why not? # Tosses each die separately, and so gives correct odds. let "die1 = $RANDOM % $SPOTS +1" # Roll first one. let "die2 = $RANDOM % $SPOTS +1" # Roll second one. # Which arithmetic operation, above, has greater precedence -- #+ modulo (%) or addition (+)? let "throw = $die1 + $die2" echo "Throw of the dice = $throw" echo exit 0 Example 9-27. Picking a random card from a deck #!/bin/bash # pick-card.sh # This is an example of choosing random elements of an array. # Pick a card, any card. Suites="Clubs Diamonds Hearts Spades" Denominations="2 3 4 5 6 7 8 9 10 Jack Queen King Ace" # Note variables spread over multiple lines. suite=($Suites) # Read into array variable. denomination=($Denominations) num_suites=${#suite[*]} # Count how many elements. num_denominations=${#denomination[*]} echo -n "${denomination[$((RANDOM%num_denominations))]} of " echo ${suite[$((RANDOM%num_suites))]} # $bozo sh pick-cards.sh # Jack of Clubs # Thank you, "jipe," for pointing out this use of $RANDOM. exit 0 Example 9-28. Brownian Motion Simulation #!/bin/bash # brownian.sh # Author: Mendel Cooper # Reldate: 10/26/07 # License: GPL3 # ---------------------------------------------------------------- # This script models Brownian motion: #+ the random wanderings of tiny particles in a fluid, #+ as they are buffeted by random currents and collisions. #+ This is colloquially known as the "Drunkard's Walk." # It can also be considered as a stripped-down simulation of a #+ Galton Board, a slanted board with a pattern of pegs, #+ down which rolls a succession of marbles, one at a time. #+ At the bottom is a row of slots or catch basins in which #+ the marbles come to rest at the end of their journey. # Think of it as a kind of bare-bones Pachinko game. # As you see by running the script, #+ most of the marbles cluster around the center slot. #+ This is consistent with the expected binomial distribution. # As a Galton Board simulation, the script #+ disregards such parameters as #+ board tilt-angle, rolling friction of the marbles, #+ angles of impact, and elasticity of the pegs. # To what extent does this affect the accuracy of the simulation? # ---------------------------------------------------------------- PASSES=500 # Number of particle interactions / marbles. ROWS=10 # Number of "collisions" (or horiz. peg rows). RANGE=3 # 0 - 2 output range from $RANDOM. POS=0 # Left/right position. RANDOM=$$ # Seeds the random number generator from PID #+ of script. declare -a Slots # Array holding cumulative results of passes. NUMSLOTS=21 # Number of slots at bottom of board. Initialize_Slots () { # Zero out all elements of the array. for i in $( seq $NUMSLOTS ) do Slots[$i]=0 done echo # Blank line at beginning of run. } Show_Slots () { echo -n " " for i in $( seq $NUMSLOTS ) # Pretty-print array elements. do printf "%3d" ${Slots[$i]} # Allot three spaces per result. done echo # Row of slots: echo " |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|" echo " ^^" echo # Note that if the count within any particular slot exceeds 99, #+ it messes up the display. # Running only(!) 500 passes usually avoids this. } Move () { # Move one unit right / left, or stay put. Move=$RANDOM # How random is $RANDOM? Well, let's see ... let "Move %= RANGE" # Normalize into range of 0 - 2. case "$Move" in 0 ) ;; # Do nothing, i.e., stay in place. 1 ) ((POS--));; # Left. 2 ) ((POS++));; # Right. * ) echo -n "Error ";; # Anomaly! (Should never occur.) esac } Play () { # Single pass (inner loop). i=0 while [ "$i" -lt "$ROWS" ] # One event per row. do Move ((i++)); done SHIFT=11 # Why 11, and not 10? let "POS += $SHIFT" # Shift "zero position" to center. (( Slots[$POS]++ )) # DEBUG: echo $POS } Run () { # Outer loop. p=0 while [ "$p" -lt "$PASSES" ] do Play (( p++ )) POS=0 # Reset to zero. Why? done } # -------------- # main () Initialize_Slots Run Show_Slots # -------------- exit $? # Exercises: # --------- # 1) Show the results in a vertical bar graph, or as an alternative, #+ a scattergram. # 2) Alter the script to use /dev/urandom instead of $RANDOM. # Will this make the results more random? Jipe points out a set of techniques for generating random numbers within a range. # Generate random number between 6 and 30. rnumber=$((RANDOM%25+6)) # Generate random number in the same 6 - 30 range, #+ but the number must be evenly divisible by 3. rnumber=$(((RANDOM%30/3+1)*3)) # Note that this will not work all the time. # It fails if $RANDOM%30 returns 0. # Frank Wang suggests the following alternative: rnumber=$(( RANDOM%27/3*3+6 )) Bill Gradwohl came up with an improved formula that works for positive numbers. rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min)) Here Bill presents a versatile function that returns a random number between two specified values. Example 9-29. Random between values #!/bin/bash # random-between.sh # Random number between two specified values. # Script by Bill Gradwohl, with minor modifications by the document author. # Used with permission. randomBetween() { # Generates a positive or negative random number #+ between $min and $max #+ and divisible by $divisibleBy. # Gives a "reasonably random" distribution of return values. # # Bill Gradwohl - Oct 1, 2003 syntax() { # Function embedded within function. echo echo "Syntax: randomBetween [min] [max] [multiple]" echo echo -n "Expects up to 3 passed parameters, " echo "but all are completely optional." echo "min is the minimum value" echo "max is the maximum value" echo -n "multiple specifies that the answer must be " echo "a multiple of this value." echo " i.e. answer must be evenly divisible by this number." echo echo "If any value is missing, defaults area supplied as: 0 32767 1" echo -n "Successful completion returns 0, " echo "unsuccessful completion returns" echo "function syntax and 1." echo -n "The answer is returned in the global variable " echo "randomBetweenAnswer" echo -n "Negative values for any passed parameter are " echo "handled correctly." } local min=${1:-0} local max=${2:-32767} local divisibleBy=${3:-1} # Default values assigned, in case parameters not passed to function. local x local spread # Let's make sure the divisibleBy value is positive. [ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy)) # Sanity check. if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o ${min} -eq ${max} ]; then syntax return 1 fi # See if the min and max are reversed. if [ ${min} -gt ${max} ]; then # Swap them. x=${min} min=${max} max=${x} fi # If min is itself not evenly divisible by $divisibleBy, #+ then fix the min to be within range. if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then if [ ${min} -lt 0 ]; then min=$((min/divisibleBy*divisibleBy)) else min=$((((min/divisibleBy)+1)*divisibleBy)) fi fi # If max is itself not evenly divisible by $divisibleBy, #+ then fix the max to be within range. if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then if [ ${max} -lt 0 ]; then max=$((((max/divisibleBy)-1)*divisibleBy)) else max=$((max/divisibleBy*divisibleBy)) fi fi # --------------------------------------------------------------------- # Now, to do the real work. # Note that to get a proper distribution for the end points, #+ the range of random values has to be allowed to go between #+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1. # The slight increase will produce the proper distribution for the #+ end points. # Changing the formula to use abs(max-min)+1 will still produce #+ correct answers, but the randomness of those answers is faulty in #+ that the number of times the end points ($min and $max) are returned #+ is considerably lower than when the correct formula is used. # --------------------------------------------------------------------- spread=$((max-min)) # Omair Eshkenazi points out that this test is unnecessary, #+ since max and min have already been switched around. [ ${spread} -lt 0 ] && spread=$((0-spread)) let spread+=divisibleBy randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min)) return 0 # However, Paulo Marcel Coelho Aragao points out that #+ when $max and $min are not divisible by $divisibleBy, #+ the formula fails. # # He suggests instead the following formula: # rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy)) } # Let's test the function. min=-14 max=20 divisibleBy=3 # Generate an array of expected answers and check to make sure we get #+ at least one of each answer if we loop long enough. declare -a answer minimum=${min} maximum=${max} if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then if [ ${minimum} -lt 0 ]; then minimum=$((minimum/divisibleBy*divisibleBy)) else minimum=$((((minimum/divisibleBy)+1)*divisibleBy)) fi fi # If max is itself not evenly divisible by $divisibleBy, #+ then fix the max to be within range. if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then if [ ${maximum} -lt 0 ]; then maximum=$((((maximum/divisibleBy)-1)*divisibleBy)) else maximum=$((maximum/divisibleBy*divisibleBy)) fi fi # We need to generate only positive array subscripts, #+ so we need a displacement that that will guarantee #+ positive results. disp=$((0-minimum)) for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do answer[i+disp]=0 done # Now loop a large number of times to see what we get. loopIt=1000 # The script author suggests 100000, #+ but that takes a good long while. for ((i=0; i<${loopIt}; ++i)); do # Note that we are specifying min and max in reversed order here to #+ make the function correct for this case. randomBetween ${max} ${min} ${divisibleBy} # Report an error if an answer is unexpected. [ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ] \ && echo MIN or MAX error - ${randomBetweenAnswer}! [ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] \ && echo DIVISIBLE BY error - ${randomBetweenAnswer}! # Store the answer away statistically. answer[randomBetweenAnswer+disp]=$((answer[randomBetweenAnswer+disp]+1)) done # Let's check the results for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do [ ${answer[i+displacement]} -eq 0 ] \ && echo "We never got an answer of $i." \ || echo "${i} occurred ${answer[i+displacement]} times." done exit 0 Just how random is $RANDOM? The best way to test this is to write a script that tracks the distribution of "random" numbers generated by $RANDOM. Let's roll a $RANDOM die a few times . . . Example 9-30. Rolling a single die with RANDOM #!/bin/bash # How random is RANDOM? RANDOM=$$ # Reseed the random number generator using script process ID. PIPS=6 # A die has 6 pips. MAXTHROWS=600 # Increase this if you have nothing better to do with your time. throw=0 # Throw count. ones=0 # Must initialize counts to zero, twos=0 #+ since an uninitialized variable is null, not zero. threes=0 fours=0 fives=0 sixes=0 print_result () { echo echo "ones = $ones" echo "twos = $twos" echo "threes = $threes" echo "fours = $fours" echo "fives = $fives" echo "sixes = $sixes" echo } update_count() { case "$1" in 0) let "ones += 1";; # Since die has no "zero", this corresponds to 1. 1) let "twos += 1";; # And this to 2, etc. 2) let "threes += 1";; 3) let "fours += 1";; 4) let "fives += 1";; 5) let "sixes += 1";; esac } echo while [ "$throw" -lt "$MAXTHROWS" ] do let "die1 = RANDOM % $PIPS" update_count $die1 let "throw += 1" done print_result exit 0 # The scores should distribute fairly evenly, assuming RANDOM is fairly random. # With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so. # # Keep in mind that RANDOM is a pseudorandom generator, #+ and not a spectacularly good one at that. # Randomness is a deep and complex subject. # Sufficiently long "random" sequences may exhibit #+ chaotic and other "non-random" behavior. # Exercise (easy): # --------------- # Rewrite this script to flip a coin 1000 times. # Choices are "HEADS" and "TAILS". As we have seen in the last example, it is best to reseed the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. [45] (This mirrors the behavior of the random() function in C.) Example 9-31. Reseeding RANDOM #!/bin/bash # seeding-random.sh: Seeding the RANDOM variable. MAXCOUNT=25 # How many numbers to generate. random_numbers () { count=0 while [ "$count" -lt "$MAXCOUNT" ] do number=$RANDOM echo -n "$number " let "count += 1" done } echo; echo RANDOM=1 # Setting RANDOM seeds the random number generator. random_numbers echo; echo RANDOM=1 # Same seed for RANDOM... random_numbers # ...reproduces the exact same number series. # # When is it useful to duplicate a "random" number series? echo; echo RANDOM=2 # Trying again, but with a different seed... random_numbers # gives a different number series. echo; echo # RANDOM=$$ seeds RANDOM from process id of script. # It is also possible to seed RANDOM from 'time' or 'date' commands. # Getting fancy... SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }') # Pseudo-random output fetched #+ from /dev/urandom (system pseudo-random device-file), #+ then converted to line of printable (octal) numbers by "od", #+ finally "awk" retrieves just one number for SEED. RANDOM=$SEED random_numbers echo; echo exit 0 Note The /dev/urandom pseudo-device file provides a method of generating much more "random" pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example, Example 15-14, and Example A-36), or even piping to md5sum (see Example 33-14). There are also other ways to generate pseudorandom numbers in a script. Awk provides a convenient means of doing this. Example 9-32. Pseudorandom numbers, using awk #!/bin/bash # random2.sh: Returns a pseudorandom number in the range 0 - 1. # Uses the awk rand() function. AWKSCRIPT=' { srand(); print rand() } ' # Command(s) / parameters passed to awk # Note that srand() reseeds awk's random number generator. echo -n "Random number between 0 and 1 = " echo | awk "$AWKSCRIPT" # What happens if you leave out the 'echo'? exit 0 # Exercises: # --------- # 1) Using a loop construct, print out 10 different random numbers. # (Hint: you must reseed the "srand()" function with a different seed #+ in each pass through the loop. What happens if you fail to do this?) # 2) Using an integer multiplier as a scaling factor, generate random numbers #+ in the range between 10 and 100. # 3) Same as exercise #2, above, but generate random integers this time. The date command also lends itself to generating pseudorandom integer sequences. ------------------------------------------------------------------------ 9.7. The Double-Parentheses Construct Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ )). Example 9-33. C-style manipulation of variables #!/bin/bash # c-vars.sh # Manipulating a variable, C-style, using the (( ... )) construct. echo (( a = 23 )) # Setting a value, C-style, #+ with spaces on both sides of the "=". echo "a (initial value) = $a" # 23 (( a++ )) # Post-increment 'a', C-style. echo "a (after a++) = $a" # 24 (( a-- )) # Post-decrement 'a', C-style. echo "a (after a--) = $a" # 23 (( ++a )) # Pre-increment 'a', C-style. echo "a (after ++a) = $a" # 24 (( --a )) # Pre-decrement 'a', C-style. echo "a (after --a) = $a" # 23 echo ######################################################## # Note that, as in C, pre- and post-decrement operators #+ have different side-effects. n=1; let --n && echo "True" || echo "False" # False n=1; let n-- && echo "True" || echo "False" # True # Thanks, Jeroen Domburg. ######################################################## echo (( t = a<45?7:11 )) # C-style trinary operator. # ^ ^ ^ echo "If a < 45, then t = 7, else t = 11." # a = 23 echo "t = $t " # t = 7 echo # ----------------- # Easter Egg alert! # ----------------- # Chet Ramey seems to have snuck a bunch of undocumented C-style #+ constructs into Bash (actually adapted from ksh, pretty much). # In the Bash docs, Ramey calls (( ... )) shell arithmetic, #+ but it goes far beyond that. # Sorry, Chet, the secret is out. # See also "for" and "while" loops using the (( ... )) construct. # These work only with version 2.04 or later of Bash. exit See also Example 10-12 and Example 8-4. ------------------------------------------------------------------------ Chapter 10. Loops and Branches What needs this iteration, woman? --Shakespeare, Othello Operations on code blocks are the key to structured and organized shell scripts. Looping and branching constructs provide the tools for accomplishing this. ------------------------------------------------------------------------ 10.1. Loops A loop is a block of code that iterates [46] a list of commands as long as the loop control condition is true. for loops for arg in [list] This is the basic looping construct. It differs significantly from its C counterpart. for arg in [list] do  command(s)... done Note During each pass through the loop, arg takes on the value of each successive variable in the list. for arg in "$var1" "$var2" "$var3" ... "$varN" # In pass 1 of the loop, arg = $var1 # In pass 2 of the loop, arg = $var2 # In pass 3 of the loop, arg = $var3 # ... # In pass N of the loop, arg = $varN # Arguments in [list] quoted to prevent possible word splitting. The argument list may contain wild cards. If do is on same line as for, there needs to be a semicolon after list. for arg in [list] ; do Example 10-1. Simple for loops #!/bin/bash # Listing the planets. for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto do echo $planet # Each planet on a separate line. done echo; echo for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto" # All planets on same line. # Entire 'list' enclosed in quotes creates a single variable. # Why? Whitespace incorporated into the variable. do echo $planet done echo; echo "Whoops! Pluto is no longer a planet!" exit 0 Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see Example 14-16) to force parsing of each [list] element and assignment of each component to the positional parameters. Example 10-2. for loop with two parameters in each [list] element #!/bin/bash # Planets revisited. # Associate the name of each planet with its distance from the sun. for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483" do set -- $planet # Parses variable "planet" #+ and sets positional parameters. # The "--" prevents nasty surprises if $planet is null or #+ begins with a dash. # May need to save original positional parameters, #+ since they get overwritten. # One way of doing this is to use an array, # original_params=("$@") echo "$1 $2,000,000 miles from the sun" #-------two tabs---concatenate zeroes onto parameter $2 done # (Thanks, S.C., for additional clarification.) exit 0 A variable may supply the [list] in a for loop. Example 10-3. Fileinfo: operating on a file list contained in a variable #!/bin/bash # fileinfo.sh FILES="/usr/sbin/accept /usr/sbin/pwck /usr/sbin/chroot /usr/bin/fakefile /sbin/badblocks /sbin/ypbind" # List of files you are curious about. # Threw in a dummy file, /usr/bin/fakefile. echo for file in $FILES do if [ ! -e "$file" ] # Check if file exists. then echo "$file does not exist."; echo continue # On to next. fi ls -l $file | awk '{ print $8 " file size: " $5 }' # Print 2 fields. whatis `basename $file` # File info. # Note that the whatis database needs to have been set up for this to work. # To do this, as root run /usr/bin/makewhatis. echo done exit 0 If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing takes place. Example 10-4. Operating on files with a for loop #!/bin/bash # list-glob.sh: Generating [list] in a for-loop, using "globbing" echo for file in * # ^ Bash performs filename expansion #+ on expressions that globbing recognizes. do ls -l "$file" # Lists all files in $PWD (current directory). # Recall that the wild card character "*" matches every filename, #+ however, in "globbing," it doesn't match dot-files. # If the pattern matches no file, it is expanded to itself. # To prevent this, set the nullglob option #+ (shopt -s nullglob). # Thanks, S.C. done echo; echo for file in [jx]* do rm -f $file # Removes only files beginning with "j" or "x" in $PWD. echo "Removed file \"$file\"". done echo exit 0 Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is Example A-15. See also Example 14-17. Example 10-5. Missing in [list] in a for loop #!/bin/bash # Invoke this script both with and without arguments, #+ and see what happens. for a do echo -n "$a " done # The 'in list' missing, therefore the loop operates on '$@' #+ (command-line argument list, including whitespace). echo exit 0 It is possible to use command substitution to generate the [list] in a for loop. See also Example 15-54, Example 10-10 and Example 15-48. Example 10-6. Generating the [list] in a for loop with command substitution #!/bin/bash # for-loopcmd.sh: for-loop with [list] #+ generated by command substitution. NUMBERS="9 7 3 8 37.53" for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53 do echo -n "$number " done echo exit 0 Here is a somewhat more complex example of using command substitution to create the [list]. Example 10-7. A grep replacement for binary files #!/bin/bash # bin-grep.sh: Locates matching strings in a binary file. # A "grep" replacement for binary files. # Similar effect to "grep -a" E_BADARGS=65 E_NOFILE=66 if [ $# -ne 2 ] then echo "Usage: `basename $0` search_string filename" exit $E_BADARGS fi if [ ! -f "$2" ] then echo "File \"$2\" does not exist." exit $E_NOFILE fi IFS=$'\012' # Per suggestion of Anton Filippov. # was: IFS="\n" for word in $( strings "$2" | grep "$1" ) # The "strings" command lists strings in binary files. # Output then piped to "grep", which tests for desired string. do echo $word done # As S.C. points out, lines 23 - 30 could be replaced with the simpler # strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]' # Try something like "./bin-grep.sh mem /bin/ls" #+ to exercise this script. exit 0 More of the same. Example 10-8. Listing all users on the system #!/bin/bash # userlist.sh PASSWORD_FILE=/etc/passwd n=1 # User number for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" ) # Field separator = : ^^^^^^ # Print first field ^^^^^^^^ # Get input from password file ^^^^^^^^^^^^^^^^^ do echo "USER #$n = $name" let "n += 1" done # USER #1 = root # USER #2 = bin # USER #3 = daemon # ... # USER #30 = bozo exit 0 # Exercise: # -------- # How is it that an ordinary user (or a script run by same) #+ can read /etc/passwd? # Isn't this a security hole? Why or why not? Yet another example of the [list] resulting from command substitution. Example 10-9. Checking all the binaries in a directory for authorship #!/bin/bash # findstring.sh: # Find a particular string in the binaries in a specified directory. directory=/usr/bin/ fstring="Free Software Foundation" # See which files come from the FSF. for file in $( find $directory -type f -name '*' | sort ) do strings -f $file | grep "$fstring" | sed -e "s%$directory%%" # In the "sed" expression, #+ it is necessary to substitute for the normal "/" delimiter #+ because "/" happens to be one of the characters filtered out. # Failure to do so gives an error message. (Try it.) done exit $? # Exercise (easy): # --------------- # Convert this script to take command-line parameters #+ for $directory and $fstring. A final example of [list] / command substitution, but this time the "command" is a function. generate_list () { echo "one two three" } for word in $(generate_list) # Let "word" grab output of function. do echo "$word" done # one # two # three The output of a for loop may be piped to a command or commands. Example 10-10. Listing the symbolic links in a directory #!/bin/bash # symlinks.sh: Lists symbolic links in a directory. directory=${1-`pwd`} # Defaults to current working directory, #+ if not otherwise specified. # Equivalent to code block below. # ---------------------------------------------------------- # ARGS=1 # Expect one command-line argument. # # if [ $# -ne "$ARGS" ] # If not 1 arg... # then # directory=`pwd` # current working directory # else # directory=$1 # fi # ---------------------------------------------------------- echo "symbolic links in directory \"$directory\"" for file in "$( find $directory -type l )" # -type l = symbolic links do echo "$file" done | sort # Otherwise file list is unsorted. # Strictly speaking, a loop isn't really necessary here, #+ since the output of the "find" command is expanded into a single word. # However, it's easy to understand and illustrative this way. # As Dominik 'Aeneas' Schnitzer points out, #+ failing to quote $( find $directory -type l ) #+ will choke on filenames with embedded whitespace. # containing whitespace. exit 0 # -------------------------------------------------------- # Jean Helou proposes the following alternative: echo "symbolic links in directory \"$directory\"" # Backup of the current IFS. One can never be too cautious. OLDIFS=$IFS IFS=: for file in $(find $directory -type l -printf "%p$IFS") do # ^^^^^^^^^^^^^^^^ echo "$file" done|sort # And, James "Mike" Conley suggests modifying Helou's code thusly: OLDIFS=$IFS IFS='' # Null IFS means no word breaks for file in $( find $directory -type l ) do echo $file done | sort # This works in the "pathological" case of a directory name having #+ an embedded colon. # "This also fixes the pathological case of the directory name having #+ a colon (or space in earlier example) as well." The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows. Example 10-11. Symbolic links in a directory, saved to a file #!/bin/bash # symlinks.sh: Lists symbolic links in a directory. OUTFILE=symlinks.list # save file directory=${1-`pwd`} # Defaults to current working directory, #+ if not otherwise specified. echo "symbolic links in directory \"$directory\"" > "$OUTFILE" echo "---------------------------" >> "$OUTFILE" for file in "$( find $directory -type l )" # -type l = symbolic links do echo "$file" done | sort >> "$OUTFILE" # stdout of loop # ^^^^^^^^^^^^^ redirected to save file. exit 0 There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses. Example 10-12. A C-style for loop #!/bin/bash # Multiple ways to count up to 10. echo # Standard syntax. for a in 1 2 3 4 5 6 7 8 9 10 do echo -n "$a " done echo; echo # +==========================================+ # Using "seq" ... for a in `seq 10` do echo -n "$a " done echo; echo # +==========================================+ # Using brace expansion ... # Bash, version 3+. for a in {1..10} do echo -n "$a " done echo; echo # +==========================================+ # Now, let's do the same, using C-like syntax. LIMIT=10 for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and "LIMIT" with no "$". do echo -n "$a " done # A construct borrowed from 'ksh93'. echo; echo # +=========================================================================+ # Let's use the C "comma operator" to increment two variables simultaneously. for ((a=1, b=1; a <= LIMIT ; a++, b++)) do # The comma chains together operations. echo -n "$a-$b " done echo; echo exit 0 See also Example 26-16, Example 26-17, and Example A-6. --- Now, a for loop used in a "real-life" context. Example 10-13. Using efax in batch mode #!/bin/bash # Faxing (must have 'efax' package installed). EXPECTED_ARGS=2 E_BADARGS=85 MODEM_PORT="/dev/ttyS2" # May be different on your machine. # ^^^^^ PCMCIA modem card default port. if [ $# -ne $EXPECTED_ARGS ] # Check for proper number of command-line args. then echo "Usage: `basename $0` phone# text-file" exit $E_BADARGS fi if [ ! -f "$2" ] then echo "File $2 is not a text file." # File is not a regular file, or does not exist. exit $E_BADARGS fi fax make $2 # Create fax-formatted files from text files. for file in $(ls $2.0*) # Concatenate the converted files. # Uses wild card (filename "globbing") #+ in variable list. do fil="$fil $file" done efax -d "$MODEM_PORT" -t "T$1" $fil # Finally, do the work. # Trying adding -o1 if above line fails. # As S.C. points out, the for-loop can be eliminated with # efax -d /dev/ttyS2 -o1 -t "T$1" $2.0* #+ but it's not quite as instructive [grin]. exit $? # Also, efax sends diagnostic messages to stdout. while This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand. while [ condition ] do  command(s)... done The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]). As is the case with for loops, placing the do on the same line as the condition test requires a semicolon. while [ condition ] ; do Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct. Example 10-14. Simple while loop #!/bin/bash var0=0 LIMIT=10 while [ "$var0" -lt "$LIMIT" ] # ^ ^ # Spaces, because these are "test-brackets" . . . do echo -n "$var0 " # -n suppresses newline. # ^ Space, to separate printed out numbers. var0=`expr $var0 + 1` # var0=$(($var0+1)) also works. # var0=$((var0 + 1)) also works. # let "var0 += 1" also works. done # Various other methods also work. echo exit 0 Example 10-15. Another while loop #!/bin/bash echo # Equivalent to: while [ "$var1" != "end" ] # while test "$var1" != "end" do echo "Input variable #1 (end to exit) " read var1 # Not 'read $var1' (why?). echo "variable #1 = $var1" # Need quotes because of "#" . . . # If input is 'end', echoes it here. # Does not test for termination condition until top of loop. echo done exit 0 A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however. Example 10-16. while loop with multiple conditions #!/bin/bash var1=unset previous=$var1 while echo "previous-variable = $previous" echo previous=$var1 [ "$var1" != end ] # Keeps track of what $var1 was previously. # Four conditions on "while", but only last one controls loop. # The *last* exit status is the one that counts. do echo "Input variable #1 (end to exit) " read var1 echo "variable #1 = $var1" done # Try to figure out how this all works. # It's a wee bit tricky. exit 0 As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct (see also Example 9-33). Example 10-17. C-style syntax in a while loop #!/bin/bash # wh-loopc.sh: Count to 10 in a "while" loop. LIMIT=10 a=1 while [ "$a" -le $LIMIT ] do echo -n "$a " let "a+=1" done # No surprises, so far. echo; echo # +=================================================================+ # Now, repeat with C-like syntax. ((a = 1)) # a=1 # Double parentheses permit space when setting a variable, as in C. while (( a <= LIMIT )) # Double parentheses, and no "$" preceding variables. do echo -n "$a " ((a += 1)) # let "a+=1" # Yes, indeed. # Double parentheses permit incrementing a variable with C-like syntax. done echo # C programmers can feel right at home in Bash. exit 0 Inside its test brackets, a while loop can call a function. t=0 condition () { ((t++)) if [ $t -lt 5 ] then return 0 # true else return 1 # false fi } while condition # ^^^^^^^^^ # Function call -- four loop iterations. do echo "Still going: t = $t" done # Still going: t = 1 # Still going: t = 2 # Still going: t = 3 # Still going: t = 4 +--------------------------------------------------------------+ | Similar to the if-test construct, a while loop can omit the | | test brackets. | | | | while condition | | do | | command(s) ... | | done | +--------------------------------------------------------------+ By coupling the power of the read command with a while loop, we get the handy while read construct, useful for reading and parsing files. cat $filename | # Supply input from a file. while read line # As long as there is another line to read ... do ... done # =========== Snippet from "sd.sh" example script ========== # while read value # Read one data point at a time. do rt=$(echo "scale=$SC; $rt + $value" | bc) (( ct++ )) done am=$(echo "scale=$SC; $rt / $ct" | bc) echo $am; return $ct # This function "returns" TWO values! # Caution: This little trick will not work if $ct > 255! # To handle a larger number of data points, #+ simply comment out the "return $ct" above. } <"$datafile" # Feed in data file. Note A while loop may have its stdin redirected to a file by a < at its end. A while loop may have its stdin supplied by a pipe. until This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop). until [ condition-is-true ] do  command(s)... done Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages. As is the case with for loops, placing the do on the same line as the condition test requires a semicolon. until [ condition-is-true ] ; do Example 10-18. until loop #!/bin/bash END_CONDITION=end until [ "$var1" = "$END_CONDITION" ] # Tests condition here, at top of loop. do echo "Input variable #1 " echo "($END_CONDITION to exit)" read var1 echo "variable #1 = $var1" echo done # ------------------------------------------- # # As with "for" and "while" loops, #+ an "until" loop permits C-like test constructs. LIMIT=10 var=0 until (( var > LIMIT )) do # ^^ ^ ^ ^^ No brackets, no $ prefixing variables. echo -n "$var " (( var++ )) done # 0 1 2 3 4 5 6 7 8 9 10 exit 0 How to choose between a for loop or a while loop or until loop? In C, you would typically use a for loop when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore, feel free to use whatever type of loop gets the job done in the simplest way. ------------------------------------------------------------------------ 10.2. Nested Loops A nested loop is a loop within a loop, an inner loop within the body of an outer one. How this works is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break within either the inner or outer loop would interrupt this process. Example 10-19. Nested Loop #!/bin/bash # nested-loop.sh: Nested "for" loops. outer=1 # Set outer loop counter. # Beginning of outer loop. for a in 1 2 3 4 5 do echo "Pass $outer in outer loop." echo "---------------------" inner=1 # Reset inner loop counter. # =============================================== # Beginning of inner loop. for b in 1 2 3 4 5 do echo "Pass $inner in inner loop." let "inner+=1" # Increment inner loop counter. done # End of inner loop. # =============================================== let "outer+=1" # Increment outer loop counter. echo # Space between output blocks in pass of outer loop. done # End of outer loop. exit 0 See Example 26-11 for an illustration of nested while loops, and Example 26-13 to see a while loop nested inside an until loop. ------------------------------------------------------------------------ 10.3. Loop Control Tournez cent tours, tournez mille tours, Tournez souvent et tournez toujours . . . --Verlaine, "Chevaux de bois" Commands affecting loop behavior break, continue The break and continue loop control commands [47] correspond exactly to their counterparts in other programming languages. The break command terminates the loop (breaks out of it), while continue causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle. Example 10-20. Effects of break and continue in a loop #!/bin/bash LIMIT=19 # Upper limit echo echo "Printing Numbers 1 through 20 (but not 3 and 11)." a=0 while [ $a -le "$LIMIT" ] do a=$(($a+1)) if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11. then continue # Skip rest of this particular loop iteration. fi echo -n "$a " # This will not execute for 3 and 11. done # Exercise: # Why does the loop print up to 20? echo; echo echo Printing Numbers 1 through 20, but something happens after 2. ################################################################## # Same loop, but substituting 'break' for 'continue'. a=0 while [ "$a" -le "$LIMIT" ] do a=$(($a+1)) if [ "$a" -gt 2 ] then break # Skip entire rest of loop. fi echo -n "$a " done echo; echo; echo exit 0 The break command may optionally take a parameter. A plain break terminates only the innermost loop in which it is embedded, but a break N breaks out of N levels of loop. Example 10-21. Breaking out of multiple loop levels #!/bin/bash # break-levels.sh: Breaking out of loops. # "break N" breaks out of N level loops. for outerloop in 1 2 3 4 5 do echo -n "Group $outerloop: " # -------------------------------------------------------- for innerloop in 1 2 3 4 5 do echo -n "$innerloop " if [ "$innerloop" -eq 3 ] then break # Try break 2 to see what happens. # ("Breaks" out of both inner and outer loops.) fi done # -------------------------------------------------------- echo done echo exit 0 The continue command, similar to break, optionally takes a parameter. A plain continue cuts short the current iteration within its loop and begins the next. A continue N terminates all remaining iterations at its loop level and continues with the next iteration at the loop, N levels above. Example 10-22. Continuing at a higher loop level #!/bin/bash # The "continue N" command, continuing at the Nth level loop. for outer in I II III IV V # outer loop do echo; echo -n "Group $outer: " # -------------------------------------------------------------------- for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop do if [[ "$inner" -eq 7 && "$outer" = "III" ]] then continue 2 # Continue at loop on 2nd level, that is "outer loop". # Replace above line with a simple "continue" # to see normal loop behavior. fi echo -n "$inner " # 7 8 9 10 will not echo on "Group III." done # -------------------------------------------------------------------- done echo; echo # Exercise: # Come up with a meaningful use for "continue N" in a script. exit 0 Example 10-23. Using continue N in an actual task # Albert Reiner gives an example of how to use "continue N": # --------------------------------------------------------- # Suppose I have a large number of jobs that need to be run, with #+ any data that is to be treated in files of a given name pattern in a #+ directory. There are several machines that access this directory, and #+ I want to distribute the work over these different boxen. Then I #+ usually nohup something like the following on every box: while true do for n in .iso.* do [ "$n" = ".iso.opts" ] && continue beta=${n#.iso.} [ -r .Iso.$beta ] && continue [ -r .lock.$beta ] && sleep 10 && continue lockfile -r0 .lock.$beta || continue echo -n "$beta: " `date` run-isotherm $beta date ls -alF .Iso.$beta [ -r .Iso.$beta ] && rm -f .lock.$beta continue 2 done break done # The details, in particular the sleep N, are particular to my #+ application, but the general pattern is: while true do for job in {pattern} do {job already done or running} && continue {mark job as running, do job, mark job as done} continue 2 done break # Or something like `sleep 600' to avoid termination. done # This way the script will stop only when there are no more jobs to do #+ (including jobs that were added during runtime). Through the use #+ of appropriate lockfiles it can be run on several machines #+ concurrently without duplication of calculations [which run a couple #+ of hours in my case, so I really want to avoid this]. Also, as search #+ always starts again from the beginning, one can encode priorities in #+ the file names. Of course, one could also do this without `continue 2', #+ but then one would have to actually check whether or not some job #+ was done (so that we should immediately look for the next job) or not #+ (in which case we terminate or sleep for a long time before checking #+ for a new job). Caution The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided. ------------------------------------------------------------------------ 10.4. Testing and Branching The case and select constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block. Controlling program flow in a code block case (in) / esac The case construct is the shell scripting analog to switch in C/C++. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus. case "$variable" in  "$condition1" )  command...  ;;  "$condition2" )  command...  ;; esac Note * Quoting the variables is not mandatory, since word splitting does not take place. * Each test line ends with a right paren ). * Each condition block ends with a double semicolon ;;. * If a condition tests true, then the associated commands execute and the case block terminates. * The entire case block ends with an esac (case spelled backwards). Example 10-24. Using case #!/bin/bash # Testing ranges of characters. echo; echo "Hit a key, then hit return." read Keypress case "$Keypress" in [[:lower:]] ) echo "Lowercase letter";; [[:upper:]] ) echo "Uppercase letter";; [0-9] ) echo "Digit";; * ) echo "Punctuation, whitespace, or other";; esac # Allows ranges of characters in [square brackets], #+ or POSIX ranges in [[double square brackets. # In the first version of this example, #+ the tests for lowercase and uppercase characters were #+ [a-z] and [A-Z]. # This no longer works in certain locales and/or Linux distros. # POSIX is more portable. # Thanks to Frank Wang for pointing this out. # Exercise: # -------- # As the script stands, it accepts a single keystroke, then terminates. # Change the script so it accepts repeated input, #+ reports on each keystroke, and terminates only when "X" is hit. # Hint: enclose everything in a "while" loop. exit 0 Example 10-25. Creating menus using case #!/bin/bash # Crude address database clear # Clear the screen. echo " Contact List" echo " ------- ----" echo "Choose one of the following persons:" echo echo "[E]vans, Roland" echo "[J]ones, Mildred" echo "[S]mith, Julie" echo "[Z]ane, Morris" echo read person case "$person" in # Note variable is quoted. "E" | "e" ) # Accept upper or lowercase input. echo echo "Roland Evans" echo "4321 Flash Dr." echo "Hardscrabble, CO 80753" echo "(303) 734-9874" echo "(303) 734-9892 fax" echo "revans@zzy.net" echo "Business partner & old friend" ;; # Note double semicolon to terminate each option. "J" | "j" ) echo echo "Mildred Jones" echo "249 E. 7th St., Apt. 19" echo "New York, NY 10009" echo "(212) 533-2814" echo "(212) 533-9972 fax" echo "milliej@loisaida.com" echo "Ex-girlfriend" echo "Birthday: Feb. 11" ;; # Add info for Smith & Zane later. * ) # Default option. # Empty input (hitting RETURN) fits here, too. echo echo "Not yet in database." ;; esac echo # Exercise: # -------- # Change the script so it accepts multiple inputs, #+ instead of terminating after displaying just one address. exit 0 An exceptionally clever use of case involves testing for command-line parameters. #! /bin/bash case "$1" in "") echo "Usage: ${0##*/} "; exit $E_PARAM;; # No command-line parameters, # or first parameter empty. # Note that ${0##*/} is ${var##pattern} param substitution. # Net result is $0. -*) FILENAME=./$1;; # If filename passed as argument ($1) #+ starts with a dash, #+ replace it with ./$1 #+ so further commands don't interpret it #+ as an option. * ) FILENAME=$1;; # Otherwise, $1. esac Here is an more straightforward example of command-line parameter handling: #! /bin/bash while [ $# -gt 0 ]; do # Until you run out of parameters . . . case "$1" in -d|--debug) # "-d" or "--debug" parameter? DEBUG=1 ;; -c|--conf) CONFFILE="$2" shift if [ ! -f $CONFFILE ]; then echo "Error: Supplied file doesn't exist!" exit $E_CONFFILE # File not found error. fi ;; esac shift # Check next set of parameters. done # From Stefano Falsetto's "Log2Rot" script, #+ part of his "rottlog" package. # Used with permission. Example 10-26. Using command substitution to generate the case variable #!/bin/bash # case-cmd.sh: Using command substitution to generate a "case" variable. case $( arch ) in # "arch" returns machine architecture. # Equivalent to 'uname -m' ... i386 ) echo "80386-based machine";; i486 ) echo "80486-based machine";; i586 ) echo "Pentium-based machine";; i686 ) echo "Pentium2+-based machine";; * ) echo "Other type of machine";; esac exit 0 A case construct can filter strings for globbing patterns. Example 10-27. Simple string matching #!/bin/bash # match-string.sh: Simple string matching. match_string () { # Exact string match. MATCH=0 E_NOMATCH=90 PARAMS=2 # Function requires 2 arguments. E_BAD_PARAMS=91 [ $# -eq $PARAMS ] || return $E_BAD_PARAMS case "$1" in "$2") return $MATCH;; * ) return $E_NOMATCH;; esac } a=one b=two c=three d=two match_string $a # wrong number of parameters echo $? # 91 match_string $a $b # no match echo $? # 90 match_string $b $d # match echo $? # 0 exit 0 Example 10-28. Checking for alphabetic input #!/bin/bash # isalpha.sh: Using a "case" structure to filter a string. SUCCESS=0 FAILURE=-1 isalpha () # Tests whether *first character* of input string is alphabetic. { if [ -z "$1" ] # No argument passed? then return $FAILURE fi case "$1" in [a-zA-Z]*) return $SUCCESS;; # Begins with a letter? * ) return $FAILURE;; esac } # Compare this with "isalpha ()" function in C. isalpha2 () # Tests whether *entire string* is alphabetic. { [ $# -eq 1 ] || return $FAILURE case $1 in *[!a-zA-Z]*|"") return $FAILURE;; *) return $SUCCESS;; esac } isdigit () # Tests whether *entire string* is numerical. { # In other words, tests for integer variable. [ $# -eq 1 ] || return $FAILURE case $1 in *[!0-9]*|"") return $FAILURE;; *) return $SUCCESS;; esac } check_var () # Front-end to isalpha (). { if isalpha "$@" then echo "\"$*\" begins with an alpha character." if isalpha2 "$@" then # No point in testing if first char is non-alpha. echo "\"$*\" contains only alpha characters." else echo "\"$*\" contains at least one non-alpha character." fi else echo "\"$*\" begins with a non-alpha character." # Also "non-alpha" if no argument passed. fi echo } digit_check () # Front-end to isdigit (). { if isdigit "$@" then echo "\"$*\" contains only digits [0 - 9]." else echo "\"$*\" has at least one non-digit character." fi echo } a=23skidoo b=H3llo c=-What? d=What? e=`echo $b` # Command substitution. f=AbcDef g=27234 h=27a34 i=27.34 check_var $a check_var $b check_var $c check_var $d check_var $e check_var $f check_var # No argument passed, so what happens? # digit_check $g digit_check $h digit_check $i exit 0 # Script improved by S.C. # Exercise: # -------- # Write an 'isfloat ()' function that tests for floating point numbers. # Hint: The function duplicates 'isdigit ()', #+ but adds a test for a mandatory decimal point. select The select construct, adopted from the Korn Shell, is yet another tool for building menus. select variable [in list] do  command...  break done This prompts the user to enter one of the choices presented in the variable list. Note that select uses the $PS3 prompt (#? ) by default, but this may be changed. Example 10-29. Creating menus using select #!/bin/bash PS3='Choose your favorite vegetable: ' # Sets the prompt string. # Otherwise it defaults to #? . echo select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas" do echo echo "Your favorite veggie is $vegetable." echo "Yuck!" echo break # What happens if there is no 'break' here? done exit # Exercise: # -------- # Fix this script to accept user input not specified in #+ the "select" statement. # For example, if the user inputs "peas," #+ The script would respond "Sorry. That is not on the menu." If in list is omitted, then select uses the list of command line arguments ($@) passed to the script or to the function in which the select construct is embedded. Compare this to the behavior of a for variable [in list] construct with the in list omitted. Example 10-30. Creating menus using select in a function #!/bin/bash PS3='Choose your favorite vegetable: ' echo choice_of() { select vegetable # [in list] omitted, so 'select' uses arguments passed to function. do echo echo "Your favorite veggie is $vegetable." echo "Yuck!" echo break done } choice_of beans rice carrots radishes tomatoes spinach # $1 $2 $3 $4 $5 $6 # passed to choice_of() function exit 0 See also Example 34-3. ------------------------------------------------------------------------ Chapter 11. Command Substitution Command substitution reassigns the output of a command [48] or even multiple commands; it literally plugs the command output into another context. [49] The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command-line text. script_name=`basename $0` echo "The name of this script is $script_name." The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for loop. rm `cat filename` # "filename" contains a list of files to delete. # # S. C. points out that "arg list too long" error might result. # Better is xargs rm -- < filename # ( -- covers those cases where "filename" begins with a "-" ) textfile_listing=`ls *.txt` # Variable contains names of all *.txt files in current working directory. echo $textfile_listing textfile_listing2=$(ls *.txt) # The alternative form of command substitution. echo $textfile_listing2 # Same result. # A possible problem with putting a list of files into a single string # is that a newline may creep in. # # A safer way to assign a list of files to a parameter is with an array. # shopt -s nullglob # If no match, filename expands to nothing. # textfile_listing=( *.txt ) # # Thanks, S.C. Note Command substitution invokes a subshell. Caution Command substitution may result in word splitting. COMMAND `echo a b` # 2 args: a and b COMMAND "`echo a b`" # 1 arg: "a b" COMMAND `echo` # no arg COMMAND "`echo`" # one empty arg # Thanks, S.C. Even when there is no word splitting, command substitution can remove trailing newlines. # cd "`pwd`" # This should always work. # However... mkdir 'dir with trailing newline ' cd 'dir with trailing newline ' cd "`pwd`" # Error message: # bash: cd: /tmp/file with trailing newline: No such file or directory cd "$PWD" # Works fine. old_tty_setting=$(stty -g) # Save old terminal setting. echo "Hit a key " stty -icanon -echo # Disable "canonical" mode for terminal. # Also, disable *local* echo. key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress. stty "$old_tty_setting" # Restore old setting. echo "You hit ${#key} key." # ${#variable} = number of characters in $variable # # Hit any key except RETURN, and the output is "You hit 1 key." # Hit RETURN, and it's "You hit 0 key." # The newline gets eaten in the command substitution. Thanks, S.C. Caution Using echo to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises. dir_listing=`ls -l` echo $dir_listing # unquoted # Expecting a nicely ordered directory listing. # However, what you get is: # total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo # bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh # The newlines disappeared. echo "$dir_listing" # quoted # -rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt # -rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh # -rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command. variable1=`/dev/null|grep -E "^I.*Cls=03.*Prot=02"` kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"` ... fi Caution Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke. Example 11-1. Stupid script tricks #!/bin/bash # stupid-script-tricks.sh: Don't try this at home, folks. # From "Stupid Script Tricks," Volume I. dangerous_variable=`cat /boot/vmlinuz` # The compressed Linux kernel itself. echo "string-length of \$dangerous_variable = ${#dangerous_variable}" # string-length of $dangerous_variable = 794151 # (Does not give same count as 'wc -c /boot/vmlinuz'.) # echo "$dangerous_variable" # Don't try this! It would hang the script. # The document author is aware of no useful applications for #+ setting a variable to the contents of a binary file. exit 0 Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as Bash, provides more protection from programmer mistakes than a compiled language. Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop. Example 11-2. Generating a variable from a loop #!/bin/bash # csubloop.sh: Setting a variable to the output of a loop. variable1=`for i in 1 2 3 4 5 do echo -n "$i" # The 'echo' command is critical done` #+ to command substitution here. echo "variable1 = $variable1" # variable1 = 12345 i=0 variable2=`while [ "$i" -lt 10 ] do echo -n "$i" # Again, the necessary 'echo'. let "i += 1" # Increment. done` echo "variable2 = $variable2" # variable2 = 0123456789 # Demonstrates that it's possible to embed a loop #+ within a variable declaration. exit 0 +----------------------------------------------------------------------+ | Command substitution makes it possible to extend the toolset | | available to Bash. It is simply a matter of writing a program or | | script that outputs to stdout (like a well-behaved UNIX tool should) | | and assigning that output to a variable. | | | | #include | | | | /* "Hello, world." C program */ | | | | int main() | | { | | printf( "Hello, world." ); | | return (0); | | } | | | | +------------------------------------------------------------------+ | | |bash$ gcc -o hello hello.c | | | | | | | +------------------------------------------------------------------+ | | | | #!/bin/bash | | # hello.sh | | | | greeting=`./hello` | | echo $greeting | | | | +------------------------------------------------------------------+ | | |bash$ sh hello.sh | | | |Hello, world. | | | | | | | +------------------------------------------------------------------+ | +----------------------------------------------------------------------+ Note The $(...) form has superseded backticks for command substitution. output=$(sed -n /"$1"/p $file) # From "grp.sh" example. # Setting a variable to the contents of a text file. File_contents1=$(cat $file1) File_contents2=$(<$file2) # Bash permits this also. The $(...) form of command substitution treats a double backslash in a different way than `...`. +------------------------------------------------------------------------+ |bash$ echo `echo \\` | | | | | |bash$ echo $(echo \\) | |\ | | | +------------------------------------------------------------------------+ The $(...) form of command substitution permits nesting. [50] word_count=$( wc -w $(echo * | awk '{print $8}') ) Or, for something a bit more elaborate . . . Example 11-3. Finding anagrams #!/bin/bash # agram2.sh # Example of nested command substitution. # Uses "anagram" utility #+ that is part of the author's "yawl" word list package. # http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz # http://bash.neuralshortcircuit.com/yawl-0.3.2.tar.gz E_NOARGS=66 E_BADARG=67 MINLEN=7 if [ -z "$1" ] then echo "Usage $0 LETTERSET" exit $E_NOARGS # Script needs a command-line argument. elif [ ${#1} -lt $MINLEN ] then echo "Argument must have at least $MINLEN letters." exit $E_BADARG fi FILTER='.......' # Must have at least 7 letters. # 1234567 Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) ) # $( $( nested command sub. ) ) # ( array assignment ) echo echo "${#Anagrams[*]} 7+ letter anagrams found" echo echo ${Anagrams[0]} # First anagram. echo ${Anagrams[1]} # Second anagram. # Etc. # echo "${Anagrams[*]}" # To list all the anagrams in a single line . . . # Look ahead to the "Arrays" chapter for enlightenment on #+ what's going on here. # See also the agram.sh script for an example of anagram finding. exit $? Examples of command substitution in shell scripts: 1. Example 10-7 2. Example 10-26 3. Example 9-31 4. Example 15-3 5. Example 15-22 6. Example 15-17 7. Example 15-54 8. Example 10-13 9. Example 10-10 10. Example 15-32 11. Example 19-8 12. Example A-16 13. Example 27-3 14. Example 15-47 15. Example 15-48 16. Example 15-49 ------------------------------------------------------------------------ Chapter 12. Arithmetic Expansion Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let. Variations Arithmetic expansion with backticks (often used in conjunction with expr) z=`expr $z + 3` # The 'expr' command performs the expansion. Arithmetic expansion with double parentheses, and using let The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses -- ((...)) and $((...)) -- and also by the very convenient let construction. z=$(($z+3)) z=$((z+3)) # Also correct. # Within double parentheses, #+ parameter dereferencing #+ is optional. # $((EXPRESSION)) is arithmetic expansion. # Not to be confused with #+ command substitution. # You may also use operations within double parentheses without assignment. n=0 echo "n = $n" # n = 0 (( n += 1 )) # Increment. # (( $n += 1 )) is incorrect! echo "n = $n" # n = 1 let z=z+3 let "z += 3" # Quotes permit the use of spaces in variable assignment. # The 'let' operator actually performs arithmetic evaluation, #+ rather than expansion. Examples of arithmetic expansion in scripts: 1. Example 15-9 2. Example 10-14 3. Example 26-1 4. Example 26-11 5. Example A-16 ------------------------------------------------------------------------ Chapter 13. Recess Time This bizarre little intermission gives the reader a chance to relax and maybe laugh a bit. Fellow Linux user, greetings! You are reading something which will bring you luck and good fortune. Just e-mail a copy of this document to 10 of your friends. Before making the copies, send a 100-line Bash script to the first person on the list at the bottom of this letter. Then delete their name and add yours to the bottom of the list. Don't break the chain! Make the copies within 48 hours. Wilfred P. of Brooklyn failed to send out his ten copies and woke the next morning to find his job description changed to "COBOL programmer." Howard L. of Newport News sent out his ten copies and within a month had enough hardware to build a 100-node Beowulf cluster dedicated to playing Tuxracer. Amelia V. of Chicago laughed at this letter and broke the chain. Shortly thereafter, a fire broke out in her terminal and she now spends her days writing documentation for MS Windows. Don't break the chain! Send out your ten copies today! Courtesy 'NIX "fortune cookies", with some alterations and many apologies Part 4. Commands Mastering the commands on your Linux machine is an indispensable prelude to writing effective shell scripts. This section covers the following commands: * . (See also source) * ac * adduser * agetty * agrep * ar * arch * at * autoload * awk (See also Using awk for math operations) * badblocks * banner * basename * batch * bc * bg * bind * bison * builtin * bzgrep * bzip2 * cal * caller * cat * cd * chattr * chfn * chgrp * chkconfig * chmod * chown * chroot * cksum * clear * clock * cmp * col * colrm * column * comm * command * compgen * complete * compress * coproc * cp * cpio * cron * crypt * csplit * cu * cut * date * dc * dd * debugfs * declare * depmod * df * dialog * diff * diff3 * diffstat * dig * dirname * dirs * disown * dmesg * doexec * dos2unix * du * dump * dumpe2fs * e2fsck * echo * egrep * enable * enscript * env * eqn * eval * exec * exit (Related topic: exit status) * expand * export * expr * factor * false * fdformat * fdisk * fg * fgrep * file * find * finger * flex * flock * fmt * fold * free * fsck * ftp * fuser * getfacl * getopt * getopts * gettext * getty * gnome-mount * grep * groff * groupmod * groups (Related topic: the $GROUPS variable) * gs * gzip * halt * hash * hdparm * head * help * hexdump * host * hostid * hostname (Related topic: the $HOSTNAME variable) * hwclock * iconv * id (Related topic: the $UID variable) * ifconfig * info * infocmp * init * insmod * install * ip * ipcalc * iwconfig * jobs * join * jot * kill * killall * last * lastcomm * lastlog * ldd * less * let * lex * lid * ln * locate * lockfile * logger * logname * logout * logrotate * look * losetup * lp * ls * lsdev * lsmod * lsof * lspci * lsusb * ltrace * lynx * lzcat * lzma * m4 * mail * mailstats * mailto * make * MAKEDEV * man * mapfile * mcookie * md5sum * merge * mesg * mimencode * mkbootdisk * mkdir * mke2fs * mkfifo * mkisofs * mknod * mkswap * mktemp * mmencode * modinfo * modprobe * more * mount * msgfmt * mv * nc * netconfig * netstat * newgrp * nice * nl * nm * nmap * nohup * nslookup * objdump * od * openssl * passwd * paste * patch (Related topic: diff) * pathchk * pax * pgrep * pidof * ping * pkill * popd * pr * printenv * printf * procinfo * ps * pstree * ptx * pushd * pwd (Related topic: the $PWD variable) * quota * rcp * rdev * rdist * read * readelf * readlink * readonly * reboot * recode * renice * reset * resize * restore * rev * rlogin * rm * rmdir * rmmod * route * rpm * rpm2cpio * rsh * rsync * runlevel * run-parts * rx * rz * sar * scp * script * sdiff * sed * seq * service * set * setfacl * setquota * setserial * setterm * sha1sum * shar * shopt * shred * shutdown * size * skill * sleep * slocate * snice * sort * source * sox * split * sq * ssh * stat * strace * strings * strip * stty * su * sudo * sum * suspend * swapoff * swapon * sx * sync * sz * tac * tail * tar * tbl * tcpdump * tee * telinit * telnet * Tex * texexec * time * times * tmpwatch * top * touch * tput * tr * traceroute * true * tset * tsort * tty * tune2fs * type * typeset * ulimit * umask * umount * uname * unarc * unarj * uncompress * unexpand * uniq * units * unlzma * unrar * unset * unsq * unzip * uptime * usbmodules * useradd * userdel * usermod * users * usleep * uucp * uudecode * uuencode * uux * vacation * vdir * vmstat * vrfy * w * wait * wall * watch * wc * wget * whatis * whereis * which * who * whoami * whois * write * xargs * yacc * yes * zcat * zdiff * zdump * zegrep * zfgrep * zgrep * zip Table of Contents 14. Internal Commands and Builtins 14.1. Job Control Commands 15. External Filters, Programs and Commands 15.1. Basic Commands 15.2. Complex Commands 15.3. Time / Date Commands 15.4. Text Processing Commands 15.5. File and Archiving Commands 15.6. Communications Commands 15.7. Terminal Control Commands 15.8. Math Commands 15.9. Miscellaneous Commands 16. System and Administrative Commands 16.1. Analyzing a System Script ------------------------------------------------------------------------ Chapter 14. Internal Commands and Builtins A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off [51] a separate process -- or because a particular builtin needs direct access to the shell internals. +-----------------------------------------------------------------------------+ |When a command or the shell itself initiates (or spawns) a new subprocess to | |carry out a task, this is called forking. This new process is the child, and | |the process that forked it off is the parent. While the child process is | |doing its work, the parent process is still executing. | | | |Note that while a parent process gets the process ID of the child process, | |and can thus pass arguments to it, the reverse is not true. This can create | |problems that are subtle and hard to track down. | | | |Example 14-1. A script that spawns multiple instances of itself | | | |#!/bin/bash | |# spawn.sh | | | | | |PIDS=$(pidof sh $0) # Process IDs of the various instances of this script. | |P_array=( $PIDS ) # Put them in an array (why?). | |echo $PIDS # Show process IDs of parent and child processes. | |let "instances = ${#P_array[*]} - 1" # Count elements, less 1. | | # Why subtract 1? | |echo "$instances instance(s) of this script running." | |echo "[Hit Ctl-C to exit.]"; echo | | | | | |sleep 1 # Wait. | |sh $0 # Play it again, Sam. | | | |exit 0 # Not necessary; script will never get to here. | | # Why not? | | | |# After exiting with a Ctl-C, | |#+ do all the spawned instances of the script die? | |# If so, why? | | | |# Note: | |# ---- | |# Be careful not to run this script too long. | |# It will eventually eat up too many system resources. | | | |# Is having a script spawn multiple instances of itself | |#+ an advisable scripting technique. | |# Why or why not? | | | |Generally, a Bash builtin does not fork a subprocess when it executes within | |a script. An external system command or filter in a script usually will fork | |a subprocess. | +-----------------------------------------------------------------------------+ A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For example, the Bash echo command is not the same as /bin/echo, although their behavior is almost identical. #!/bin/bash echo "This line uses the \"echo\" builtin." /bin/echo "This line uses the /bin/echo system command." A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a subunit of a command construct. [52] I/O echo prints (to stdout) an expression or variable (see Example 4-1). echo Hello echo $a An echo requires the -e option to print escaped characters. See Example 5-2. Normally, each echo command prints a terminal newline, but the -n option suppresses this. Note An echo can be used to feed a sequence of commands down a pipe. if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] then echo "$VAR contains the substring sequence \"txt\"" fi Note An echo, in combination with command substitution can set a variable. a=`echo "HELLO" | tr A-Z a-z` See also Example 15-22, Example 15-3, Example 15-47, and Example 15-48. Be aware that echo `command` deletes any linefeeds that the output of command generates. The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to echo. Then echo outputs these arguments, separated by spaces. +------------------------------------------------------------------------------------+ |bash$ ls -l /usr/share/apps/kjezz/sounds | |-rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au | | -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au | | | | | | | | | |bash$ echo `ls -l /usr/share/apps/kjezz/sounds` | |total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root ...| | | +------------------------------------------------------------------------------------+ So, how can we embed a linefeed within an echoed character string? # Embedding a linefeed? echo "Why doesn't this string \n split on two lines?" # Doesn't split. # Let's try something else. echo echo $"A line of text containing a linefeed." # Prints as two distinct lines (embedded linefeed). # But, is the "$" variable prefix really necessary? echo echo "This string splits on two lines." # No, the "$" is not needed. echo echo "---------------" echo echo -n $"Another line of text containing a linefeed." # Prints as two distinct lines (embedded linefeed). # Even the -n option fails to suppress the linefeed here. echo echo echo "---------------" echo echo # However, the following doesn't work as expected. # Why not? Hint: Assignment to a variable. string1=$"Yet another line of text containing a linefeed (maybe)." echo $string1 # Yet another line of text containing a linefeed (maybe). # ^ # Linefeed becomes a space. # Thanks, Steve Parker, for pointing this out. Note This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar. +--------------------------------------------+ |bash$ type -a echo | |echo is a shell builtin | | echo is /bin/echo | | | +--------------------------------------------+ printf The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language printf() library function, and its syntax is somewhat different. printf format-string... parameter... This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the printf manpage (of the system command) for in-depth coverage. Caution Older versions of Bash may not support printf. Example 14-2. printf in action #!/bin/bash # printf demo declare -r PI=3.14159265358979 # Read-only variable, i.e., a constant. declare -r DecimalConstant=31373 Message1="Greetings," Message2="Earthling." echo printf "Pi to 2 decimal places = %1.2f" $PI echo printf "Pi to 9 decimal places = %1.9f" $PI # It even rounds off correctly. printf "\n" # Prints a line feed, # Equivalent to 'echo' . . . printf "Constant = \t%d\n" $DecimalConstant # Inserts tab (\t). printf "%s %s \n" $Message1 $Message2 echo # ==========================================# # Simulation of C function, sprintf(). # Loading a variable with a formatted string. echo Pi12=$(printf "%1.12f" $PI) echo "Pi to 12 decimal places = $Pi12" # Roundoff error! Msg=`printf "%s %s \n" $Message1 $Message2` echo $Msg; echo $Msg # As it happens, the 'sprintf' function can now be accessed #+ as a loadable module to Bash, #+ but this is not portable. exit 0 Formatting error messages is a useful application of printf E_BADDIR=85 var=nonexistent_directory error() { printf "$@" >&2 # Formats positional params passed, and sends them to stderr. echo exit $E_BADDIR } cd $var || error $"Can't cd to %s." "$var" # Thanks, S.C. See also Example 33-15. read "Reads" the value of a variable from stdin, that is, interactively fetches input from the keyboard. The -a option lets read get array variables (see Example 26-6). Example 14-3. Variable assignment, using read #!/bin/bash # "Reading" variables. echo -n "Enter the value of variable 'var1': " # The -n option to echo suppresses newline. read var1 # Note no '$' in front of var1, since it is being set. echo "var1 = $var1" echo # A single 'read' statement can set multiple variables. echo -n "Enter the values of variables 'var2' and 'var3' " echo =n "(separated by a space or tab): " read var2 var3 echo "var2 = $var2 var3 = $var3" # If you input only one value, #+ the other variable(s) will remain unset (null). exit 0 A read without an associated variable assigns its input to the dedicated variable $REPLY. Example 14-4. What happens when read has no variable #!/bin/bash # read-novar.sh echo # -------------------------- # echo -n "Enter a value: " read var echo "\"var\" = "$var"" # Everything as expected here. # -------------------------- # echo # ------------------------------------------------------------------- # echo -n "Enter another value: " read # No variable supplied for 'read', therefore... #+ Input to 'read' assigned to default variable, $REPLY. var="$REPLY" echo "\"var\" = "$var"" # This is equivalent to the first code block. # ------------------------------------------------------------------- # echo echo "=========================" echo # This example is similar to the "reply.sh" script. # However, this one shows that $REPLY is available #+ even after a 'read' to a variable in the conventional way. # ================================================================= # # In some instances, you might wish to discard the first value read. # In such cases, simply ignore the $REPLY variable. { # Code block. read # Line 1, to be discarded. read line2 # Line 2, saved in variable. } <$0 echo "Line 2 of this script is:" echo "$line2" # # read-novar.sh echo # #!/bin/bash line discarded. # See also the soundcard-on.sh script. exit 0 Normally, inputting a \ suppresses a newline during input to a read. The -r option causes an inputted \ to be interpreted literally. Example 14-5. Multi-line input to read #!/bin/bash echo echo "Enter a string terminated by a \\, then press ." echo "Then, enter a second string (no \\ this time), and again press ." read var1 # The "\" suppresses the newline, when reading $var1. # first line \ # second line echo "var1 = $var1" # var1 = first line second line # For each line terminated by a "\" #+ you get a prompt on the next line to continue feeding characters into var1. echo; echo echo "Enter another string terminated by a \\ , then press ." read -r var2 # The -r option causes the "\" to be read literally. # first line \ echo "var2 = $var2" # var2 = first line \ # Data entry terminates with the first . echo exit 0 The read command has some interesting options that permit echoing a prompt and even reading keystrokes without hitting ENTER. # Read a keypress without hitting ENTER. read -s -n1 -p "Hit a key " keypress echo; echo "Keypress was "\"$keypress\""." # -s option means do not echo input. # -n N option means accept only N characters of input. # -p option means echo the following prompt before reading input. # Using these options is tricky, since they need to be in the correct order. The -n option to read also allows detection of the arrow keys and certain of the other unusual keys. Example 14-6. Detecting the arrow keys #!/bin/bash # arrow-detect.sh: Detects the arrow keys, and a few more. # Thank you, Sandro Magi, for showing me how. # -------------------------------------------- # Character codes generated by the keypresses. arrowup='\[A' arrowdown='\[B' arrowrt='\[C' arrowleft='\[D' insert='\[2' delete='\[3' # -------------------------------------------- SUCCESS=0 OTHER=65 echo -n "Press a key... " # May need to also press ENTER if a key not listed above pressed. read -n3 key # Read 3 characters. echo -n "$key" | grep "$arrowup" #Check if character code detected. if [ "$?" -eq $SUCCESS ] then echo "Up-arrow key pressed." exit $SUCCESS fi echo -n "$key" | grep "$arrowdown" if [ "$?" -eq $SUCCESS ] then echo "Down-arrow key pressed." exit $SUCCESS fi echo -n "$key" | grep "$arrowrt" if [ "$?" -eq $SUCCESS ] then echo "Right-arrow key pressed." exit $SUCCESS fi echo -n "$key" | grep "$arrowleft" if [ "$?" -eq $SUCCESS ] then echo "Left-arrow key pressed." exit $SUCCESS fi echo -n "$key" | grep "$insert" if [ "$?" -eq $SUCCESS ] then echo "\"Insert\" key pressed." exit $SUCCESS fi echo -n "$key" | grep "$delete" if [ "$?" -eq $SUCCESS ] then echo "\"Delete\" key pressed." exit $SUCCESS fi echo " Some other key pressed." exit $OTHER # ========================================= # # Mark Alexander came up with a simplified #+ version of the above script (Thank you!). # It eliminates the need for grep. #!/bin/bash uparrow=$'\x1b[A' downarrow=$'\x1b[B' leftarrow=$'\x1b[D' rightarrow=$'\x1b[C' read -s -n3 -p "Hit an arrow key: " x case "$x" in $uparrow) echo "You pressed up-arrow" ;; $downarrow) echo "You pressed down-arrow" ;; $leftarrow) echo "You pressed left-arrow" ;; $rightarrow) echo "You pressed right-arrow" ;; esac exit $? # ========================================= # # Antonio Macchi has a simpler alternative. #!/bin/bash while true do read -sn1 a test "$a" == `echo -en "\e"` || continue read -sn1 a test "$a" == "[" || continue read -sn1 a case "$a" in A) echo "up";; B) echo "down";; C) echo "right";; D) echo "left";; esac done # ========================================= # # Exercise: # -------- # 1) Add detection of the "Home," "End," "PgUp," and "PgDn" keys. Note The -n option to read will not detect the ENTER (newline) key. The -t option to read permits timed input (see Example 9-4 and Example A-41). The -u option takes the file descriptor of the target file. The read command may also "read" its variable value from a file redirected to stdin. If the file contains more than one line, only the first line is assigned to the variable. If read has more than one parameter, then each of these variables gets assigned a successive whitespace-delineated string. Caution! Example 14-7. Using read with file redirection #!/bin/bash read var1 . Example 14-21. Using getopts to read the options/arguments passed to a script #!/bin/bash # ex33.sh: Exercising getopts and OPTIND # Script modified 10/09/03 at the suggestion of Bill Gradwohl. # Here we observe how 'getopts' processes command-line arguments to script. # The arguments are parsed as "options" (flags) and associated arguments. # Try invoking this script with: # 'scriptname -mn' # 'scriptname -oq qOption' (qOption can be some arbitrary string.) # 'scriptname -qXXX -r' # # 'scriptname -qr' #+ - Unexpected result, takes "r" as the argument to option "q" # 'scriptname -q -r' #+ - Unexpected result, same as above # 'scriptname -mnop -mnop' - Unexpected result # (OPTIND is unreliable at stating where an option came from.) # # If an option expects an argument ("flag:"), then it will grab #+ whatever is next on the command-line. NO_ARGS=0 E_OPTERROR=85 if [ $# -eq "$NO_ARGS" ] # Script invoked with no command-line args? then echo "Usage: `basename $0` options (-mnopqrs)" exit $E_OPTERROR # Exit and explain usage. # Usage: scriptname -options # Note: dash (-) necessary fi while getopts ":mnopq:rs" Option do case $Option in m ) echo "Scenario #1: option -m- [OPTIND=${OPTIND}]";; n | o ) echo "Scenario #2: option -$Option- [OPTIND=${OPTIND}]";; p ) echo "Scenario #3: option -p- [OPTIND=${OPTIND}]";; q ) echo "Scenario #4: option -q-\ with argument \"$OPTARG\" [OPTIND=${OPTIND}]";; # Note that option 'q' must have an associated argument, #+ otherwise it falls through to the default. r | s ) echo "Scenario #5: option -$Option-";; * ) echo "Unimplemented option chosen.";; # Default. esac done shift $(($OPTIND - 1)) # Decrements the argument pointer so it points to next argument. # $1 now references the first non-option item supplied on the command-line #+ if one exists. exit $? # As Bill Gradwohl states, # "The getopts mechanism allows one to specify: scriptname -mnop -mnop #+ but there is no reliable way to differentiate what came #+ from where by using OPTIND." # There are, however, workarounds. Script Behavior source, . (dot command) This command, when invoked from the command-line, executes a script. Within a script, a source file-name loads the file file-name. Sourcing a file (dot-command) imports code into the script, appending to the script (same effect as the #include directive in a C program). The net result is the same as if the "sourced" lines of code were physically present in the body of the script. This is useful in situations when multiple scripts use a common data file or function library. Example 14-22. "Including" a data file #!/bin/bash . data-file # Load a data file. # Same effect as "source data-file", but more portable. # The file "data-file" must be present in current working directory, #+ since it is referred to by its 'basename'. # Now, reference some data from that file. echo "variable1 (from data-file) = $variable1" echo "variable3 (from data-file) = $variable3" let "sum = $variable2 + $variable4" echo "Sum of variable2 + variable4 (from data-file) = $sum" echo "message1 (from data-file) is \"$message1\"" # Note: escaped quotes print_message This is the message-print function in the data-file. exit 0 File data-file for Example 14-22, above. Must be present in same directory. # This is a data file loaded by a script. # Files of this type may contain variables, functions, etc. # It may be loaded with a 'source' or '.' command by a shell script. # Let's initialize some variables. variable1=22 variable2=474 variable3=5 variable4=97 message1="Hello, how are you?" message2="Enough for now. Goodbye." print_message () { # Echoes any message passed to it. if [ -z "$1" ] then return 1 # Error, if argument missing. fi echo until [ -z "$1" ] do # Step through arguments passed to function. echo -n "$1" # Echo args one at a time, suppressing line feeds. echo -n " " # Insert spaces between words. shift # Next one. done echo return 0 } If the sourced file is itself an executable script, then it will run, then return control to the script that called it. A sourced executable script may use a return for this purpose. Arguments may be (optionally) passed to the sourced file as positional parameters. source $filename $arg1 arg2 It is even possible for a script to source itself, though this does not seem to have any practical applications. Example 14-23. A (useless) script that sources itself #!/bin/bash # self-source.sh: a script sourcing itself "recursively." # From "Stupid Script Tricks," Volume II. MAXPASSCNT=100 # Maximum number of execution passes. echo -n "$pass_count " # At first execution pass, this just echoes two blank spaces, #+ since $pass_count still uninitialized. let "pass_count += 1" # Assumes the uninitialized variable $pass_count #+ can be incremented the first time around. # This works with Bash and pdksh, but #+ it relies on non-portable (and possibly dangerous) behavior. # Better would be to initialize $pass_count to 0 before incrementing. while [ "$pass_count" -le $MAXPASSCNT ] do . $0 # Script "sources" itself, rather than calling itself. # ./$0 (which would be true recursion) doesn't work here. Why? done # What occurs here is not actually recursion, #+ since the script effectively "expands" itself, i.e., #+ generates a new section of code #+ with each pass through the 'while' loop', # with each 'source' in line 20. # # Of course, the script interprets each newly 'sourced' "#!" line #+ as a comment, and not as the start of a new script. echo exit 0 # The net effect is counting from 1 to 100. # Very impressive. # Exercise: # -------- # Write a script that uses this trick to actually do something useful. exit Unconditionally terminates a script. [56] The exit command may optionally take an integer argument, which is returned to the shell as the exit status of the script. It is good practice to end all but the simplest scripts with an exit 0, indicating a successful run. Note If a script terminates with an exit lacking an argument, the exit status of the script is the exit status of the last command executed in the script, not counting the exit. This is equivalent to an exit $?. Note An exit command may also be used to terminate a subshell. exec This shell builtin replaces the current process with a specified command. Normally, when the shell encounters a command, it forks off a child process to actually execute the command. Using the exec builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script, therefore, it forces an exit from the script when the exec'ed command terminates. [57] Example 14-24. Effects of exec #!/bin/bash exec echo "Exiting \"$0\"." # Exit from script here. # ---------------------------------- # The following lines never execute. echo "This echo will never echo." exit 99 # This script will not exit here. # Check exit value after script terminates #+ with an 'echo $?'. # It will *not* be 99. Example 14-25. A script that exec's itself #!/bin/bash # self-exec.sh # Note: Set permissions on this script to 555 or 755, # then call it with ./self-exec.sh or sh ./self-exec.sh. echo echo "This line appears ONCE in the script, yet it keeps echoing." echo "The PID of this instance of the script is still $$." # Demonstrates that a subshell is not forked off. echo "==================== Hit Ctl-C to exit ====================" sleep 1 exec $0 # Spawns another instance of this same script #+ that replaces the previous one. echo "This line will never echo!" # Why not? exit 99 # Will not exit here! # Exit code will not be 99! An exec also serves to reassign file descriptors. For example, exec $IMAGE_DIRECTORY/$CONTENTSFILE # The "l" option gives a "long" file listing. # The "R" option makes the listing recursive. # The "F" option marks the file types (directories get a trailing /). echo "Creating table of contents." # Create an image file preparatory to burning it onto the CDR. mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY echo "Creating ISO9660 file system image ($IMAGEFILE)." # Burn the CDR. echo "Burning the disk." echo "Please be patient, this will take a while." wodim -v -isosize dev=$DEVICE $IMAGEFILE # In newer Linux distros, the "wodim" utility assumes the #+ functionality of "cdrecord." exitcode=$? echo "Exit code = $exitcode" exit $exitcode cat, tac cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files. # Uses of 'cat' cat filename # Lists the file. cat file.1 file.2 file.3 > file.123 # Combines three files into one. The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line. See also Example 15-28 and Example 15-24. Note In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat the file. cat filename | tr a-z A-Z tr a-z A-Z < filename # Same effect, but starts one less process, #+ and also dispenses with the pipe. tac, is the inverse of cat, listing a file backwards from its end. rev reverses each line of a file, and outputs to stdout. This does not have the same effect as tac, as it preserves the order of the lines, but flips each one around (mirror image). +-------------------------------------------------------+ |bash$ cat file1.txt | |This is line 1. | | This is line 2. | | | | | |bash$ tac file1.txt | |This is line 2. | | This is line 1. | | | | | |bash$ rev file1.txt | |.1 enil si sihT | | .2 enil si sihT | | | +-------------------------------------------------------+ cp This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see Example 15-6). Tip Particularly useful are the -a archive flag (for copying an entire directory tree), the -u update flag (which prevents overwriting identically-named newer files), and the -r and -R recursive flags. cp -u source_dir/* dest_dir # "Synchronize" dest_dir to source_dir #+ by copying over all newer and not previously existing files. mv This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see Example 9-20 and Example A-2. Note When used in a non-interactive script, mv takes the -f (force) option to bypass user input. When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory. +--------------------------------------------------------------------+ |bash$ mv source_directory target_directory | | | |bash$ ls -lF target_directory | |total 1 | | drwxrwxr-x 2 bozo bozo 1024 May 28 19:20 source_directory/| | | +--------------------------------------------------------------------+ rm Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for bypassing user input in a script. Note The rm command will, by itself, fail to remove filenames beginning with a dash. Why? Because rm sees a dash-prefixed filename as an option. +--------------------------------------------+ |bash$ rm -badname | |rm: invalid option -- b | | Try `rm --help' for more information. | +--------------------------------------------+ One clever workaround is to precede the filename with a " -- " (the end-of-options flag). +--------------------------------------------+ |bash$ rm -- -badname | +--------------------------------------------+ Another method to is to preface the filename to be removed with a dot-slash . +--------------------------------------------+ |bash$ rm ./-badname | +--------------------------------------------+ Warning When used with the recursive flag -r, this command removes files all the way down the directory tree from the current directory. A careless rm -rf * can wipe out a big chunk of a directory structure. rmdir Remove directory. The directory must be empty of all files -- including "invisible" dotfiles [62] -- for this command to succeed. mkdir Make directory, creates a new directory. For example, mkdir -p project/programs/December creates the named directory. The -p option automatically creates any necessary parent directories. chmod Changes the attributes of an existing file or directory (see Example 14-14). chmod +x filename # Makes "filename" executable for all users. chmod u+s filename # Sets "suid" bit on "filename" permissions. # An ordinary user may execute "filename" with same privileges as the file's owner. # (This does not apply to shell scripts.) chmod 644 filename # Makes "filename" readable/writable to owner, readable to others # (octal mode). chmod 444 filename # Makes "filename" read-only for all. # Modifying the file (for example, with a text editor) #+ not allowed for a user who does not own the file (except for root), #+ and even the file owner must force a file-save #+ if she modifies the file. # Same restrictions apply for deleting the file. chmod 1777 directory-name # Gives everyone read, write, and execute permission in directory, #+ however also sets the "sticky bit". # This means that only the owner of the directory, #+ owner of the file, and, of course, root #+ can delete any particular file in that directory. chmod 111 directory-name # Gives everyone execute-only permission in a directory. # This means that you can execute and READ the files in that directory #+ (execute permission necessarily includes read permission #+ because you can't execute a file without being able to read it). # But you can't list the files or search for them with the "find" command. # These restrictions do not apply to root. chmod 000 directory-name # No permissions at all for that directory. # Can't read, write, or execute files in it. # Can't even list files in it or "cd" to it. # But, you can rename (mv) the directory #+ or delete it (rmdir) if it is empty. # You can even symlink to files in the directory, #+ but you can't read, write, or execute the symlinks. # These restrictions do not apply to root. chattr Change file attributes. This is analogous to chmod above, but with different options and a different invocation syntax, and it works only on ext2/ext3 filesystems. One particularly interesting chattr option is i. A chattr +i filename marks the file as immutable. The file cannot be modified, linked to, or deleted, not even by root. This file attribute can be set or removed only by root. In a similar fashion, the a option marks the file as append only. +-------------------------------------------------------+ |root# chattr +i file1.txt | | | | | |root# rm file1.txt | | | |rm: remove write-protected regular file `file1.txt'? y | | rm: cannot remove `file1.txt': Operation not permitted| | | +-------------------------------------------------------+ If a file has the s (secure) attribute set, then when it is deleted its block is overwritten with binary zeroes. [63] If a file has the u (undelete) attribute set, then when it is deleted, its contents can still be retrieved (undeleted). If a file has the c (compress) attribute set, then it will automatically be compressed on writes to disk, and uncompressed on reads. Note The file attributes set with chattr do not show in a file listing (ls -l). ln Creates links to pre-existings files. A "link" is a reference to a file, an alternate name for it. The ln command permits referencing the linked file by more than one name and is a superior alternative to aliasing (see Example 4-6). The ln creates only a reference, a pointer to the file only a few bytes in size. The ln command is most often used with the -s, symbolic or "soft" link flag. Advantages of using the -s flag are that it permits linking across file systems or to directories. The syntax of the command is a bit tricky. For example: ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile. Caution If a file named newfile has previously existed, an error message will result. +--------------------------------------------------------------+ | Which type of link to use? | | | | As John Macdonald explains it: | | | | Both of these [types of links] provide a certain measure of | | dual reference -- if you edit the contents of the file using | | any name, your changes will affect both the original name | | and either a hard or soft new name. The differences between | | them occurs when you work at a higher level. The advantage | | of a hard link is that the new name is totally independent | | of the old name -- if you remove or rename the old name, | | that does not affect the hard link, which continues to point | | to the data while it would leave a soft link hanging | | pointing to the old name which is no longer there. The | | advantage of a soft link is that it can refer to a different | | file system (since it is just a reference to a file name, | | not to actual data). And, unlike a hard link, a symbolic | | link can refer to a directory. | +--------------------------------------------------------------+ Links give the ability to invoke a script (or any other type of executable) with multiple names, and having that script behave according to how it was invoked. Example 15-2. Hello or Good-bye #!/bin/bash # hello.sh: Saying "hello" or "goodbye" #+ depending on how script is invoked. # Make a link in current working directory ($PWD) to this script: # ln -s hello.sh goodbye # Now, try invoking this script both ways: # ./hello.sh # ./goodbye HELLO_CALL=65 GOODBYE_CALL=66 if [ $0 = "./goodbye" ] then echo "Good-bye!" # Some other goodbye-type commands, as appropriate. exit $GOODBYE_CALL fi echo "Hello!" # Some other hello-type commands, as appropriate. exit $HELLO_CALL man, info These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain more detailed descriptions than do the man pages. There have been various attempts at "automating" the writing of man pages. For a script that makes a tentative first step in that direction, see Example A-39. ------------------------------------------------------------------------ 15.2. Complex Commands Commands for more advanced users find -exec COMMAND \; Carries out COMMAND on each file that find matches. The command sequence terminates with ; (the ";" is escaped to make certain the shell passes it to find literally, without interpreting it as a special character). +-------------------------------------------------------+ |bash$ find ~/ -name '*.txt' | |/home/bozo/.kde/share/apps/karm/karmdata.txt | | /home/bozo/misc/irmeyc.txt | | /home/bozo/test-scripts/1.txt | | | +-------------------------------------------------------+ If COMMAND contains {}, then find substitutes the full path name of the selected file for "{}". find ~/ -name 'core*' -exec rm {} \; # Removes all core dump files from user's home directory. find /home/bozo/projects -mtime -1 # ^ Note minus sign! # Lists all files in /home/bozo/projects directory tree #+ that were modified within the last day (current_day - 1). # find /home/bozo/projects -mtime 1 # Same as above, but modified *exactly* one day ago. # # mtime = last modification time of the target file # ctime = last status change time (via 'chmod' or otherwise) # atime = last access time DIR=/home/bozo/junk_files find "$DIR" -type f -atime +5 -exec rm {} \; # ^ ^^ # Curly brackets are placeholder for the path name output by "find." # # Deletes all files in "/home/bozo/junk_files" #+ that have not been accessed in *at least* 5 days (plus sign ... +5). # # "-type filetype", where # f = regular file # d = directory # l = symbolic link, etc. # # (The 'find' manpage and info page have complete option listings.) find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \; # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. # There a few extraneous hits. Can they be filtered out? # Possibly by: find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \ | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' # # [:digit:] is one of the character classes #+ introduced with the POSIX 1003.2 standard. # Thanks, Stéphane Chazelas. Note The -exec option to find should not be confused with the exec shell builtin. Example 15-3. Badname, eliminate file names in current directory containing bad characters and whitespace. #!/bin/bash # badname.sh # Delete filenames in current directory containing bad characters. for filename in * do badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p` # badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'` also works. # Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $ # rm $badname 2>/dev/null # ^^^^^^^^^^^ Error messages deep-sixed. done # Now, take care of files containing all manner of whitespace. find . -name "* *" -exec rm -f {} \; # The path name of the file that _find_ finds replaces the "{}". # The '\' ensures that the ';' is interpreted literally, as end of command. exit 0 #--------------------------------------------------------------------- # Commands below this line will not execute because of _exit_ command. # An alternative to the above script: find . -name '*[+{;"\\=?~()<>&*|$ ]*' -maxdepth 0 \ -exec rm -f '{}' \; # The "-maxdepth 0" option ensures that _find_ will not search #+ subdirectories below $PWD. # (Thanks, S.C.) Example 15-4. Deleting a file by its inode number #!/bin/bash # idelete.sh: Deleting a file by its inode number. # This is useful when a filename starts with an illegal character, #+ such as ? or -. ARGCOUNT=1 # Filename arg must be passed to script. E_WRONGARGS=70 E_FILE_NOT_EXIST=71 E_CHANGED_MIND=72 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` filename" exit $E_WRONGARGS fi if [ ! -e "$1" ] then echo "File \""$1"\" does not exist." exit $E_FILE_NOT_EXIST fi inum=`ls -i | grep "$1" | awk '{print $1}'` # inum = inode (index node) number of file # ----------------------------------------------------------------------- # Every file has an inode, a record that holds its physical address info. # ----------------------------------------------------------------------- echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? " # The '-v' option to 'rm' also asks this. read answer case "$answer" in [nN]) echo "Changed your mind, huh?" exit $E_CHANGED_MIND ;; *) echo "Deleting file \"$1\".";; esac find . -inum $inum -exec rm {} \; # ^^ # Curly brackets are placeholder #+ for text output by "find." echo "File "\"$1"\" deleted!" exit 0 The find command also works without the -exec option. #!/bin/bash # Find suid root files. # A strange suid file might indicate a security hole, #+ or even a system intrusion. directory="/usr/sbin" # Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc. permissions="+4000" # suid root (dangerous!) for file in $( find "$directory" -perm "$permissions" ) do ls -ltF --author "$file" done See Example 15-30, Example 3-4, and Example 10-9 for scripts using find. Its manpage provides more detail on this complex and powerful command. xargs A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where command substitution fails with a too many arguments error, substituting xargs often works. [64] Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file. The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out. +-----------------------------------------------------------------------------------+ |bash$ ls -l | |total 0 | | -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 | | -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 | | | | | | | |bash$ ls -l | xargs | |total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan...| | | | | | | |bash$ find ~/mail -type f | xargs grep "Linux" | |./misc:User-Agent: slrn/0.9.8.1 (Linux) | | ./sent-mail-jul-2005: hosted by the Linux Documentation Project. | | ./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version) | | ./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article | | ./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem | | . . . | | | +-----------------------------------------------------------------------------------+ ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation. Note Note that xargs processes the arguments passed to it sequentially, one at a time. +--------------------------------------------------------------------+ |bash$ find /usr/bin | xargs file | |/usr/bin: directory | | /usr/bin/foomatic-ppd-options: perl script text executable| | . . . | | | +--------------------------------------------------------------------+ Tip An interesting xargs option is -n NN, which limits to NN the number of arguments passed. ls | xargs -n 8 echo lists the files in the current directory in 8 columns. Tip Another useful option is -0, in combination with find -print0 or grep -lZ. This allows handling arguments containing whitespace or quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f grep -rliwZ GUI / | xargs -0 rm -f Either of the above will remove any file containing "GUI". (Thanks, S.C.) Or: cat /proc/"$pid"/"$OPTION" | xargs -0 echo # Formats output: ^^^^^^^^^^^^^^^ # From Han Holl's fixup of "get-commandline.sh" #+ script in "/dev and /proc" chapter. Example 15-5. Logfile: Using xargs to monitor system log #!/bin/bash # Generates a log file in current directory # from the tail end of /var/log/messages. # Note: /var/log/messages must be world readable # if this script invoked by an ordinary user. # #root chmod 644 /var/log/messages LINES=5 ( date; uname -a ) >>logfile # Time and machine name echo ---------------------------------------------------------- >>logfile tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile echo >>logfile echo >>logfile exit 0 # Note: # ---- # As Frank Wang points out, #+ unmatched quotes (either single or double quotes) in the source file #+ may give xargs indigestion. # # He suggests the following substitution for line 15: # tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile # Exercise: # -------- # Modify this script to track changes in /var/log/messages at intervals #+ of 20 minutes. # Hint: Use the "watch" command. As in find, a curly bracket pair serves as a placeholder for replacement text. Example 15-6. Copying files in current directory to another #!/bin/bash # copydir.sh # Copy (verbose) all files in current directory ($PWD) #+ to directory specified on command-line. E_NOARGS=85 if [ -z "$1" ] # Exit if no argument given. then echo "Usage: `basename $0` directory-to-copy-to" exit $E_NOARGS fi ls . | xargs -i -t cp ./{} $1 # ^^ ^^ ^^ # -t is "verbose" (output command-line to stderr) option. # -i is "replace strings" option. # {} is a placeholder for output text. # This is similar to the use of a curly-bracket pair in "find." # # List the files in current directory (ls .), #+ pass the output of "ls" as arguments to "xargs" (-i -t options), #+ then copy (cp) these arguments ({}) to new directory ($1). # # The net result is the exact equivalent of #+ cp * $1 #+ unless any of the filenames has embedded "whitespace" characters. exit 0 Example 15-7. Killing processes by name #!/bin/bash # kill-byname.sh: Killing processes by name. # Compare this script with kill-process.sh. # For instance, #+ try "./kill-byname.sh xterm" -- #+ and watch all the xterms on your desktop disappear. # Warning: # ------- # This is a fairly dangerous script. # Running it carelessly (especially as root) #+ can cause data loss and other undesirable effects. E_BADARGS=66 if test -z "$1" # No command-line arg supplied? then echo "Usage: `basename $0` Process(es)_to_kill" exit $E_BADARGS fi PROCESS_NAME="$1" ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/null # ^^ ^^ # --------------------------------------------------------------- # Notes: # -i is the "replace strings" option to xargs. # The curly brackets are the placeholder for the replacement. # 2&>/dev/null suppresses unwanted error messages. # # Can grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"? # --------------------------------------------------------------- exit $? # The "killall" command has the same effect as this script, #+ but using it is not quite as educational. Example 15-8. Word frequency analysis using xargs #!/bin/bash # wf2.sh: Crude word frequency analysis on a text file. # Uses 'xargs' to decompose lines of text into single words. # Compare this example to the "wf.sh" script later on. # Check for input file on command-line. ARGS=1 E_BADARGS=85 E_NOFILE=86 if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script? then echo "Usage: `basename $0` filename" exit $E_BADARGS fi if [ ! -f "$1" ] # Check if file exists. then echo "File \"$1\" does not exist." exit $E_NOFILE fi ##################################################### cat "$1" | xargs -n1 | \ # List the file, one word per line. tr A-Z a-z | \ # Shift characters to lowercase. sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\ /g' | \ # Filter out periods and commas, and #+ change space between words to linefeed, sort | uniq -c | sort -nr # Finally remove duplicates, prefix occurrence count #+ and sort numerically. ##################################################### # This does the same job as the "wf.sh" example, #+ but a bit more ponderously, and it runs more slowly (why?). exit $? expr All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical. expr 3 + 5 returns 8 expr 5 % 3 returns 2 expr 1 / 0 returns the error message, expr: division by zero Illegal arithmetic operations not allowed. expr 5 \* 3 returns 15 The multiplication operator must be escaped when used in an arithmetic expression with expr. y=`expr $y + 1` Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion. z=`expr substr $string $position $length` Extract substring of $length characters, starting at $position. Example 15-9. Using expr #!/bin/bash # Demonstrating some of the uses of 'expr' # ======================================= echo # Arithmetic Operators # ---------- --------- echo "Arithmetic Operators" echo a=`expr 5 + 3` echo "5 + 3 = $a" a=`expr $a + 1` echo echo "a + 1 = $a" echo "(incrementing a variable)" a=`expr 5 % 3` # modulo echo echo "5 mod 3 = $a" echo echo # Logical Operators # ------- --------- # Returns 1 if true, 0 if false, #+ opposite of normal Bash convention. echo "Logical Operators" echo x=24 y=25 b=`expr $x = $y` # Test equality. echo "b = $b" # 0 ( $x -ne $y ) echo a=3 b=`expr $a \> 10` echo 'b=`expr $a \> 10`, therefore...' echo "If a > 10, b = 0 (false)" echo "b = $b" # 0 ( 3 ! -gt 10 ) echo b=`expr $a \< 10` echo "If a < 10, b = 1 (true)" echo "b = $b" # 1 ( 3 -lt 10 ) echo # Note escaping of operators. b=`expr $a \<= 3` echo "If a <= 3, b = 1 (true)" echo "b = $b" # 1 ( 3 -le 3 ) # There is also a "\>=" operator (greater than or equal to). echo echo # String Operators # ------ --------- echo "String Operators" echo a=1234zipper43231 echo "The string being operated upon is \"$a\"." # length: length of string b=`expr length $a` echo "Length of \"$a\" is $b." # index: position of first character in substring # that matches a character in string b=`expr index $a 23` echo "Numerical position of first \"2\" in \"$a\" is \"$b\"." # substr: extract substring, starting position & length specified b=`expr substr $a 2 6` echo "Substring of \"$a\", starting at position 2,\ and 6 chars long is \"$b\"." # The default behavior of the 'match' operations is to #+ search for the specified match at the BEGINNING of the string. # # Using Regular Expressions ... b=`expr match "$a" '[0-9]*'` # Numerical count. echo Number of digits at the beginning of \"$a\" is $b. b=`expr match "$a" '\([0-9]*\)'` # Note that escaped parentheses # == == #+ trigger substring match. echo "The digits at the beginning of \"$a\" are \"$b\"." echo exit 0 Important The : (null) operator can substitute for match. For example, b=`expr $a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in the above listing. #!/bin/bash echo echo "String operations using \"expr \$string : \" construct" echo "===================================================" echo a=1234zipper5FLIPPER43231 echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"." # Escaped parentheses grouping operator. == == # *************************** #+ Escaped parentheses #+ match a substring # *************************** # If no escaped parentheses... #+ then 'expr' converts the string operand to an integer. echo "Length of \"$a\" is `expr "$a" : '.*'`." # Length of string echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`." # ------------------------------------------------------------------------- # echo echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`." # == == echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`." # ===== == == # Again, escaped parentheses force a substring match. # echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`." # ==== end of string operator ^^ # (actually means skip over one or more of any characters until specified #+ substring) echo exit 0 The above script illustrates how expr uses the escaped parentheses -- \( ... \) -- grouping operator in tandem with regular expression parsing to match a substring. Here is a another example, this time from "real life." # Strip the whitespace from the beginning and end. LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'` # From Peter Knowles' "booklistgen.sh" script #+ for converting files to Sony Librie/PRS-50X format. # (http://booklistgensh.peterknowles.com) Perl, sed, and awk have far superior string parsing facilities. A short sed or awk "subroutine" within a script (see Section 33.3) is an attractive alternative to expr. See Section 9.2 for more on using expr in string operations. ------------------------------------------------------------------------ 15.3. Time / Date Commands Time/date and timing date Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options. Example 15-10. Using date #!/bin/bash # Exercising the 'date' command echo "The number of days since the year's beginning is `date +%j`." # Needs a leading '+' to invoke formatting. # %j gives day of year. echo "The number of seconds elapsed since 01/01/1970 is `date +%s`." # %s yields number of seconds since "UNIX epoch" began, #+ but how is this useful? prefix=temp suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific. filename=$prefix.$suffix echo "Temporary filename = $filename" # It's great for creating "unique and random" temp filenames, #+ even better than using $$. # Read the 'date' man page for more formatting options. exit 0 The -u option gives the UTC (Universal Coordinated Time). +-------------------------------------------------------+ |bash$ date | |Fri Mar 29 21:07:39 MST 2002 | | | | | | | |bash$ date -u | |Sat Mar 30 04:07:42 UTC 2002 | | | +-------------------------------------------------------+ This option facilitates calculating the time between different dates. Example 15-11. Date calculations #!/bin/bash # date-calc.sh # Author: Nathan Coulter # Used in ABS Guide with permission (thanks!). MPHR=60 # Minutes per hour. HPD=24 # Hours per day. diff () { printf '%s' $(( $(date -u -d"$TARGET" +%s) - $(date -u -d"$CURRENT" +%s))) # %d = day of month. } CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z') TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z') # %F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone. printf '\nIn 2007, %s ' \ "$(date -d"$CURRENT + $(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')" # %B = name of month ^ halfway printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')" printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')" printf '\nOn %s at %s, there were\n' \ $(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T) DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD )) CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z') HOURS=$(( $(diff) / $MPHR / $MPHR )) CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z') MINUTES=$(( $(diff) / $MPHR )) CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z') printf '%s days, %s hours, ' "$DAYS" "$HOURS" printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)" printf 'until Christmas Dinner!\n\n' # Exercise: # -------- # Rewrite the diff () function to accept passed parameters, #+ rather than using global variables. The date command has quite a number of output options. For example %N gives the nanosecond portion of the current time. One interesting use for this is to generate random integers. date +%N | sed -e 's/000$//' -e 's/^0//' ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ # Strip off leading and trailing zeroes, if present. # Length of generated integer depends on #+ how many zeroes stripped off. # 115281032 # 63408725 # 394504284 There are many more options (try man date). date +%j # Echoes day of the year (days elapsed since January 1). date +%k%M # Echoes hour and minute in 24-hour format, as a single digit string. # The 'TZ' parameter permits overriding the default time zone. date # Mon Mar 28 21:42:16 MST 2005 TZ=EST date # Mon Mar 28 23:42:16 EST 2005 # Thanks, Frank Kannemann and Pete Sjoberg, for the tip. SixDaysAgo=$(date --date='6 days ago') OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month!) OneYearAgo=$(date --date='1 year ago') See also Example 3-4 and Example A-43. zdump Time zone dump: echoes the time in a specified time zone. +-------------------------------------------------------+ |bash$ zdump EST | |EST Tue Sep 18 22:09:22 2001 EST | | | +-------------------------------------------------------+ time Outputs verbose timing statistics for executing a command. time ls -l / gives something like this: +-------------------------------------------------------+ |real 0m0.067s | | user 0m0.004s | | sys 0m0.005s | +-------------------------------------------------------+ See also the very similar times command in the previous section. Note As of version 2.0 of Bash, time became a shell reserved word, with slightly altered behavior in a pipeline. touch Utility for updating access/modification times of a file to current system time or other specified time, but also useful for creating a new file. The command touch zzz will create a new file of zero length, named zzz, assuming that zzz did not previously exist. Time-stamping empty files in this way is useful for storing date information, for example in keeping track of modification times on a project. Note The touch command is equivalent to : >> newfile or >> newfile (for ordinary files). Tip Before doing a cp -u (copy/update), use touch to update the time stamp of files you don't wish overwritten. As an example, if the directory /home/bozo/tax_audit contains the files spreadsheet-051606.data, spreadsheet-051706.data, and spreadsheet-051806.data, then doing a touch spreadsheet*.data will protect these files from being overwritten by files with the same names during a cp -u /home/bozo/financial_info/spreadsheet*data /home/bozo/tax_audit. at The at job control command executes a given set of commands at a specified time. Superficially, it resembles cron, however, at is chiefly useful for one-time execution of a command set. at 2pm January 15 prompts for a set of commands to execute at that time. These commands should be shell-script compatible, since, for all practical purposes, the user is typing in an executable shell script a line at a time. Input terminates with a Ctl-D. Using either the -f option or input redirection (<), at reads a command list from a file. This file is an executable shell script, though it should, of course, be non-interactive. Particularly clever is including the run-parts command in the file to execute a different set of scripts. +-------------------------------------------------------+ |bash$ at 2:30 am Friday < at-jobs.list | |job 2 at 2000-10-27 02:30 | | | +-------------------------------------------------------+ batch The batch job control command is similar to at, but it runs a command list when the system load drops below .8. Like at, it can read commands from a file with the -f option. +--------------------------------------------------------------+ | The concept of batch processing dates back to the era of | | mainframe computers. It means running a set of commands | | without user intervention. | +--------------------------------------------------------------+ cal Prints a neatly formatted monthly calendar to stdout. Will do current year or a large range of past and future years. sleep This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing. It can be useful for timing or in processes running in the background, checking for a specific event every so often (polling), as in Example 29-6. sleep 3 # Pauses 3 seconds. Note The sleep command defaults to seconds, but minute, hours, or days may also be specified. sleep 3 h # Pauses 3 hours! Note The watch command may be a better choice than sleep for running commands at timed intervals. usleep Microsleep (the u may be read as the Greek mu, or micro- prefix). This is the same as sleep, above, but "sleeps" in microsecond intervals. It can be used for fine-grained timing, or for polling an ongoing process at very frequent intervals. usleep 30 # Pauses 30 microseconds. This command is part of the Red Hat initscripts / rc-scripts package. Caution The usleep command does not provide particularly accurate timing, and is therefore unsuitable for critical timing loops. hwclock, clock The hwclock command accesses or adjusts the machine's hardware clock. Some options require root privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from the hardware clock at bootup. The clock command is a synonym for hwclock. ------------------------------------------------------------------------ 15.4. Text Processing Commands Commands affecting text and text files sort File sort utility, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the -m option, it merges presorted input files. The info page lists its many capabilities and options. See Example 10-9, Example 10-10, and Example A-8. tsort Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns. The original purpose of tsort was to sort a list of dependencies for an obsolete version of the ld linker in an "ancient" version of UNIX. The results of a tsort will usually differ markedly from those of the standard sort command, above. uniq This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort. cat list-1 list-2 list-3 | sort | uniq > final.list # Concatenates the list files, # sorts them, # removes duplicate lines, # and finally writes the result to an output file. The useful -c option prefixes each line of the input file with its number of occurrences. +-------------------------------------------------------+ |bash$ cat testfile | |This line occurs only once. | | This line occurs twice. | | This line occurs twice. | | This line occurs three times. | | This line occurs three times. | | This line occurs three times. | | | | | |bash$ uniq -c testfile | | 1 This line occurs only once. | | 2 This line occurs twice. | | 3 This line occurs three times. | | | | | |bash$ sort testfile | uniq -c | sort -nr | | 3 This line occurs three times. | | 2 This line occurs twice. | | 1 This line occurs only once. | | | +-------------------------------------------------------+ The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined. Example 15-12. Word Frequency Analysis #!/bin/bash # wf.sh: Crude word frequency analysis on a text file. # This is a more efficient version of the "wf2.sh" script. # Check for input file on command-line. ARGS=1 E_BADARGS=85 E_NOFILE=86 if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script? then echo "Usage: `basename $0` filename" exit $E_BADARGS fi if [ ! -f "$1" ] # Check if file exists. then echo "File \"$1\" does not exist." exit $E_NOFILE fi ######################################################## # main () sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\ /g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr # ========================= # Frequency of occurrence # Filter out periods and commas, and #+ change space between words to linefeed, #+ then shift characters to lowercase, and #+ finally prefix occurrence count and sort numerically. # Arun Giridhar suggests modifying the above to: # . . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr # This adds a secondary sort key, so instances of #+ equal occurrence are sorted alphabetically. # As he explains it: # "This is effectively a radix sort, first on the #+ least significant column #+ (word or string, optionally case-insensitive) #+ and last on the most significant column (frequency)." # # As Frank Wang explains, the above is equivalent to #+ . . . | sort | uniq -c | sort +0 -nr #+ and the following also works: #+ . . . | sort | uniq -c | sort -k1nr -k ######################################################## exit 0 # Exercises: # --------- # 1) Add 'sed' commands to filter out other punctuation, #+ such as semicolons. # 2) Modify the script to also filter out multiple spaces and #+ other whitespace. +-------------------------------------------------------+ |bash$ cat testfile | |This line occurs only once. | | This line occurs twice. | | This line occurs twice. | | This line occurs three times. | | This line occurs three times. | | This line occurs three times. | | | | | |bash$ ./wf.sh testfile | | 6 this | | 6 occurs | | 6 line | | 3 times | | 3 three | | 2 twice | | 1 only | | 1 once | | | +-------------------------------------------------------+ expand, unexpand The expand filter converts tabs to spaces. It is often used in a pipe. The unexpand filter converts spaces to tabs. This reverses the effect of expand. cut A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter) and -f (field specifier) options. Using cut to obtain a listing of the mounted filesystems: cut -d ' ' -f1,2 /etc/mtab Using cut to list the OS and kernel version: uname -a | cut -d" " -f1,3,11,12 Using cut to extract message headers from an e-mail folder: +-------------------------------------------------------+ |bash$ grep '^Subject:' read-messages | cut -c10-80 | |Re: Linux suitable for mission-critical apps? | | MAKE MILLIONS WORKING AT HOME!!! | | Spam complaint | | Re: Spam complaint | +-------------------------------------------------------+ Using cut to parse a file: # List all the users in /etc/passwd. FILENAME=/etc/passwd for user in $(cut -d: -f1 $FILENAME) do echo $user done # Thanks, Oleg Philon for suggesting this. cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }' filename Note It is even possible to specify a linefeed as a delimiter. The trick is to actually embed a linefeed (RETURN) in the command sequence. +--------------------------------------------+ |bash$ cut -d' | | ' -f3,7,19 testfile | |This is line 3 of testfile. | | This is line 7 of testfile. | | This is line 19 of testfile. | | | +--------------------------------------------+ Thank you, Jaka Kranjc, for pointing this out. See also Example 15-48. paste Tool for merging together different files into a single, multi-column file. In combination with cut, useful for creating system log files. join Consider this a special-purpose cousin of paste. This powerful utility allows merging two files in a meaningful fashion, which essentially creates a simple version of a relational database. The join command operates on exactly two files, but pastes together only those lines with a common tagged field (usually a numerical label), and writes the result to stdout. The files to be joined should be sorted according to the tagged field for the matchups to work properly. File: 1.data 100 Shoes 200 Laces 300 Socks File: 2.data 100 $40.00 200 $1.00 300 $2.00 +-------------------------------------------------------+ |bash$ join 1.data 2.data | |File: 1.data 2.data | | | | 100 Shoes $40.00 | | 200 Laces $1.00 | | 300 Socks $2.00 | | | +-------------------------------------------------------+ Note The tagged field appears only once in the output. head lists the beginning of a file to stdout. The default is 10 lines, but a different number can be specified. The command has a number of interesting options. Example 15-13. Which files are scripts? #!/bin/bash # script-detector.sh: Detects scripts within a directory. TESTCHARS=2 # Test first 2 characters. SHABANG='#!' # Scripts begin with a "sha-bang." for file in * # Traverse all the files in current directory. do if [[ `head -c$TESTCHARS "$file"` = "$SHABANG" ]] # head -c2 #! # The '-c' option to "head" outputs a specified #+ number of characters, rather than lines (the default). then echo "File \"$file\" is a script." else echo "File \"$file\" is *not* a script." fi done exit 0 # Exercises: # --------- # 1) Modify this script to take as an optional argument #+ the directory to scan for scripts #+ (rather than just the current working directory). # # 2) As it stands, this script gives "false positives" for #+ Perl, awk, and other scripting language scripts. # Correct this. Example 15-14. Generating 10-digit random numbers #!/bin/bash # rnd.sh: Outputs a 10-digit random number # Script by Stephane Chazelas. head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p' # =================================================================== # # Analysis # -------- # head: # -c4 option takes first 4 bytes. # od: # -N4 option limits output to 4 bytes. # -tu4 option selects unsigned decimal format for output. # sed: # -n option, in combination with "p" flag to the "s" command, # outputs only matched lines. # The author of this script explains the action of 'sed', as follows. # head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p' # ----------------------------------> | # Assume output up to "sed" --------> | # is 0000000 1198195154\n # sed begins reading characters: 0000000 1198195154\n. # Here it finds a newline character, #+ so it is ready to process the first line (0000000 1198195154). # It looks at its s. The first and only one is # range action # 1 s/.* //p # The line number is in the range, so it executes the action: #+ tries to substitute the longest string ending with a space in the line # ("0000000 ") with nothing (//), and if it succeeds, prints the result # ("p" is a flag to the "s" command here, this is different #+ from the "p" command). # sed is now ready to continue reading its input. (Note that before #+ continuing, if -n option had not been passed, sed would have printed #+ the line once again). # Now, sed reads the remainder of the characters, and finds the #+ end of the file. # It is now ready to process its 2nd line (which is also numbered '$' as #+ it's the last one). # It sees it is not matched by any , so its job is done. # In few word this sed commmand means: # "On the first line only, remove any character up to the right-most space, #+ then print it." # A better way to do this would have been: # sed -e 's/.* //;q' # Here, two s (could have been written # sed -e 's/.* //' -e q): # range action # nothing (matches line) s/.* // # nothing (matches line) q (quit) # Here, sed only reads its first line of input. # It performs both actions, and prints the line (substituted) before #+ quitting (because of the "q" action) since the "-n" option is not passed. # =================================================================== # # An even simpler altenative to the above one-line script would be: # head -c4 /dev/urandom| od -An -tu4 exit See also Example 15-39. tail lists the (tail) end of a file to stdout. The default is 10 lines, but this can be changed with the -n option. Commonly used to keep track of changes to a system logfile, using the -f option, which outputs lines appended to the file. Example 15-15. Using tail to monitor the system log #!/bin/bash filename=sys.log cat /dev/null > $filename; echo "Creating / cleaning out file." # Creates file if it does not already exist, #+ and truncates it to zero length if it does. # : > filename and > filename also work. tail /var/log/messages > $filename # /var/log/messages must have world read permission for this to work. echo "$filename contains tail end of system log." exit 0 Tip To list a specific line of a text file, pipe the output of head to tail -n 1. For example head -n 8 database.txt | tail -n 1 lists the 8th line of the file database.txt. To set a variable to a given block of a text file: var=$(head -n $m $filename | tail -n $n) # filename = name of file # m = from beginning of file, number of lines to end of block # n = number of lines to set variable to (trim from end of block) Note Newer implementations of tail deprecate the older tail -$LINES filename usage. The standard tail -n $LINES filename is correct. See also Example 15-5, Example 15-39 and Example 29-6. grep A multi-purpose file search tool that uses Regular Expressions. It was originally a command/filter in the venerable ed line editor: g/re/p -- global - regular expression - print. grep pattern [file...] Search the target file(s) for occurrences of pattern, where pattern may be literal text or a Regular Expression. +---------------------------------------------------------------+ |bash$ grep '[rst]ystem.$' osinfo.txt | |The GPL governs the distribution of the Linux operating system.| | | +---------------------------------------------------------------+ If no target file(s) specified, grep works as a filter on stdout, as in a pipe. +-------------------------------------------------------+ |bash$ ps ax | grep clock | |765 tty1 S 0:00 xclock | | 901 pts/1 S 0:00 grep clock | | | +-------------------------------------------------------+ The -i option causes a case-insensitive search. The -w option matches only whole words. The -l option lists only the files in which matches were found, but not the matching lines. The -r (recursive) option searches files in the current working directory and all subdirectories below it. The -n option lists the matching lines, together with line numbers. +------------------------------------------------------------------+ |bash$ grep -n Linux osinfo.txt | |2:This is a file containing information about Linux. | | 6:The GPL governs the distribution of the Linux operating system.| | | +------------------------------------------------------------------+ The -v (or --invert-match) option filters out matches. grep pattern1 *.txt | grep -v pattern2 # Matches all lines in "*.txt" files containing "pattern1", # but ***not*** "pattern2". The -c (--count) option gives a numerical count of matches, rather than actually listing the matches. grep -c txt *.sgml # (number of occurrences of "txt" in "*.sgml" files) # grep -cz . # ^ dot # means count (-c) zero-separated (-z) items matching "." # that is, non-empty ones (containing at least 1 character). # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 3 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$' # 5 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5 # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$' # 9 # By default, newline chars (\n) separate items to match. # Note that the -z option is GNU "grep" specific. # Thanks, S.C. The --color (or --colour) option marks the matching string in color (on the console or in an xterm window). Since grep prints out each entire line containing the matching pattern, this lets you see exactly what is being matched. See also the -o option, which shows only the matching portion of the line(s). Example 15-16. Printing out the From lines in stored e-mail messages #!/bin/bash # from.sh # Emulates the useful "from" utility in Solaris, BSD, etc. # Echoes the "From" header line in all messages #+ in your e-mail directory. MAILDIR=~/mail/* # No quoting of variable. Why? GREP_OPTS="-H -A 5 --color" # Show file, plus extra context lines #+ and display "From" in color. TARGETSTR="^From" # "From" at beginning of line. for file in $MAILDIR # No quoting of variable. do grep $GREP_OPTS "$TARGETSTR" "$file" # ^^^^^^^^^^ # Again, do not quote this variable. echo done exit $? # Might wish to pipe the output of this script to 'more' or #+ redirect it to a file . . . When invoked with more than one target file given, grep specifies which file contains matches. +---------------------------------------------------------------------------+ |bash$ grep Linux osinfo.txt misc.txt | |osinfo.txt:This is a file containing information about Linux. | | osinfo.txt:The GPL governs the distribution of the Linux operating system.| | misc.txt:The Linux operating system is steadily gaining in popularity. | | | +---------------------------------------------------------------------------+ Tip To force grep to show the filename when searching only one target file, simply give /dev/null as the second file. +---------------------------------------------------------------------------+ |bash$ grep Linux osinfo.txt /dev/null | |osinfo.txt:This is a file containing information about Linux. | | osinfo.txt:The GPL governs the distribution of the Linux operating system.| | | +---------------------------------------------------------------------------+ If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the -q option to suppress output. SUCCESS=0 # if grep lookup succeeds word=Linux filename=data.file grep -q "$word" "$filename" # The "-q" option #+ causes nothing to echo to stdout. if [ $? -eq $SUCCESS ] # if grep -q "$word" "$filename" can replace lines 5 - 7. then echo "$word found in $filename" else echo "$word not found in $filename" fi Example 29-6 demonstrates how to use grep to search for a word pattern in a system logfile. Example 15-17. Emulating grep in a script #!/bin/bash # grp.sh: Rudimentary reimplementation of grep. E_BADARGS=85 if [ -z "$1" ] # Check for argument to script. then echo "Usage: `basename $0` pattern" exit $E_BADARGS fi echo for file in * # Traverse all files in $PWD. do output=$(sed -n /"$1"/p $file) # Command substitution. if [ ! -z "$output" ] # What happens if "$output" is not quoted? then echo -n "$file: " echo "$output" fi # sed -ne "/$1/s|^|${file}: |p" is equivalent to above. echo done echo exit 0 # Exercises: # --------- # 1) Add newlines to output, if more than one match in any given file. # 2) Add features. How can grep search for two (or more) separate patterns? What if you want grep to display all lines in a file or files that contain both "pattern1" and "pattern2"? One method is to pipe the result of grep pattern1 to grep pattern2. For example, given the following file: # Filename: tstfile This is a sample file. This is an ordinary text file. This file does not contain any unusual text. This file is not unusual. Here is some text. Now, let's search this file for lines containing both "file" and "text" . . . +-------------------------------------------------------+ |bash$ grep file tstfile | |# Filename: tstfile | | This is a sample file. | | This is an ordinary text file. | | This file does not contain any unusual text. | | This file is not unusual. | | | |bash$ grep file tstfile | grep text | |This is an ordinary text file. | | This file does not contain any unusual text. | +-------------------------------------------------------+ Now, for an interesting recreational use of grep . . . Example 15-18. Crossword puzzle solver #!/bin/bash # cw-solver.sh # This is actually a wrapper around a one-liner (line 46). # Crossword puzzle and anagramming word game solver. # You know *some* of the letters in the word you're looking for, #+ so you need a list of all valid words #+ with the known letters in given positions. # For example: w...i....n # 1???5????10 # w in position 1, 3 unknowns, i in the 5th, 4 unknowns, n at the end. # (See comments at end of script.) E_NOPATT=71 DICT=/usr/share/dict/word.lst # ^^^^^^^^ Looks for word list here. # ASCII word list, one word per line. # If you happen to need an appropriate list, #+ download the author's "yawl" word list package. # http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz # or # http://bash.neuralshortcircuit.com/yawl-0.3.2.tar.gz if [ -z "$1" ] # If no word pattern specified then #+ as a command-line argument . . . echo #+ . . . then . . . echo "Usage:" #+ Usage message. echo echo ""$0" \"pattern,\"" echo "where \"pattern\" is in the form" echo "xxx..x.x..." echo echo "The x's represent known letters," echo "and the periods are unknown letters (blanks)." echo "Letters and periods can be in any position." echo "For example, try: sh cw-solver.sh w...i....n" echo exit $E_NOPATT fi echo # =============================================== # This is where all the work gets done. grep ^"$1"$ "$DICT" # Yes, only one line! # | | # ^ is start-of-word regex anchor. # $ is end-of-word regex anchor. # From _Stupid Grep Tricks_, vol. 1, #+ a book the ABS Guide author may yet get around #+ to writing . . . one of these days . . . # =============================================== echo exit $? # Script terminates here. # If there are too many words generated, #+ redirect the output to a file. $ sh cw-solver.sh w...i....n wellington workingman workingmen egrep -- extended grep -- is the same as grep -E. This uses a somewhat different, extended set of Regular Expressions, which can make the search a bit more flexible. It also allows the boolean | (or) operator. +-------------------------------------------------------+ |bash $ egrep 'matches|Matches' file.txt | |Line 1 matches. | | Line 3 Matches. | | Line 4 contains matches, but also Matches | | | +-------------------------------------------------------+ fgrep -- fast grep -- is the same as grep -F. It does a literal string search (no Regular Expressions), which generally speeds things up a bit. Note On some Linux distros, egrep and fgrep are symbolic links to, or aliases for grep, but invoked with the -E and -F options, respectively. Example 15-19. Looking up definitions in Webster's 1913 Dictionary #!/bin/bash # dict-lookup.sh # This script looks up definitions in the 1913 Webster's Dictionary. # This Public Domain dictionary is available for download #+ from various sites, including #+ Project Gutenberg (http://www.gutenberg.org/etext/247). # # Convert it from DOS to UNIX format (with only LF at end of line) #+ before using it with this script. # Store the file in plain, uncompressed ASCII text. # Set DEFAULT_DICTFILE variable below to path/filename. E_BADARGS=85 MAXCONTEXTLINES=50 # Maximum number of lines to show. DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt" # Default dictionary file pathname. # Change this as necessary. # Note: # ---- # This particular edition of the 1913 Webster's #+ begins each entry with an uppercase letter #+ (lowercase for the remaining characters). # Only the *very first line* of an entry begins this way, #+ and that's why the search algorithm below works. if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]] # Must at least specify word to look up, and #+ it must start with an uppercase letter. then echo "Usage: `basename $0` Word-to-define [dictionary-file]" echo echo "Note: Word to look up must start with capital letter," echo "with the rest of the word in lowercase." echo "--------------------------------------------" echo "Examples: Abandon, Dictionary, Marking, etc." exit $E_BADARGS fi if [ -z "$2" ] # May specify different dictionary #+ as an argument to this script. then dictfile=$DEFAULT_DICTFILE else dictfile="$2" fi # --------------------------------------------------------- Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \\" "$dictfile") # Definitions in form "Word \..." # # And, yes, "fgrep" is fast enough #+ to search even a very large text file. # Now, snip out just the definition block. echo "$Definition" | sed -n '1,/^[A-Z]/p' | # Print from first line of output #+ to the first line of the next entry. sed '$d' | sed '$d' # Delete last two lines of output #+ (blank line and first line of next entry). # --------------------------------------------------------- exit $? # Exercises: # --------- # 1) Modify the script to accept any type of alphabetic input # + (uppercase, lowercase, mixed case), and convert it # + to an acceptable format for processing. # # 2) Convert the script to a GUI application, # + using something like 'gdialog' or 'zenity' . . . # The script will then no longer take its argument(s) # + from the command-line. # # 3) Modify the script to parse one of the other available # + Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer. Note See also Example A-41 for an example of speedy fgrep lookup on a large text file. agrep (approximate grep) extends the capabilities of grep to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution. Tip To search compressed files, use zgrep, zegrep, or zfgrep. These also work on non-compressed files, though slower than plain grep, egrep, fgrep. They are handy for searching through a mixed set of files, some compressed, some not. To search bzipped files, use bzgrep. look The command look works like grep, but does a lookup on a "dictionary," a sorted word list. By default, look searches for a match in /usr/dict/words, but a different dictionary file may be specified. Example 15-20. Checking words in a list for validity #!/bin/bash # lookup: Does a dictionary lookup on each word in a data file. file=words.data # Data file from which to read words to test. echo while [ "$word" != end ] # Last word in data file. do # ^^^ read word # From data file, because of redirection at end of loop. look $word > /dev/null # Don't want to display lines in dictionary file. lookup=$? # Exit status of 'look' command. if [ "$lookup" -eq 0 ] then echo "\"$word\" is valid." else echo "\"$word\" is invalid." fi done <"$file" # Redirects stdin to $file, so "reads" come from there. echo exit 0 # ---------------------------------------------------------------- # Code below line will not execute because of "exit" command above. # Stephane Chazelas proposes the following, more concise alternative: while read word && [[ $word != end ]] do if look "$word" > /dev/null then echo "\"$word\" is valid." else echo "\"$word\" is invalid." fi done <"$file" exit 0 sed, awk Scripting languages especially suited for parsing text files and command output. May be embedded singly or in combination in pipes and shell scripts. sed Non-interactive "stream editor", permits using many ex commands in batch mode. It finds many uses in shell scripts. awk Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns) in structured text files. Its syntax is similar to C. wc wc gives a "word count" on a file or I/O stream: +-------------------------------------------------------+ |bash $ wc /usr/share/doc/sed-4.1.2/README | |13 70 447 README | |[13 lines 70 words 447 characters] | +-------------------------------------------------------+ wc -w gives only the word count. wc -l gives only the line count. wc -c gives only the byte count. wc -m gives only the character count. wc -L gives only the length of the longest line. Using wc to count how many .txt files are in current working directory: $ ls *.txt | wc -l # Will work as long as none of the "*.txt" files #+ have a linefeed embedded in their name. # Alternative ways of doing this are: # find . -maxdepth 1 -name \*.txt -print0 | grep -cz . # (shopt -s nullglob; set -- *.txt; echo $#) # Thanks, S.C. Using wc to total up the size of all the files whose names begin with letters in the range d - h +-------------------------------------------------------+ |bash$ wc [d-h]* | grep total | awk '{print $3}' | |71832 | | | +-------------------------------------------------------+ Using wc to count the instances of the word "Linux" in the main source file for this book. +-------------------------------------------------------+ |bash$ grep Linux abs-book.sgml | wc -l | |50 | | | +-------------------------------------------------------+ See also Example 15-39 and Example 19-8. Certain commands include some of the functionality of wc as options. ... | grep foo | wc -l # This frequently used construct can be more concisely rendered. ... | grep -c foo # Just use the "-c" (or "--count") option of grep. # Thanks, S.C. tr character translation filter. Caution Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from reinterpreting the special characters in tr command sequences. Brackets should be quoted to prevent expansion by the shell. Either tr "A-Z" "*" $NEWFILENAME # Delete CR's and write to new file. echo "Original DOS text file is \"$1\"." echo "Converted UNIX text file is \"$NEWFILENAME\"." exit 0 # Exercise: # -------- # Change the above script to convert from UNIX to DOS. Example 15-24. rot13: ultra-weak encryption. #!/bin/bash # rot13.sh: Classic rot13 algorithm, # encryption that might fool a 3-year old. # Usage: ./rot13.sh filename # or ./rot13.sh > "$BOOKLIST" } # From Peter Knowles' "booklistgen.sh" script #+ for converting files to Sony Librie/PRS-50X format. # (http://booklistgensh.peterknowles.com) recode Consider this a fancier version of iconv, above. This very versatile utility for converting a file to a different encoding scheme. Note that recode is not part of the standard Linux installation. TeX, gs TeX and Postscript are text markup languages used for preparing copy for printing or formatted video display. TeX is Donald Knuth's elaborate typsetting system. It is often convenient to write a shell script encapsulating all the options and arguments passed to one of these markup languages. Ghostscript (gs) is a GPL-ed Postscript interpreter. texexec Utility for processing TeX and pdf files. Found in /usr/bin on many Linux distros, it is actually a shell wrapper that calls Perl to invoke Tex. texexec --pdfarrange --result=Concatenated.pdf *pdf # Concatenates all the pdf files in the current working directory #+ into the merged file, Concatenated.pdf . . . # (The --pdfarrange option repaginates a pdf file. See also --pdfcombine.) # The above command-line could be parameterized and put into a shell script. enscript Utility for converting plain text file to PostScript For example, enscript filename.txt -p filename.ps produces the PostScript output file filename.ps. groff, tbl, eqn Yet another text markup and display formatting language is groff. This is the enhanced GNU version of the venerable UNIX roff/troff display and typesetting package. Manpages use groff. The tbl table processing utility is considered part of groff, as its function is to convert table markup into groff commands. The eqn equation processing utility is likewise part of groff, and its function is to convert equation markup into groff commands. Example 15-29. manview: Viewing formatted manpages #!/bin/bash # manview.sh: Formats the source of a man page for viewing. # This script is useful when writing man page source. # It lets you look at the intermediate results on the fly #+ while working on it. E_WRONGARGS=85 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_WRONGARGS fi # --------------------------- groff -Tascii -man $1 | less # From the man page for groff. # --------------------------- # If the man page includes tables and/or equations, #+ then the above code will barf. # The following line can handle such cases. # # gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man # # Thanks, S.C. exit $? # See also the "maned.sh" script. See also Example A-39. lex, yacc The lex lexical analyzer produces programs for pattern matching. This has been replaced by the nonproprietary flex on Linux systems. The yacc utility creates a parser based on a set of specifications. This has been replaced by the nonproprietary bison on Linux systems. ------------------------------------------------------------------------ 15.5. File and Archiving Commands Archiving tar The standard UNIX archiving utility. [66] Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even stdout (see Example 3-4). GNU tar has been patched to accept various compression filters, for example: tar czvf archive_name.tar.gz *, which recursively archives and gzips all files in a directory tree except dotfiles in the current working directory ($PWD). [67] Some useful tar options: 1. -c create (a new archive) 2. -x extract (files from existing archive) 3. --delete delete (files from existing archive) Caution This option will not work on magnetic tape devices. 4. -r append (files to existing archive) 5. -A append (tar files to existing archive) 6. -t list (contents of existing archive) 7. -u update archive 8. -d compare archive with specified filesystem 9. --after-date only process files with a date stamp after specified date 10. -z gzip the archive (compress or uncompress, depending on whether combined with the -c or -x) option 11. -j bzip2 the archive Caution It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups. shar Shell archiving utility. The text files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, containing all the necessary unarchiving commands, as well as the files themselves. Shar archives still show up in Usenet newsgroups, but otherwise shar has been replaced by tar/gzip. The unshar command unpacks shar archives. The mailshar command is a Bash script that uses shar to concatenate multiple files into a single one for e-mailing. This script supports compression and uuencoding. ar Creation and manipulation utility for archives, mainly used for binary object file libraries. rpm The Red Hat Package Manager, or rpm utility provides a wrapper for source or binary archives. It includes commands for installing and checking the integrity of packages, among other things. A simple rpm -i package_name.rpm usually suffices to install a package, though there are many more options available. Tip rpm -qf identifies which package a file originates from. +---------------------------------------------+ |bash$ rpm -qf /bin/ls | |coreutils-5.2.1-31 | | | +---------------------------------------------+ Tip rpm -qa gives a complete list of all installed rpm packages on a given system. An rpm -qa package_name lists only the package(s) corresponding to package_name. +---------------------------------------------+ |bash$ rpm -qa | |redhat-logos-1.1.3-1 | | glibc-2.2.4-13 | | cracklib-2.7-12 | | dosfstools-2.7-1 | | gdbm-1.8.0-10 | | ksymoops-2.4.1-1 | | mktemp-1.5-11 | | perl-5.6.0-17 | | reiserfs-utils-3.x.0j-2 | | ... | | | | | |bash$ rpm -qa docbook-utils | |docbook-utils-0.6.9-2 | | | | | |bash$ rpm -qa docbook | grep docbook | |docbook-dtd31-sgml-1.0-10 | | docbook-style-dsssl-1.64-3 | | docbook-dtd30-sgml-1.0-10 | | docbook-dtd40-sgml-1.0-11 | | docbook-utils-pdf-0.6.9-2 | | docbook-dtd41-sgml-1.0-10 | | docbook-utils-0.6.9-2 | | | +---------------------------------------------+ cpio This specialized archiving copy command (copy input and output) is rarely seen any more, having been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. With an appropriate block size (for copying) specified, it can be appreciably faster than tar. Example 15-30. Using cpio to move a directory tree #!/bin/bash # Copying a directory tree using cpio. # Advantages of using 'cpio': # Speed of copying. It's faster than 'tar' with pipes. # Well suited for copying special files (named pipes, etc.) #+ that 'cp' may choke on. ARGS=2 E_BADARGS=65 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` source destination" exit $E_BADARGS fi source="$1" destination="$2" ################################################################### find "$source" -depth | cpio -admvp "$destination" # ^^^^^ ^^^^^ # Read the 'find' and 'cpio' info pages to decipher these options. # The above works only relative to $PWD (current directory) . . . #+ full pathnames are specified. ################################################################### # Exercise: # -------- # Add code to check the exit status ($?) of the 'find | cpio' pipe #+ and output appropriate error messages if anything went wrong. exit $? rpm2cpio This command extracts a cpio archive from an rpm one. Example 15-31. Unpacking an rpm archive #!/bin/bash # de-rpm.sh: Unpack an 'rpm' archive : ${1?"Usage: `basename $0` target-file"} # Must specify 'rpm' archive name as an argument. TEMPFILE=$$.cpio # Tempfile with "unique" name. # $$ is process ID of script. rpm2cpio < $1 > $TEMPFILE # Converts rpm archive into #+ cpio archive. cpio --make-directories -F $TEMPFILE -i # Unpacks cpio archive. rm -f $TEMPFILE # Deletes cpio archive. exit 0 # Exercise: # Add check for whether 1) "target-file" exists and #+ 2) it is an rpm archive. # Hint: Parse output of 'file' command. pax The pax portable archive exchange toolkit facilitates periodic file backups and is designed to be cross-compatible between various flavors of UNIX. It was ported from BSD to Linux. pax -wf daily_backup.pax ~/linux-server/files # Creates a tar archive of all files in the target directory. # Note that the options to pax must be in the correct order -- #+ pax -fw has an entirely different effect. pax -f daily_backup.pax # Lists the files in the archive. pax -rf daily_backup.pax ~/bsd-server/files # Restores the backed-up files from the Linux machine #+ onto a BSD one. Note that pax handles many of the standard archiving and compression commands. Compression gzip The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The corresponding decompression command is gunzip, which is the equivalent of gzip -d. Note The -c option sends the output of gzip to stdout. This is useful when piping to other commands. The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This is, in effect, a cat command that works on compressed files (including files processed with the older compress utility). The zcat command is equivalent to gzip -dc. Caution On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will not work on gzipped files. See also Example 7-7. bzip2 An alternate compression utility, usually more efficient (but slower) than gzip, especially on large files. The corresponding decompression command is bunzip2. Similar to the zcat command, bzcat decompresses a bzipped2-ed file to stdout. Note Newer versions of tar have been patched with bzip2 support. compress, uncompress This is an older, proprietary compression utility found in commercial UNIX distributions. The more efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for compatibility, although gunzip can unarchive files treated with compress. Tip The znew command transforms compressed files into gzipped ones. sq Yet another compression (squeeze) utility, a filter that works only on sorted ASCII word lists. It uses the standard invocation syntax for a filter, sq < input-file > output-file. Fast, but not nearly as efficient as gzip. The corresponding uncompression filter is unsq, invoked like sq. Tip The output of sq may be piped to gzip for further compression. zip, unzip Cross-platform file archiving and compression utility compatible with DOS pkzip.exe. "Zipped" archives seem to be a more common medium of file exchange on the Internet than "tarballs." unarc, unarj, unrar These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and rar.exe programs. lzma, unlzma, lzcat Highly efficient Lempel-Ziv-Markov compression. The syntax of lzma is similar to that of gzip. The [http://www.7-zip.org/sdk.html] 7-zip Website has more information. File Information file A utility for identifying file types. The command file file-name will return a file specification for file-name, such as ascii text or data. It references the magic numbers found in /usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX distribution. The -f option causes file to run in batch mode, to read from a designated file a list of filenames to analyze. The -z option, when used on a compressed target file, forces an attempt to analyze the uncompressed file type. +-------------------------------------------------------------+ |bash$ file test.tar.gz | |test.tar.gz: gzip compressed data, deflated, | | last modified: Sun Sep 16 13:34:51 2001, os: Unix | | | |bash file -z test.tar.gz | |test.tar.gz: GNU tar archive (gzip compressed data, deflated,| | last modified: Sun Sep 16 13:34:51 2001, os: Unix) | | | +-------------------------------------------------------------+ # Find sh and Bash scripts in a given directory: DIRECTORY=/usr/local/bin KEYWORD=Bourne # Bourne and Bourne-Again shell scripts file $DIRECTORY/* | fgrep $KEYWORD # Output: # /usr/local/bin/burn-cd: Bourne-Again shell script text executable # /usr/local/bin/burnit: Bourne-Again shell script text executable # /usr/local/bin/cassette.sh: Bourne shell script text executable # /usr/local/bin/copy-cd: Bourne-Again shell script text executable # . . . Example 15-32. Stripping comments from C program files #!/bin/bash # strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program. E_NOARGS=0 E_ARGERROR=66 E_WRONG_FILE_TYPE=67 if [ $# -eq "$E_NOARGS" ] then echo "Usage: `basename $0` C-program-file" >&2 # Error message to stderr. exit $E_ARGERROR fi # Test for correct file type. type=`file $1 | awk '{ print $2, $3, $4, $5 }'` # "file $1" echoes file type . . . # Then awk removes the first field, the filename . . . # Then the result is fed into the variable "type." correct_type="ASCII C program text" if [ "$type" != "$correct_type" ] then echo echo "This script works on C program files only." echo exit $E_WRONG_FILE_TYPE fi # Rather cryptic sed script: #-------- sed ' /^\/\*/d /.*\*\//d ' $1 #-------- # Easy to understand if you take several hours to learn sed fundamentals. # Need to add one more line to the sed script to deal with #+ case where line of code has a comment following it on same line. # This is left as a non-trivial exercise. # Also, the above code deletes non-comment lines with a "*/" . . . #+ not a desirable result. exit 0 # ---------------------------------------------------------------- # Code below this line will not execute because of 'exit 0' above. # Stephane Chazelas suggests the following alternative: usage() { echo "Usage: `basename $0` C-program-file" >&2 exit 1 } WEIRD=`echo -n -e '\377'` # or WEIRD=$'\377' [[ $# -eq 1 ]] || usage case `file "$1"` in *"C program text"*) sed -e "s%/\*%${WEIRD}%g;s%\*/%${WEIRD}%g" "$1" \ | tr '\377\n' '\n\377' \ | sed -ne 'p;n' \ | tr -d '\n' | tr '\377' '\n';; *) usage;; esac # This is still fooled by things like: # printf("/*"); # or # /* /* buggy embedded comment */ # # To handle all special cases (comments in strings, comments in string #+ where there is a \", \\" ...), #+ the only way is to write a C parser (using lex or yacc perhaps?). exit 0 which which command gives the full path to "command." This is useful for finding out whether a particular command or utility is installed on the system. $bash which rm +-------------------------------------------------------+ |/usr/bin/rm | +-------------------------------------------------------+ For an interesting use of this command, see Example 33-14. whereis Similar to which, above, whereis command gives the full path to "command," but also to its manpage. $bash whereis rm +-------------------------------------------------------+ |rm: /bin/rm /usr/share/man/man1/rm.1.bz2 | +-------------------------------------------------------+ whatis whatis command looks up "command" in the whatis database. This is useful for identifying system commands and important configuration files. Consider it a simplified man command. $bash whatis whatis +-------------------------------------------------------------------------+ |whatis (1) - search the whatis database for complete words| +-------------------------------------------------------------------------+ Example 15-33. Exploring /usr/X11R6/bin #!/bin/bash # What are all those mysterious binaries in /usr/X11R6/bin? DIRECTORY="/usr/X11R6/bin" # Try also "/bin", "/usr/bin", "/usr/local/bin", etc. for file in $DIRECTORY/* do whatis `basename $file` # Echoes info about the binary. done exit 0 # You may wish to redirect output of this script, like so: # ./what.sh >>whatis.db # or view it a page at a time on stdout, # ./what.sh | less See also Example 10-3. vdir Show a detailed directory listing. The effect is similar to ls -lb. This is one of the GNU fileutils. +-------------------------------------------------------------------+ |bash$ vdir | |total 10 | | -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo | | -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak | | -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo| | | |bash ls -l | |total 10 | | -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo | | -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak | | -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo| | | +-------------------------------------------------------------------+ locate, slocate The locate command searches for files using a database stored for just that purpose. The slocate command is the secure version of locate (which may be aliased to slocate). $bash locate hickson +-------------------------------------------------------+ |/usr/lib/xephem/catalogs/hickson.edb | +-------------------------------------------------------+ getfacl, setfacl These commands retrieve or set the file access control list -- the owner, group, and file permissions. +-------------------------------------------------------+ |bash$ getfacl * | |# file: test1.txt | | # owner: bozo | | # group: bozgrp | | user::rw- | | group::rw- | | other::r-- | | | | # file: test2.txt | | # owner: bozo | | # group: bozgrp | | user::rw- | | group::rw- | | other::r-- | | | | | | | |bash$ setfacl -m u:bozo:rw yearly_budget.csv | |bash$ getfacl yearly_budget.csv | |# file: yearly_budget.csv | | # owner: accountant | | # group: budgetgrp | | user::rw- | | user:bozo:rw- | | user:accountant:rw- | | group::rw- | | mask::rw- | | other::r-- | | | +-------------------------------------------------------+ readlink Disclose the file that a symbolic link points to. +-------------------------------------------------------+ |bash$ readlink /usr/bin/awk | |../../bin/gawk | | | +-------------------------------------------------------+ strings Use the strings command to find printable strings in a binary or data file. It will list sequences of printable characters found in the target file. This might be handy for a quick 'n dirty examination of a core dump or for looking at an unknown graphic image file (strings image-file | more might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you would probably parse the output of strings with grep or sed. See Example 10-7 and Example 10-9. Example 15-34. An "improved" strings command #!/bin/bash # wstrings.sh: "word-strings" (enhanced "strings" command) # # This script filters the output of "strings" by checking it #+ against a standard word list file. # This effectively eliminates gibberish and noise, #+ and outputs only recognized words. # =========================================================== # Standard Check for Script Argument(s) ARGS=1 E_BADARGS=85 E_NOFILE=86 if [ $# -ne $ARGS ] then echo "Usage: `basename $0` filename" exit $E_BADARGS fi if [ ! -f "$1" ] # Check if file exists. then echo "File \"$1\" does not exist." exit $E_NOFILE fi # =========================================================== MINSTRLEN=3 # Minimum string length. WORDFILE=/usr/share/dict/linux.words # Dictionary file. # May specify a different word list file #+ of one-word-per-line format. # For example, the "yawl" word-list package, # http://bash.neuralshortcircuit.com/yawl-0.3.2.tar.gz wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \ tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '` # Translate output of 'strings' command with multiple passes of 'tr'. # "tr A-Z a-z" converts to lowercase. # "tr '[:space:]'" converts whitespace characters to Z's. # "tr -cs '[:alpha:]' Z" converts non-alphabetic characters to Z's, #+ and squeezes multiple consecutive Z's. # "tr -s '\173-\377' Z" converts all characters past 'z' to Z's #+ and squeezes multiple consecutive Z's, #+ which gets rid of all the weird characters that the previous #+ translation failed to deal with. # Finally, "tr Z ' '" converts all those Z's to whitespace, #+ which will be seen as word separators in the loop below. # **************************************************************** # Note the technique of feeding the output of 'tr' back to itself, #+ but with different arguments and/or options on each pass. # **************************************************************** for word in $wlist # Important: # $wlist must not be quoted here. # "$wlist" does not work. # Why not? do strlen=${#word} # String length. if [ "$strlen" -lt "$MINSTRLEN" ] # Skip over short strings. then continue fi grep -Fw $word "$WORDFILE" # Match whole words only. # ^^^ # "Fixed strings" and #+ "whole words" options. done exit $? Comparison diff, patch diff: flexible file comparison utility. It compares the target files line-by-line sequentially. In some applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and uniq before piping them to diff. diff file-1 file-2 outputs the lines in the files that differ, with carets showing which file each particular line belongs to. The --side-by-side option to diff outputs each compared file, line by line, in separate columns, with non-matching lines marked. The -c and -u options likewise make the output of the command easier to interpret. There are available various fancy frontends for diff, such as sdiff, wdiff, xdiff, and mgdiff. Tip The diff command returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use of diff in a test construct within a shell script (see below). A common use for diff is generating difference files to be used with patch The -e option outputs files suitable for ed or ex scripts. patch: flexible versioning utility. Given a difference file generated by diff, patch can upgrade a previous version of a package to a newer version. It is much more convenient to distribute a relatively small "diff" file than the entire body of a newly revised package. Kernel "patches" have become the preferred method of distributing the frequent releases of the Linux kernel. patch -p1 /dev/null # /dev/null buries the output of the "cmp" command. # cmp -s $1 $2 has same result ("-s" silent flag to "cmp") # Thank you Anders Gustavsson for pointing this out. # # Also works with 'diff', i.e., diff $1 $2 &> /dev/null if [ $? -eq 0 ] # Test exit status of "cmp" command. then echo "File \"$1\" is identical to file \"$2\"." else echo "File \"$1\" differs from file \"$2\"." fi exit 0 Tip Use zcmp on gzipped files. comm Versatile file comparison utility. The files must be sorted for this to be useful. comm -options first-file second-file comm file-1 file-2 outputs three columns: * column 1 = lines unique to file-1 * column 2 = lines unique to file-2 * column 3 = lines common to both. The options allow suppressing output of one or more columns. * -1 suppresses column 1 * -2 suppresses column 2 * -3 suppresses column 3 * -12 suppresses both columns 1 and 2, etc. This command is useful for comparing "dictionaries" or word lists -- sorted text files with one word per line. Utilities basename Strips the path information from a file name, printing only the file name. The construction basename $0 lets the script know its name, that is, the name it was invoked by. This can be used for "usage" messages if, for example a script is called with missing arguments: echo "Usage: `basename $0` arg1 arg2 ... argn" dirname Strips the basename from a filename, printing only the path information. Note basename and dirname can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see Example A-7). Example 15-36. basename and dirname #!/bin/bash a=/home/bozo/daily-journal.txt echo "Basename of /home/bozo/daily-journal.txt = `basename $a`" echo "Dirname of /home/bozo/daily-journal.txt = `dirname $a`" echo echo "My own home is `basename ~/`." # `basename ~` also works. echo "The home of my home is `dirname ~/`." # `dirname ~` also works. exit 0 split, csplit These are utilities for splitting a file into smaller chunks. Their usual use is for splitting up large files in order to back them up on floppies or preparatory to e-mailing or uploading them. The csplit command splits a file according to context, the split occuring where patterns are matched. Example 15-37. A script that copies itself in sections #!/bin/bash # splitcopy.sh # A script that splits itself into chunks, #+ then reassembles the chunks into an exact copy #+ of the original script. CHUNKSIZE=4 # Size of first chunk of split files. OUTPREFIX=xx # csplit prefixes, by default, #+ files with "xx" ... csplit "$0" "$CHUNKSIZE" # Some comment lines for padding . . . # Line 15 # Line 16 # Line 17 # Line 18 # Line 19 # Line 20 cat "$OUTPREFIX"* > "$0.copy" # Concatenate the chunks. rm "$OUTPREFIX"* # Get rid of the chunks. exit $? Encoding and Encryption sum, cksum, md5sum, sha1sum These are utilities for generating checksums. A checksum is a number [68] mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. For security applications, use the md5sum (message digest 5 checksum) command, or better yet, the newer sha1sum (Secure Hash Algorithm). [69] +-------------------------------------------------------+ |bash$ cksum /boot/vmlinuz | |1670054224 804083 /boot/vmlinuz | | | |bash$ echo -n "Top Secret" | cksum | |3391003827 10 | | | | | | | |bash$ md5sum /boot/vmlinuz | |0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz | | | |bash$ echo -n "Top Secret" | md5sum | |8babc97a6f62a4649716f4df8d61728f - | | | +-------------------------------------------------------+ Note The cksum command shows the size, in bytes, of its target, whether file or stdout. The md5sum and sha1sum commands display a dash when they receive their input from stdout. Example 15-38. Checking file integrity #!/bin/bash # file-integrity.sh: Checking whether files in a given directory # have been tampered with. E_DIR_NOMATCH=70 E_BAD_DBFILE=71 dbfile=File_record.md5 # Filename for storing records (database file). set_up_database () { echo ""$directory"" > "$dbfile" # Write directory name to first line of file. md5sum "$directory"/* >> "$dbfile" # Append md5 checksums and filenames. } check_database () { local n=0 local filename local checksum # ------------------------------------------- # # This file check should be unnecessary, #+ but better safe than sorry. if [ ! -r "$dbfile" ] then echo "Unable to read checksum database file!" exit $E_BAD_DBFILE fi # ------------------------------------------- # while read record[n] do directory_checked="${record[0]}" if [ "$directory_checked" != "$directory" ] then echo "Directories do not match up!" # Tried to use file for a different directory. exit $E_DIR_NOMATCH fi if [ "$n" -gt 0 ] # Not directory name. then filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' ) # md5sum writes records backwards, #+ checksum first, then filename. checksum[n]=$( md5sum "${filename[n]}" ) if [ "${record[n]}" = "${checksum[n]}" ] then echo "${filename[n]} unchanged." elif [ "`basename ${filename[n]}`" != "$dbfile" ] # Skip over checksum database file, #+ as it will change with each invocation of script. # --- # This unfortunately means that when running #+ this script on $PWD, tampering with the #+ checksum database file will not be detected. # Exercise: Fix this. then echo "${filename[n]} : CHECKSUM ERROR!" # File has been changed since last checked. fi fi let "n+=1" done <"$dbfile" # Read from checksum database file. } # =================================================== # # main () if [ -z "$1" ] then directory="$PWD" # If not specified, else #+ use current working directory. directory="$1" fi clear # Clear screen. echo " Running file integrity check on $directory" echo # ------------------------------------------------------------------ # if [ ! -r "$dbfile" ] # Need to create database file? then echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo set_up_database fi # ------------------------------------------------------------------ # check_database # Do the actual work. echo # You may wish to redirect the stdout of this script to a file, #+ especially if the directory checked has many files in it. exit 0 # For a much more thorough file integrity check, #+ consider the "Tripwire" package, #+ http://sourceforge.net/projects/tripwire/. Also see Example A-19, Example 33-14, and Example 9-11 for creative uses of the md5sum command. Note There have been reports that the 128-bit md5sum can be cracked, so the more secure 160-bit sha1sum is a welcome new addition to the checksum toolkit. +--------------------------------------------------+ |bash$ md5sum testfile | |e181e2c8720c60522c4c4c981108e367 testfile | | | | | |bash$ sha1sum testfile | |5d7425a9c08a66c3177f1e31286fa40986ffc996 testfile| | | +--------------------------------------------------+ Security consultants have demonstrated that even sha1sum can be compromised. Fortunately, newer Linux distros include longer bit-length sha224sum, sha256sum, sha384sum, and sha512sum commands. uuencode This utility encodes binary files (images, sound files, compressed files, etc.) into ASCII characters, making them suitable for transmission in the body of an e-mail message or in a newsgroup posting. This is especially useful where MIME (multimedia) encoding is not available. uudecode This reverses the encoding, decoding uuencoded files back into the original binaries. Example 15-39. Uudecoding encoded files #!/bin/bash # Uudecodes all uuencoded files in current working directory. lines=35 # Allow 35 lines for the header (very generous). for File in * # Test all the files in $PWD. do search1=`head -n $lines $File | grep begin | wc -w` search2=`tail -n $lines $File | grep end | wc -w` # Uuencoded files have a "begin" near the beginning, #+ and an "end" near the end. if [ "$search1" -gt 0 ] then if [ "$search2" -gt 0 ] then echo "uudecoding - $File -" uudecode $File fi fi done # Note that running this script upon itself fools it #+ into thinking it is a uuencoded file, #+ because it contains both "begin" and "end". # Exercise: # -------- # Modify this script to check each file for a newsgroup header, #+ and skip to next if not found. exit 0 Tip The fold -s command may be useful (possibly in a pipe) to process long uudecoded text messages downloaded from Usenet newsgroups. mimencode, mmencode The mimencode and mmencode commands process multimedia-encoded e-mail attachments. Although mail user agents (such as pine or kmail) normally handle this automatically, these particular utilities permit manipulating such attachments manually from the command-line or in batch processing mode by means of a shell script. crypt At one time, this was the standard UNIX file encryption utility. [70] Politically-motivated government regulations prohibiting the export of encryption software resulted in the disappearance of crypt from much of the UNIX world, and it is still missing from most Linux distributions. Fortunately, programmers have come up with a number of decent alternatives to it, among them the author's very own [ftp://metalab.unc.edu/pub/Linux/utils/file/cruft-0.2.tar.gz] cruft (see Example A-4). openssl This is an Open Source implementation of Secure Sockets Layer encryption. # To encrypt a file: openssl aes-128-ecb -salt -in file.txt -out file.encrypted \ -pass pass:my_password # ^^^^^^^^^^^ User-selected password. # aes-128-ecb is the encryption method chosen. # To decrypt an openssl-encrypted file: openssl aes-128-ecb -d -salt -in file.encrypted -out file.txt \ -pass pass:my_password # ^^^^^^^^^^^ User-selected password. Piping openssl to/from tar makes it possible to encrypt an entire directory tree. # To encrypt a directory: sourcedir="/home/bozo/testfiles" encrfile="encr-dir.tar.gz" password=my_secret_password tar czvf - "$sourcedir" | openssl des3 -salt -out "$encrfile" -pass pass:"$password" # ^^^^ Uses des3 encryption. # Writes encrypted file "encr-dir.tar.gz" in current working directory. # To decrypt the resulting tarball: openssl des3 -d -salt -in "$encrfile" -pass pass:"$password" | tar -xzv # Decrypts and unpacks into current working directory. Of course, openssl has many other uses, such as obtaining signed certificates for Web sites. See the info page. shred Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This command has the same effect as Example 15-60, but does it in a more thorough and elegant manner. This is one of the GNU fileutils. Caution Advanced forensic technology may still be able to recover the contents of a file, even after application of shred. Miscellaneous mktemp Create a temporary file [71] with a "unique" filename. When invoked from the command-line without additional arguments, it creates a zero-length file in the /tmp directory. +-------------------------------------------------------+ |bash$ mktemp | |/tmp/tmp.zzsvql3154 | | | +-------------------------------------------------------+ PREFIX=filename tempfile=`mktemp $PREFIX.XXXXXX` # ^^^^^^ Need at least 6 placeholders #+ in the filename template. # If no filename template supplied, #+ "tmp.XXXXXXXXXX" is the default. echo "tempfile name = $tempfile" # tempfile name = filename.QA2ZpY # or something similar... # Creates a file of that name in the current working directory #+ with 600 file permissions. # A "umask 177" is therefore unnecessary, #+ but it's good programming practice anyhow. make Utility for building and compiling binary packages. This can also be used for any set of operations triggered by incremental changes in source files. The make command checks a Makefile, a list of file dependencies and operations to be carried out. The make utility is, in effect, a powerful scripting language similar in many ways to Bash, but with the capability of recognizing dependencies. For in-depth coverage of this useful tool set, see the GNU software documentation site. install Special purpose file copying command, similar to cp, but capable of setting permissions and attributes of the copied files. This command seems tailormade for installing software packages, and as such it shows up frequently in Makefiles (in the make install : section). It could likewise prove useful in installation scripts. dos2unix This utility, written by Benjamin Lin and collaborators, converts DOS-formatted text files (lines terminated by CR-LF) to UNIX format (lines terminated by LF only), and vice-versa. ptx The ptx [targetfile] command outputs a permuted index (cross-reference list) of the targetfile. This may be further filtered and formatted in a pipe, if necessary. more, less Pagers that display a text file or stream to stdout, one screenful at a time. These may be used to filter the output of stdout . . . or of a script. An interesting application of more is to "test drive" a command sequence, to forestall potentially unpleasant consequences. ls /home/bozo | awk '{print "rm -rf " $1}' | more # ^^^^ # Testing the effect of the following (disastrous) command-line: # ls /home/bozo | awk '{print "rm -rf " $1}' | sh # Hand off to the shell to execute . . . ^^ The less pager has the interesting property of doing a formatted display of man page source. See Example A-39. ------------------------------------------------------------------------ 15.6. Communications Commands Certain of the following commands find use in network data transfer and analysis, as well as in chasing spammers. Information and Statistics host Searches for information about an Internet host by name or IP address, using DNS. +-------------------------------------------------------+ |bash$ host surfacemail.com | |surfacemail.com. has address 202.92.42.236 | | | +-------------------------------------------------------+ ipcalc Displays IP information for a host. With the -h option, ipcalc does a reverse DNS lookup, finding the name of the host (server) from the IP address. +-------------------------------------------------------+ |bash$ ipcalc -h 202.92.42.236 | |HOSTNAME=surfacemail.com | | | +-------------------------------------------------------+ nslookup Do an Internet "name server lookup" on a host by IP address. This is essentially equivalent to ipcalc -h or dig -x . The command may be run either interactively or noninteractively, i.e., from within a script. The nslookup command has allegedly been "deprecated," but it is still useful. +-------------------------------------------------------+ |bash$ nslookup -sil 66.97.104.180 | |nslookup kuhleersparnis.ch | | Server: 135.116.137.2 | | Address: 135.116.137.2#53 | | | | Non-authoritative answer: | | Name: kuhleersparnis.ch | | | +-------------------------------------------------------+ dig Domain Information Groper. Similar to nslookup, dig does an Internet name server lookup on a host. May be run from the command-line or from within a script. Some interesting options to dig are +time=N for setting a query timeout to N seconds, +nofail for continuing to query servers until a reply is received, and -x for doing a reverse address lookup. Compare the output of dig -x with ipcalc -h and nslookup. +-----------------------------------------------------------------------------+ |bash$ dig -x 81.9.6.2 | |;; Got answer: | | ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649 | | ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 | | | | ;; QUESTION SECTION: | | ;2.6.9.81.in-addr.arpa. IN PTR | | | | ;; AUTHORITY SECTION: | | 6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.| | 2002031705 900 600 86400 3600 | | | | ;; Query time: 537 msec | | ;; SERVER: 135.116.137.2#53(135.116.137.2) | | ;; WHEN: Wed Jun 26 08:35:24 2002 | | ;; MSG SIZE rcvd: 91 | | | +-----------------------------------------------------------------------------+ Example 15-40. Finding out where to report a spammer #!/bin/bash # spam-lookup.sh: Look up abuse contact to report a spammer. # Thanks, Michael Zick. # Check for command-line arg. ARGCOUNT=1 E_WRONGARGS=65 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` domain-name" exit $E_WRONGARGS fi dig +short $1.contacts.abuse.net -c in -t txt # Also try: # dig +nssearch $1 # Tries to find "authoritative name servers" and display SOA records. # The following also works: # whois -h whois.abuse.net $1 # ^^ ^^^^^^^^^^^^^^^ Specify host. # Can even lookup multiple spammers with this, i.e." # whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . . # Exercise: # -------- # Expand the functionality of this script #+ so that it automatically e-mails a notification #+ to the responsible ISP's contact address(es). # Hint: use the "mail" command. exit $? # spam-lookup.sh chinatietong.com # A known spam domain. # "crnet_mgr@chinatietong.com" # "crnet_tec@chinatietong.com" # "postmaster@chinatietong.com" # For a more elaborate version of this script, #+ see the SpamViz home page, http://www.spamviz.net/index.html. Example 15-41. Analyzing a spam domain #! /bin/bash # is-spammer.sh: Identifying spam domains # $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $ # Above line is RCS ID info. # # This is a simplified version of the "is_spammer.bash #+ script in the Contributed Scripts appendix. # is-spammer # Uses an external program: 'dig' # Tested with version: 9.2.4rc5 # Uses functions. # Uses IFS to parse strings by assignment into arrays. # And even does something useful: checks e-mail blacklists. # Use the domain.name(s) from the text body: # http://www.good_stuff.spammer.biz/just_ignore_everything_else # ^^^^^^^^^^^ # Or the domain.name(s) from any e-mail address: # Really_Good_Offer@spammer.biz # # as the only argument to this script. #(PS: have your Inet connection running) # # So, to invoke this script in the above two instances: # is-spammer.sh spammer.biz # Whitespace == :Space:Tab:Line Feed:Carriage Return: WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D' # No Whitespace == Line Feed:Carriage Return No_WSP=$'\x0A'$'\x0D' # Field separator for dotted decimal ip addresses ADR_IFS=${No_WSP}'.' # Get the dns text resource record. # get_txt get_txt() { # Parse $1 by assignment at the dots. local -a dns IFS=$ADR_IFS dns=( $1 ) IFS=$WSP_IFS if [ "${dns[0]}" == '127' ] then # See if there is a reason. echo $(dig +short $2 -t txt) fi } # Get the dns address resource record. # chk_adr chk_adr() { local reply local server local reason server=${1}${2} reply=$( dig +short ${server} ) # If reply might be an error code . . . if [ ${#reply} -gt 6 ] then reason=$(get_txt ${reply} ${server} ) reason=${reason:-${reply}} fi echo ${reason:-' not blacklisted.'} } # Need to get the IP address from the name. echo 'Get address of: '$1 ip_adr=$(dig +short $1) dns_reply=${ip_adr:-' no answer '} echo ' Found address: '${dns_reply} # A valid reply is at least 4 digits plus 3 dots. if [ ${#ip_adr} -gt 6 ] then echo declare query # Parse by assignment at the dots. declare -a dns IFS=$ADR_IFS dns=( ${ip_adr} ) IFS=$WSP_IFS # Reorder octets into dns query order. rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.' # See: http://www.spamhaus.org (Conservative, well maintained) echo -n 'spamhaus.org says: ' echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org') # See: http://ordb.org (Open mail relays) echo -n ' ordb.org says: ' echo $(chk_adr ${rev_dns} 'relays.ordb.org') # See: http://www.spamcop.net/ (You can report spammers here) echo -n ' spamcop.net says: ' echo $(chk_adr ${rev_dns} 'bl.spamcop.net') # # # other blacklist operations # # # # See: http://cbl.abuseat.org. echo -n ' abuseat.org says: ' echo $(chk_adr ${rev_dns} 'cbl.abuseat.org') # See: http://dsbl.org/usage (Various mail relays) echo echo 'Distributed Server Listings' echo -n ' list.dsbl.org says: ' echo $(chk_adr ${rev_dns} 'list.dsbl.org') echo -n ' multihop.dsbl.org says: ' echo $(chk_adr ${rev_dns} 'multihop.dsbl.org') echo -n 'unconfirmed.dsbl.org says: ' echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org') else echo echo 'Could not use that address.' fi exit 0 # Exercises: # -------- # 1) Check arguments to script, # and exit with appropriate error message if necessary. # 2) Check if on-line at invocation of script, # and exit with appropriate error message if necessary. # 3) Substitute generic variables for "hard-coded" BHL domains. # 4) Set a time-out for the script using the "+time=" option to the 'dig' command. For a much more elaborate version of the above script, see Example A-28. traceroute Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe. +------------------------------------------------------------------------+ |bash$ traceroute 81.9.6.2 | |traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets | | 1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms| | 2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms | | 3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms * | | ... | | | +------------------------------------------------------------------------+ ping Broadcast an ICMP ECHO_REQUEST packet to another machine, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution. +----------------------------------------------------------------------------------+ |bash$ ping localhost | |PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. | | 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec| | 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec| | | | --- localhost.localdomain ping statistics --- | | 2 packets transmitted, 2 packets received, 0% packet loss | | round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms | | | +----------------------------------------------------------------------------------+ A successful ping returns an exit status of 0. This can be tested for in a script. HNAME=nastyspammer.com # HNAME=$HOST # Debug: test for localhost. count=2 # Send only two pings. if [[ `ping -c $count "$HNAME"` ]] then echo ""$HNAME" still up and broadcasting spam your way." else echo ""$HNAME" seems to be down. Pity." fi whois Perform a DNS (Domain Name System) lookup. The -h option permits specifying which particular whois server to query. See Example 4-6 and Example 15-40. finger Retrieve information about users on a network. Optionally, this command can display a user's ~/.plan, ~/.project, and ~/.forward files, if present. +-------------------------------------------------------------------------+ |bash$ finger | |Login Name Tty Idle Login Time Office Office Phone| | bozo Bozo Bozeman tty1 8 Jun 25 16:59 (:0) | | bozo Bozo Bozeman ttyp0 Jun 25 16:59 (:0.0) | | bozo Bozo Bozeman ttyp1 Jun 25 17:07 (:0.0) | | | | | | | |bash$ finger bozo | |Login: bozo Name: Bozo Bozeman | | Directory: /home/bozo Shell: /bin/bash | | Office: 2355 Clown St., 543-1234 | | On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle | | On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle | | On since Fri Aug 31 20:13 (MST) on pts/1 | | On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle | | Mail last read Tue Jul 3 10:08 2007 (MST) | | No Plan. | | | +-------------------------------------------------------------------------+ Out of security considerations, many networks disable finger and its associated daemon. [72] chfn Change information disclosed by the finger command. vrfy Verify an Internet e-mail address. This command seems to be missing from newer Linux distros. Remote Host Access sx, rx The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom. sz, rz The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package. ftp Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see Example 18-6 and Example A-4). uucp, uux, cu uucp: UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence. Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate. The advantage of uucp is that it is fault-tolerant, so even if there is a service interruption the copy operation will resume where it left off when the connection is restored. --- uux: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the uucp package. --- cu: Call Up a remote system and connect as a simple terminal. It is a sort of dumbed-down version of telnet. This command is part of the uucp package. telnet Utility and protocol for connecting to a remote host. Caution The telnet protocol contains security holes and should therefore probably be avoided. Its use within a shell script is not recommended. wget The wget utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in a script. wget -p http://www.xyz23.com/file01.html # The -p or --page-requisite option causes wget to fetch all files #+ required to display the specified page. wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE # The -r option recursively follows and retrieves all links #+ on the specified site. wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2 # The -c option lets wget resume an interrupted download. # This works with ftp servers and many HTTP sites. Example 15-42. Getting a stock quote #!/bin/bash # quote-fetch.sh: Download a stock quote. E_NOPARAMS=86 if [ -z "$1" ] # Must specify a stock (symbol) to fetch. then echo "Usage: `basename $0` stock-symbol" exit $E_NOPARAMS fi stock_symbol=$1 file_suffix=.html # Fetches an HTML file, so name it appropriately. URL='http://finance.yahoo.com/q?s=' # Yahoo finance board, with stock query suffix. # ----------------------------------------------------------- wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}" # ----------------------------------------------------------- # To look up stuff on http://search.yahoo.com: # ----------------------------------------------------------- # URL="http://search.yahoo.com/search?fr=ush-news&p=${query}" # wget -O "$savefilename" "${URL}" # ----------------------------------------------------------- # Saves a list of relevant URLs. exit $? # Exercises: # --------- # # 1) Add a test to ensure the user running the script is on-line. # (Hint: parse the output of 'ps -ax' for "ppp" or "connect." # # 2) Modify this script to fetch the local weather report, #+ taking the user's zip code as an argument. See also Example A-30 and Example A-31. lynx The lynx Web and file browser can be used inside a script (with the -dump option) to retrieve a file from a Web or ftp site noninteractively. lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE With the -traversal option, lynx starts at the HTTP URL specified as an argument, then "crawls" through all links located on that particular server. Used together with the -crawl option, outputs page text to a log file. rlogin Remote login, initates a session on a remote host. This command has security issues, so use ssh instead. rsh Remote shell, executes command(s) on a remote host. This has security issues, so use ssh instead. rcp Remote copy, copies files between two different networked machines. rsync Remote synchronize, updates (synchronizes) files between two different networked machines. +-------------------------------------------------------+ |bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/ | | | +-------------------------------------------------------+ Example 15-43. Updating FC4 #!/bin/bash # fc4upd.sh # Script author: Frank Wang. # Slight stylistic modifications by ABS Guide author. # Used in ABS Guide with permission. # Download Fedora Core 4 update from mirror site using rsync. # Should also work for newer Fedora Cores -- 5, 6, . . . # Only download latest package if multiple versions exist, #+ to save space. URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/ # URL=rsync://ftp.kddilabs.jp/fedora/core/updates/ # URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/ DEST=${1:-/var/www/html/fedora/updates/} LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt PID_FILE=/var/run/${0##*/}.pid E_RETURN=85 # Something unexpected happened. # General rsync options # -r: recursive download # -t: reserve time # -v: verbose OPTS="-rtv --delete-excluded --delete-after --partial" # rsync include pattern # Leading slash causes absolute path name match. INCLUDE=( "/4/i386/kde-i18n-Chinese*" # ^ ^ # Quoting is necessary to prevent globbing. ) # rsync exclude pattern # Temporarily comment out unwanted pkgs using "#" . . . EXCLUDE=( /1 /2 /3 /testing /4/SRPMS /4/ppc /4/x86_64 /4/i386/debug "/4/i386/kde-i18n-*" "/4/i386/openoffice.org-langpack-*" "/4/i386/*i586.rpm" "/4/i386/GFS-*" "/4/i386/cman-*" "/4/i386/dlm-*" "/4/i386/gnbd-*" "/4/i386/kernel-smp*" # "/4/i386/kernel-xen*" # "/4/i386/xen-*" ) init () { # Let pipe command return possible rsync error, e.g., stalled network. set -o pipefail # Newly introduced in Bash, version 3. TMP=${TMPDIR:-/tmp}/${0##*/}.$$ # Store refined download list. trap "{ rm -f $TMP 2>/dev/null }" EXIT # Clear temporary file on exit. } check_pid () { # Check if process exists. if [ -s "$PID_FILE" ]; then echo "PID file exists. Checking ..." PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE) if /bin/ps --pid $PID &>/dev/null; then echo "Process $PID found. ${0##*/} seems to be running!" /usr/bin/logger -t ${0##*/} \ "Process $PID found. ${0##*/} seems to be running!" exit $E_RETURN fi echo "Process $PID not found. Start new process . . ." fi } # Set overall file update range starting from root or $URL, #+ according to above patterns. set_range () { include= exclude= for p in "${INCLUDE[@]}"; do include="$include --include \"$p\"" done for p in "${EXCLUDE[@]}"; do exclude="$exclude --exclude \"$p\"" done } # Retrieve and refine rsync update list. get_list () { echo $$ > $PID_FILE || { echo "Can't write to pid file $PID_FILE" exit $E_RETURN } echo -n "Retrieving and refining update list . . ." # Retrieve list -- 'eval' is needed to run rsync as a single command. # $3 and $4 is the date and time of file creation. # $5 is the full package name. previous= pre_file= pre_date=0 eval /bin/nice /usr/bin/rsync \ -r $include $exclude $URL | \ egrep '^dr.x|^-r' | \ awk '{print $3, $4, $5}' | \ sort -k3 | \ { while read line; do # Get seconds since epoch, to filter out obsolete pkgs. cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s) # echo $cur_date # Get file name. cur_file=$(echo $line | awk '{print $3}') # echo $cur_file # Get rpm pkg name from file name, if possible. if [[ $cur_file == *rpm ]]; then pkg_name=$(echo $cur_file | sed -r -e \ 's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/') else pkg_name= fi # echo $pkg_name if [ -z "$pkg_name" ]; then # If not a rpm file, echo $cur_file >> $TMP #+ then append to download list. elif [ "$pkg_name" != "$previous" ]; then # A new pkg found. echo $pre_file >> $TMP # Output latest file. previous=$pkg_name # Save current. pre_date=$cur_date pre_file=$cur_file elif [ "$cur_date" -gt "$pre_date" ]; then # If same pkg, but newer, pre_date=$cur_date #+ then update latest pointer. pre_file=$cur_file fi done echo $pre_file >> $TMP # TMP contains ALL #+ of refined list now. # echo "subshell=$BASH_SUBSHELL" } # Bracket required here to let final "echo $pre_file >> $TMP" # Remained in the same subshell ( 1 ) with the entire loop. RET=$? # Get return code of the pipe command. [ "$RET" -ne 0 ] && { echo "List retrieving failed with code $RET" exit $E_RETURN } echo "done"; echo } # Real rsync download part. get_file () { echo "Downloading..." /bin/nice /usr/bin/rsync \ $OPTS \ --filter "merge,+/ $TMP" \ --exclude '*' \ $URL $DEST \ | /usr/bin/tee $LOG RET=$? # --filter merge,+/ is crucial for the intention. # + modifier means include and / means absolute path. # Then sorted list in $TMP will contain ascending dir name and #+ prevent the following --exclude '*' from "shortcutting the circuit." echo "Done" rm -f $PID_FILE 2>/dev/null return $RET } # ------- # Main init check_pid set_range get_list get_file RET=$? # ------- if [ "$RET" -eq 0 ]; then /usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully." else /usr/bin/logger -t ${0##*/} \ "Fedora update mirrored with failure code: $RET" fi exit $RET See also Example A-32. Note Using rcp, rsync, and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh, scp, or an expect script. ssh Secure shell, logs onto a remote host and executes commands there. This secure replacement for telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details. Example 15-44. Using ssh #!/bin/bash # remote.bash: Using ssh. # This example by Michael Zick. # Used with permission. # Presumptions: # ------------ # fd-2 isn't being captured ( '2>/dev/null' ). # ssh/sshd presumes stderr ('2') will display to user. # # sshd is running on your machine. # For any 'standard' distribution, it probably is, #+ and without any funky ssh-keygen having been done. # Try ssh to your machine from the command-line: # # $ ssh $HOSTNAME # Without extra set-up you'll be asked for your password. # enter password # when done, $ exit # # Did that work? If so, you're ready for more fun. # Try ssh to your machine as 'root': # # $ ssh -l root $HOSTNAME # When asked for password, enter root's, not yours. # Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain # Enter 'exit' when done. # The above gives you an interactive shell. # It is possible for sshd to be set up in a 'single command' mode, #+ but that is beyond the scope of this example. # The only thing to note is that the following will work in #+ 'single command' mode. # A basic, write stdout (local) command. ls -l # Now the same basic command on a remote machine. # Pass a different 'USERNAME' 'HOSTNAME' if desired: USER=${USERNAME:-$(whoami)} HOST=${HOSTNAME:-$(hostname)} # Now excute the above command-line on the remote host, #+ with all transmissions encrypted. ssh -l ${USER} ${HOST} " ls -l " # The expected result is a listing of your username's home #+ directory on the remote machine. # To see any difference, run this script from somewhere #+ other than your home directory. # In other words, the Bash command is passed as a quoted line #+ to the remote shell, which executes it on the remote machine. # In this case, sshd does ' bash -c "ls -l" ' on your behalf. # For information on topics such as not having to enter a #+ password/passphrase for every command-line, see #+ man ssh #+ man ssh-keygen #+ man sshd_config. exit 0 Caution Within a loop, ssh may cause unexpected behavior. According to a [http://groups-beta.google.com/group/comp.unix.shell/msg/dcb446b5fff7d230] Usenet post in the comp.unix shell archives, ssh inherits the loop's stdin. To remedy this, pass ssh either the -n or -f option. Thanks, Jason Bechtel, for pointing this out. scp Secure copy, similar in function to rcp, copies files between two different networked machines, but does so using authentication, and with a security level similar to ssh. Local Network write This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal Since write is interactive, it would not normally find use in a script. netconfig A command-line utility for configuring a network adapter (using DHCP). This command is native to Red Hat centric Linux distros. Mail mail Send or read e-mail messages. This stripped-down command-line mail client works fine as a command embedded in a script. Example 15-45. A script that mails itself #!/bin/sh # self-mailer.sh: Self-mailing script adr=${1:-`whoami`} # Default to current user, if not specified. # Typing 'self-mailer.sh wiseguy@superdupergenius.com' #+ sends this script to that addressee. # Just 'self-mailer.sh' (no argument) sends the script #+ to the person invoking it, for example, bozo@localhost.localdomain. # # For more on the ${parameter:-default} construct, #+ see the "Parameter Substitution" section #+ of the "Variables Revisited" chapter. # ============================================================================ cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr" # ============================================================================ # -------------------------------------------- # Greetings from the self-mailing script. # A mischievous person has run this script, #+ which has caused it to mail itself to you. # Apparently, some people have nothing better #+ to do with their time. # -------------------------------------------- echo "At `date`, script \"`basename $0`\" mailed to "$adr"." exit 0 # Note that the "mailx" command (in "send" mode) may be substituted #+ for "mail" ... but with somewhat different options. mailto Similar to the mail command, mailto sends e-mail messages from the command-line or in a script. However, mailto also permits sending MIME (multimedia) messages. mailstats Show mail statistics. This command may be invoked only by root. +------------------------------------------------------------------------------+ |root# mailstats | |Statistics from Tue Jan 1 20:32:08 2008 | | M msgsfr bytes_from msgsto bytes_to msgsrej msgsdis msgsqur Mailer| | 4 1682 24118K 0 0K 0 0 0 esmtp | | 9 212 640K 1894 25131K 0 0 0 local | | ===================================================================== | | T 1894 24758K 1894 25131K 0 0 0 | | C 414 0 | | | +------------------------------------------------------------------------------+ vacation This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. It runs on a network, in conjunction with sendmail, and is not applicable to a dial-up POPmail account. ------------------------------------------------------------------------ 15.7. Terminal Control Commands Command affecting the console or terminal tput Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain terminal operations: tput clear is the equivalent of clear; tput reset is the equivalent of reset. +-------------------------------------------------------+ |bash$ tput longname | |xterm terminal emulator (X Window System) | | | +-------------------------------------------------------+ Issuing a tput cup X Y moves the cursor to the (X,Y) coordinates in the current terminal. A clear to erase the terminal screen would normally precede this. Some interesting options to tput are: * bold, for high-intensity text * smul, to underline text in the terminal * smso, to render text in reverse * sgr0, to reset the terminal parameters (to normal), without clearing the screen Example scripts using tput: 1. Example 33-13 2. Example 33-11 3. Example A-44 4. Example A-42 5. Example 26-2 Note that stty offers a more powerful command set for controlling a terminal. infocmp This command prints out extensive information about the current terminal. It references the terminfo database. +------------------------------------------------------------------+ |bash$ infocmp | |# Reconstructed via infocmp from file: | | /usr/share/terminfo/r/rxvt | | rxvt|rxvt terminal emulator (X Window System), | | am, bce, eo, km, mir, msgr, xenl, xon, | | colors#8, cols#80, it#8, lines#24, pairs#64, | | acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~, | | bel=^G, blink=\E[5m, bold=\E[1m, | | civis=\E[?25l, | | clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M, | | ... | | | +------------------------------------------------------------------+ reset Reset terminal parameters and clear text screen. As with clear, the cursor and prompt reappear in the upper lefthand corner of the terminal. clear The clear command simply clears the text screen at the console or in an xterm. The prompt and cursor reappear at the upper lefthand corner of the screen or xterm window. This command may be used either at the command line or in a script. See Example 10-25. resize Echoes commands necessary to set $TERM and $TERMCAP to duplicate the size (dimensions) of the current terminal. +-------------------------------------------------------+ |bash$ resize | |set noglob; | | setenv COLUMNS '80'; | | setenv LINES '24'; | | unset noglob; | | | +-------------------------------------------------------+ script This utility records (saves to a file) all the user keystrokes at the command-line in a console or an xterm window. This, in effect, creates a record of a session. ------------------------------------------------------------------------ 15.8. Math Commands "Doing the numbers" factor Decompose an integer into prime factors. +-------------------------------------------------------+ |bash$ factor 27417 | |27417: 3 13 19 37 | | | +-------------------------------------------------------+ Example 15-46. Generating prime numbers #!/bin/bash # primes2.sh # Generating prime numbers the quick-and-easy way, #+ without resorting to fancy algorithms. CEILING=10000 # 1 to 10000 PRIME=0 E_NOTPRIME= is_prime () { local factors factors=( $(factor $1) ) # Load output of `factor` into array. if [ -z "${factors[2]}" ] # Third element of "factors" array: #+ ${factors[2]} is 2nd factor of argument. # If it is blank, then there is no 2nd factor, #+ and the argument is therefore prime. then return $PRIME # 0 else return $E_NOTPRIME # null fi } echo for n in $(seq $CEILING) do if is_prime $n then printf %5d $n fi # ^ Five positions per number suffices. done # For a higher $CEILING, adjust upward, as necessary. echo exit bc Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, bc comes to the rescue. Not just a versatile, arbitrary precision calculation utility, bc offers many of the facilities of a programming language. bc has a syntax vaguely resembling C. Since it is a fairly well-behaved UNIX utility, and may therefore be used in a pipe, bc comes in handy in scripts. Here is a simple template for using bc to calculate a script variable. This uses command substitution. +---------------------------------------------------------+ | variable=$(echo "OPTIONS; OPERATIONS" | bc)| | | +---------------------------------------------------------+ Example 15-47. Monthly Payment on a Mortgage #!/bin/bash # monthlypmt.sh: Calculates monthly payment on a mortgage. # This is a modification of code in the #+ "mcalc" (mortgage calculator) package, #+ by Jeff Schmidt #+ and #+ Mendel Cooper (yours truly, the author of the ABS Guide). # http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz [15k] echo echo "Given the principal, interest rate, and term of a mortgage," echo "calculate the monthly payment." bottom=1.0 echo echo -n "Enter principal (no commas) " read principal echo -n "Enter interest rate (percent) " # If 12%, enter "12", not ".12". read interest_r echo -n "Enter term (months) " read term interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal. # ^^^^^^^^^^^^^^^^^ Divide by 100. # "scale" determines how many decimal places. interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc) top=$(echo "scale=9; $principal*$interest_rate^$term" | bc) # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ # Standard formula for figuring interest. echo; echo "Please be patient. This may take a while." let "months = $term - 1" # ==================================================================== for ((x=$months; x > 0; x--)) do bot=$(echo "scale=9; $interest_rate^$x" | bc) bottom=$(echo "scale=9; $bottom+$bot" | bc) # bottom = $(($bottom + $bot")) done # ==================================================================== # -------------------------------------------------------------------- # Rick Boivie pointed out a more efficient implementation #+ of the above loop, which decreases computation time by 2/3. # for ((x=1; x <= $months; x++)) # do # bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc) # done # And then he came up with an even more efficient alternative, #+ one that cuts down the run time by about 95%! # bottom=`{ # echo "scale=9; bottom=$bottom; interest_rate=$interest_rate" # for ((x=1; x <= $months; x++)) # do # echo 'bottom = bottom * interest_rate + 1' # done # echo 'bottom' # } | bc` # Embeds a 'for loop' within command substitution. # -------------------------------------------------------------------------- # On the other hand, Frank Wang suggests: # bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc) # Because . . . # The algorithm behind the loop #+ is actually a sum of geometric proportion series. # The sum formula is e0(1-q^n)/(1-q), #+ where e0 is the first element and q=e(n+1)/e(n) #+ and n is the number of elements. # -------------------------------------------------------------------------- # let "payment = $top/$bottom" payment=$(echo "scale=2; $top/$bottom" | bc) # Use two decimal places for dollars and cents. echo echo "monthly payment = \$$payment" # Echo a dollar sign in front of amount. echo exit 0 # Exercises: # 1) Filter input to permit commas in principal amount. # 2) Filter input to permit interest to be entered as percent or decimal. # 3) If you are really ambitious, #+ expand this script to print complete amortization tables. Example 15-48. Base Conversion #!/bin/bash ########################################################################### # Shellscript: base.sh - print number to different bases (Bourne Shell) # Author : Heiner Steven (heiner.steven@odn.de) # Date : 07-03-95 # Category : Desktop # $Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $ # ==> Above line is RCS ID info. ########################################################################### # Description # # Changes # 21-03-95 stv fixed error occuring with 0xb as input (0.2) ########################################################################### # ==> Used in ABS Guide with the script author's permission. # ==> Comments added by ABS Guide author. NOARGS=85 PN=`basename "$0"` # Program name VER=`echo '$Revision: 1.2 $' | cut -d' ' -f2` # ==> VER=1.2 Usage () { echo "$PN - print number to different bases, $VER (stv '95) usage: $PN [number ...] If no number is given, the numbers are read from standard input. A number may be binary (base 2) starting with 0b (i.e. 0b1100) octal (base 8) starting with 0 (i.e. 014) hexadecimal (base 16) starting with 0x (i.e. 0xc) decimal otherwise (i.e. 12)" >&2 exit $NOARGS } # ==> Prints usage message. Msg () { for i # ==> in [list] missing. Why? do echo "$PN: $i" >&2 done } Fatal () { Msg "$@"; exit 66; } PrintBases () { # Determine base of the number for i # ==> in [list] missing... do # ==> so operates on command-line arg(s). case "$i" in 0b*) ibase=2;; # binary 0x*|[a-f]*|[A-F]*) ibase=16;; # hexadecimal 0*) ibase=8;; # octal [1-9]*) ibase=10;; # decimal *) Msg "illegal number $i - ignored" continue;; esac # Remove prefix, convert hex digits to uppercase (bc needs this). number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'` # ==> Uses ":" as sed separator, rather than "/". # Convert number to decimal dec=`echo "ibase=$ibase; $number" | bc` # ==> 'bc' is calculator utility. case "$dec" in [0-9]*) ;; # number ok *) continue;; # error: ignore esac # Print all conversions in one line. # ==> 'here document' feeds command list to 'bc'. echo `bc < Is a "while loop" really necessary here, # ==>+ since all the cases either break out of the loop # ==>+ or terminate the script. # ==> (Above comment by Paulo Marcel Coelho Aragao.) do case "$1" in --) shift; break;; -h) Usage;; # ==> Help message. -*) Usage;; *) break;; # First number esac # ==> Error checking for illegal input might be appropriate. shift done if [ $# -gt 0 ] then PrintBases "$@" else # Read from stdin. while read line do PrintBases $line done fi exit An alternate method of invoking bc involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to bc. variable=`bc << LIMIT_STRING options statements operations LIMIT_STRING ` ...or... variable=$(bc << LIMIT_STRING options statements operations LIMIT_STRING ) Example 15-49. Invoking bc using a here document #!/bin/bash # Invoking 'bc' using command substitution # in combination with a 'here document'. var1=`bc << EOF 18.33 * 19.78 EOF ` echo $var1 # 362.56 # $( ... ) notation also works. v1=23.53 v2=17.881 v3=83.501 v4=171.63 var2=$(bc << EOF scale = 4 a = ( $v1 + $v2 ) b = ( $v3 * $v4 ) a * b + 15.35 EOF ) echo $var2 # 593487.8452 var3=$(bc -l << EOF scale = 9 s ( 1.7 ) EOF ) # Returns the sine of 1.7 radians. # The "-l" option calls the 'bc' math library. echo $var3 # .991664810 # Now, try it in a function... hypotenuse () # Calculate hypotenuse of a right triangle. { # c = sqrt( a^2 + b^2 ) hyp=$(bc -l << EOF scale = 9 sqrt ( $1 * $1 + $2 * $2 ) EOF ) # Can't directly return floating point values from a Bash function. # But, can echo-and-capture: echo "$hyp" } hyp=$(hypotenuse 3.68 7.31) echo "hypotenuse = $hyp" # 8.184039344 exit 0 Example 15-50. Calculating PI #!/bin/bash # cannon.sh: Approximating PI by firing cannonballs. # Author: Mendel Cooper # License: Public Domain # Version 2.2, reldate 13oct08. # This is a very simple instance of a "Monte Carlo" simulation: #+ a mathematical model of a real-life event, #+ using pseudorandom numbers to emulate random chance. # Consider a perfectly square plot of land, 10000 units on a side. # This land has a perfectly circular lake in its center, #+ with a diameter of 10000 units. # The plot is actually mostly water, except for land in the four corners. # (Think of it as a square with an inscribed circle.) # # We will fire iron cannonballs from an old-style cannon #+ at the square. # All the shots impact somewhere on the square, #+ either in the lake or on the dry corners. # Since the lake takes up most of the area, #+ most of the shots will SPLASH! into the water. # Just a few shots will THUD! into solid ground #+ in the four corners of the square. # # If we take enough random, unaimed shots at the square, #+ Then the ratio of SPLASHES to total shots will approximate #+ the value of PI/4. # # The reason for this is that the cannon is actually shooting #+ only at the upper right-hand quadrant of the square, #+ i.e., Quadrant I of the Cartesian coordinate plane. # (The previous explanation was a simplification.) # # Theoretically, the more shots taken, the better the fit. # However, a shell script, as opposed to a compiled language #+ with floating-point math built in, requires a few compromises. # This tends to lower the accuracy of the simulation. DIMENSION=10000 # Length of each side of the plot. # Also sets ceiling for random integers generated. MAXSHOTS=1000 # Fire this many shots. # 10000 or more would be better, but would take too long. PMULTIPLIER=4.0 # Scaling factor to approximate PI. declare -r M_PI=3.141592654 # Actual 9-place value of PI, for comparison purposes. get_random () { SEED=$(head -n 1 /dev/urandom | od -N 1 | awk '{ print $2 }') RANDOM=$SEED # From "seeding-random.sh" #+ example script. let "rnum = $RANDOM % $DIMENSION" # Range less than 10000. echo $rnum } distance= # Declare global variable. hypotenuse () # Calculate hypotenuse of a right triangle. { # From "alt-bc.sh" example. distance=$(bc -l << EOF scale = 0 sqrt ( $1 * $1 + $2 * $2 ) EOF ) # Setting "scale" to zero rounds down result to integer value, #+ a necessary compromise in this script. # This decreases the accuracy of the simulation. } # ========================================================== # main() { # "Main" code block, mimmicking a C-language main() function. # Initialize variables. shots=0 splashes=0 thuds=0 Pi=0 error=0 while [ "$shots" -lt "$MAXSHOTS" ] # Main loop. do xCoord=$(get_random) # Get random X and Y coords. yCoord=$(get_random) hypotenuse $xCoord $yCoord # Hypotenuse of #+ right-triangle = distance. ((shots++)) printf "#%4d " $shots printf "Xc = %4d " $xCoord printf "Yc = %4d " $yCoord printf "Distance = %5d " $distance # Distance from #+ center of lake #+ -- the "origin" -- #+ coordinate (0,0). if [ "$distance" -le "$DIMENSION" ] then echo -n "SPLASH! " ((splashes++)) else echo -n "THUD! " ((thuds++)) fi Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc) # Multiply ratio by 4.0. echo -n "PI ~ $Pi" echo done echo echo "After $shots shots, PI looks like approximately $Pi" # Tends to run a bit high, #+ probably due to round-off error and imperfect randomness of $RANDOM. # But still usually within plus-or-minus 5% . . . #+ a pretty good rough approximation. error=$(echo "scale=9; $Pi - $M_PI" | bc) pct_error=$(echo "scale=2; 100.0 * $error / $M_PI" | bc) echo -n "Deviation from mathematical value of PI = $error" echo " ($pct_error% error)" echo # End of "main" code block. # } # ========================================================== exit # One might well wonder whether a shell script is appropriate for #+ an application as complex and computation-intensive as a simulation. # # There are at least two justifications. # 1) As a proof of concept: to show it can be done. # 2) To prototype and test the algorithms before rewriting #+ it in a compiled high-level language. See also Example A-37. dc The dc (desk calculator) utility is stack-oriented and uses RPN ("Reverse Polish Notation"). Like bc, it has much of the power of a programming language. echo "7 8 * p" | dc # 56 # Pushes 7, then 8 onto the stack, #+ multiplies ("*" operator), then prints the result ("p" operator). Most persons avoid dc, because of its non-intuitive input and rather cryptic operators. Yet, it has its uses. Example 15-51. Converting a decimal number to hexadecimal #!/bin/bash # hexconvert.sh: Convert a decimal number to hexadecimal. E_NOARGS=85 # Command-line arg missing. BASE=16 # Hexadecimal. if [ -z "$1" ] then # Need a command-line argument. echo "Usage: $0 number" exit $E_NOARGS fi # Exercise: add argument validity checking. hexcvt () { if [ -z "$1" ] then echo 0 return # "Return" 0 if no arg passed to function. fi echo ""$1" "$BASE" o p" | dc # o sets radix (numerical base) of output. # p prints the top of stack. # For other options: 'man dc' ... return } hexcvt "$1" exit Studying the info page for dc is a painful path to understanding its intricacies. There seems to be a small, select group of dc wizards who delight in showing off their mastery of this powerful, but arcane utility. +----------------------------------------------------------------------+ |bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc| |Bash | | | +----------------------------------------------------------------------+ dc <<< 10k5v1+2/p # 1.6180339887 # ^^^ Feed operations to dc using a Here String. # ^^^ Pushes 10 and sets that as the precision (10k). # ^^ Pushes 5 and takes its square root (5v, v = square root). # ^^ Pushes 1 and adds it to the running total (1+). # ^^ Pushes 2 and divides the running total by that (2/). # ^ Pops and prints the result (p) # The result is 1.6180339887 ... # ... which happens to be the Pythagorean Golden Ratio, to 10 places. Example 15-52. Factoring #!/bin/bash # factr.sh: Factor a number MIN=2 # Will not work for number smaller than this. E_NOARGS=85 E_TOOSMALL=86 if [ -z $1 ] then echo "Usage: $0 number" exit $E_NOARGS fi if [ "$1" -lt "$MIN" ] then echo "Number to factor must be $MIN or greater." exit $E_TOOSMALL fi # Exercise: Add type checking (to reject non-integer arg). echo "Factors of $1:" # ------------------------------------------------------- echo "$1[p]s2[lip/dli%0=1dvsr]s12sid2%0=13sidvsr[dli%0=\ 1lrli2+dsi!>.]ds.xd1<2" | dc # ------------------------------------------------------- # Above code written by Michel Charpentier # (as a one-liner, here broken into two lines for display purposes). # Used in ABS Guide with permission (thanks!). exit # $ sh factr.sh 270138 # 2 # 3 # 11 # 4093 awk Yet another way of doing floating point math in a script is using awk's built-in math functions in a shell wrapper. Example 15-53. Calculating the hypotenuse of a triangle #!/bin/bash # hypotenuse.sh: Returns the "hypotenuse" of a right triangle. # (square root of sum of squares of the "legs") ARGS=2 # Script needs sides of triangle passed. E_BADARGS=85 # Wrong number of arguments. if [ $# -ne "$ARGS" ] # Test number of arguments to script. then echo "Usage: `basename $0` side_1 side_2" exit $E_BADARGS fi AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } ' # command(s) / parameters passed to awk # Now, pipe the parameters to awk. echo -n "Hypotenuse of $1 and $2 = " echo $1 $2 | awk "$AWKSCRIPT" # ^^^^^^^^^^^^ # An echo-and-pipe is an easy way of passing shell parameters to awk. exit # Exercise: Rewrite this script using 'bc' rather than awk. # Which method is more intuitive? ------------------------------------------------------------------------ 15.9. Miscellaneous Commands Command that fit in no special category jot, seq These utilities emit a sequence of integers, with a user-selectable increment. The default separator character between each integer is a newline, but this can be changed with the -s option. +-------------------------------------------------------+ |bash$ seq 5 | |1 | | 2 | | 3 | | 4 | | 5 | | | | | | | |bash$ seq -s : 5 | |1:2:3:4:5 | | | +-------------------------------------------------------+ Both jot and seq come in handy in a for loop. Example 15-54. Using seq to generate loop arguments #!/bin/bash # Using "seq" echo for a in `seq 80` # or for a in $( seq 80 ) # Same as for a in 1 2 3 4 5 ... 80 (saves much typing!). # May also use 'jot' (if present on system). do echo -n "$a " done # 1 2 3 4 5 ... 80 # Example of using the output of a command to generate # the [list] in a "for" loop. echo; echo COUNT=80 # Yes, 'seq' also accepts a replaceable parameter. for a in `seq $COUNT` # or for a in $( seq $COUNT ) do echo -n "$a " done # 1 2 3 4 5 ... 80 echo; echo BEGIN=75 END=80 for a in `seq $BEGIN $END` # Giving "seq" two arguments starts the count at the first one, #+ and continues until it reaches the second. do echo -n "$a " done # 75 76 77 78 79 80 echo; echo BEGIN=45 INTERVAL=5 END=80 for a in `seq $BEGIN $INTERVAL $END` # Giving "seq" three arguments starts the count at the first one, #+ uses the second for a step interval, #+ and continues until it reaches the third. do echo -n "$a " done # 45 50 55 60 65 70 75 80 echo; echo exit 0 A simpler example: # Create a set of 10 files, #+ named file.1, file.2 . . . file.10. COUNT=10 PREFIX=file for filename in `seq $COUNT` do touch $PREFIX.$filename # Or, can do other operations, #+ such as rm, grep, etc. done Example 15-55. Letter Count" #!/bin/bash # letter-count.sh: Counting letter occurrences in a text file. # Written by Stefano Palmeri. # Used in ABS Guide with permission. # Slightly modified by document author. MINARGS=2 # Script requires at least two arguments. E_BADARGS=65 FILE=$1 let LETTERS=$#-1 # How many letters specified (as command-line args). # (Subtract 1 from number of command-line args.) show_help(){ echo echo Usage: `basename $0` file letters echo Note: `basename $0` arguments are case sensitive. echo Example: `basename $0` foobar.txt G n U L i N U x. echo } # Checks number of arguments. if [ $# -lt $MINARGS ]; then echo echo "Not enough arguments." echo show_help exit $E_BADARGS fi # Checks if file exists. if [ ! -f $FILE ]; then echo "File \"$FILE\" does not exist." exit $E_BADARGS fi # Counts letter occurrences . for n in `seq $LETTERS`; do shift if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then # Checks arg. echo "$1" -\> `cat $FILE | tr -cd "$1" | wc -c` # Counting. else echo "$1 is not a single char." fi done exit $? # This script has exactly the same functionality as letter-count2.sh, #+ but executes faster. # Why? Note Somewhat more capable than seq, jot is a classic UNIX utility that is not normally included in a standard Linux distro. However, the source rpm is available for download from the [http://www.mit.edu/afs/athena/system/rhlinux/athena-9.0/free/SRPMS/athena-jot-9.0-3.src.rpm] MIT repository. Unlike seq, jot can generate a sequence of random numbers, using the -r option. +---------------------------------------------------------------------------------+ |bash$ jot -r 3 999 | |1069 | | 1272 | | 1428 | +---------------------------------------------------------------------------------+ getopt The getopt command parses command-line options preceded by a dash. This external command corresponds to the getopts Bash builtin. Using getopt permits handling long options by means of the -l flag, and this also allows parameter reshuffling. Example 15-56. Using getopt to parse command-line options #!/bin/bash # Using getopt # Try the following when invoking this script: # sh ex33a.sh -a # sh ex33a.sh -abc # sh ex33a.sh -a -b -c # sh ex33a.sh -d # sh ex33a.sh -dXYZ # sh ex33a.sh -d XYZ # sh ex33a.sh -abcd # sh ex33a.sh -abcdZ # sh ex33a.sh -z # sh ex33a.sh a # Explain the results of each of the above. E_OPTERR=65 if [ "$#" -eq 0 ] then # Script needs at least one command-line argument. echo "Usage $0 -[options a,b,c]" exit $E_OPTERR fi set -- `getopt "abcd:" "$@"` # Sets positional parameters to command-line arguments. # What happens if you use "$*" instead of "$@"? while [ ! -z "$1" ] do case "$1" in -a) echo "Option \"a\"";; -b) echo "Option \"b\"";; -c) echo "Option \"c\"";; -d) echo "Option \"d\" $2";; *) break;; esac shift done # It is usually better to use the 'getopts' builtin in a script. # See "ex33.sh." exit 0 Note As Peggy Russell points out: It is often necessary to include an eval to correctly process whitespace and quotes. args=$(getopt -o a:bc:d -- "$@") eval set -- "$args" See Example 9-14 for a simplified emulation of getopt. run-parts The run-parts command [73] executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission. The cron daemon invokes run-parts to run the scripts in the /etc/cron.* directories. yes In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A control-C terminates the run. A different output string may be specified, as in yes different string, which would continually output different string to stdout. One might well ask the purpose of this. From the command-line or in a script, the output of yes can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect. yes | fsck /dev/hda1 runs fsck non-interactively (careful!). yes | rm -r dirname has same effect as rm -rf dirname (careful!). Warning Caution advised when piping yes to a potentially dangerous system command, such as fsck or fdisk. It might have unintended consequences. Note The yes command parses variables, or more accurately, it echoes parsed variables. For example: +--------------------------------------------+ |bash$ yes $BASH_VERSION | |3.1.17(1)-release | | 3.1.17(1)-release | | 3.1.17(1)-release | | 3.1.17(1)-release | | 3.1.17(1)-release | | . . . | | | +--------------------------------------------+ This particular "feature" may be used to create a very large ASCII file on the fly: +--------------------------------------------+ |bash$ yes $PATH > huge_file.txt | |Ctl-C | | | +--------------------------------------------+ Hit Ctl-C very quickly, or you just might get more than you bargained for. . . . The yes command may be emulated in a very simple script function. yes () { # Trivial emulation of "yes" ... local DEFAULT_TEXT="y" while [ true ] # Endless loop. do if [ -z "$1" ] then echo "$DEFAULT_TEXT" else # If argument ... echo "$1" # ... expand and echo it. fi done # The only things missing are the } #+ --help and --version options. banner Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy. Note that banner has been dropped from many Linux distros. printenv Show all the environmental variables set for a particular user. +-------------------------------------------------------+ |bash$ printenv | grep HOME | |HOME=/home/bozo | | | +-------------------------------------------------------+ lp The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. [74] These commands trace the origin of their names to the line printers of another era. bash$ lp file1.txt or bash lp to file | | | | | ==========================|==================== | | command ---> command ---> |tee ---> command ---> ---> output of pipe| | =============================================== | | | +----------------------------------------------------------------------+ cat listfile* | sort | tee check.file | uniq > result.file # ^^^^^^^^^^^^^^ ^^^^ # The file "check.file" contains the concatenated sorted "listfiles," #+ before the duplicate lines are removed by 'uniq.' mkfifo This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. [75] Typically, one process writes to the FIFO, and the other reads from it. See Example A-14. #!/bin/bash # This short script by Omair Eshkenazi. # Used in ABS Guide with permission (thanks!). mkfifo pipe1 # Yes, pipes can be given names. mkfifo pipe2 # Hence the designation "named pipe." (cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2 $filename.uppercase # lcase # For lower case conversion Some basic options to dd are: * if=INFILE INFILE is the source file. * of=OUTFILE OUTFILE is the target file, the file that will have the data written to it. * bs=BLOCKSIZE This is the size of each block of data being read and written, usually a power of 2. * skip=BLOCKS How many blocks of data to skip in INFILE before starting to copy. This is useful when the INFILE has "garbage" or garbled data in its header or when it is desirable to copy only a portion of the INFILE. * seek=BLOCKS How many blocks of data to skip in OUTFILE before starting to copy, leaving blank data at beginning of OUTFILE. * count=BLOCKS Copy only this many blocks of data, rather than the entire INFILE. * conv=CONVERSION Type of conversion to be applied to INFILE data before copying operation. A dd --help lists all the options this powerful utility takes. Example 15-57. A script that copies itself #!/bin/bash # self-copy.sh # This script copies itself. file_subscript=copy dd if=$0 of=$0.$file_subscript 2>/dev/null # Suppress messages from dd: ^^^^^^^^^^^ exit $? # A program whose only output is its own source code #+ is called a "quine" per Willard Quine. # Does this script qualify as a quine? Example 15-58. Exercising dd #!/bin/bash # exercising-dd.sh # Script by Stephane Chazelas. # Somewhat modified by ABS Guide author. infile=$0 # This script. outfile=log.txt # Output file left behind. n=3 p=5 dd if=$infile of=$outfile bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null # Extracts characters n to p (3 to 5) from this script. # -------------------------------------------------------- echo -n "hello world" | dd cbs=1 conv=unblock 2> /dev/null # Echoes "hello world" vertically. # Why? A newline follows each character dd emits. exit 0 To demonstrate just how versatile dd is, let's use it to capture keystrokes. Example 15-59. Capturing Keystrokes #!/bin/bash # dd-keypress.sh: Capture keystrokes without needing to press ENTER. keypresses=4 # Number of keypresses to capture. old_tty_setting=$(stty -g) # Save old terminal settings. echo "Press $keypresses keys." stty -icanon -echo # Disable canonical mode. # Disable local echo. keys=$(dd bs=1 count=$keypresses 2> /dev/null) # 'dd' uses stdin, if "if" (input file) not specified. stty "$old_tty_setting" # Restore old terminal settings. echo "You pressed the \"$keys\" keys." # Thanks, Stephane Chazelas, for showing the way. exit 0 The dd command can do random access on a data stream. echo -n . | dd bs=1 seek=4 of=file conv=notrunc # The "conv=notrunc" option means that the output file #+ will not be truncated. # Thanks, S.C. The dd command can copy raw data and disk images to and from devices, such as floppies and tape drives (Example A-5). A common use is creating boot floppies. dd if=kernel-image of=/dev/fd0H1440 Similarly, dd can copy the entire contents of a floppy, even one formatted with a "foreign" OS, to the hard drive as an image file. dd if=/dev/fd0 of=/home/bozo/projects/floppy.img Other applications of dd include initializing temporary swap files (Example 28-2) and ramdisks (Example 28-3). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended. People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of dd. Example 15-60. Securely deleting a file #!/bin/bash # blot-out.sh: Erase "all" traces of a file. # This script overwrites a target file alternately #+ with random bytes, then zeros before finally deleting it. # After that, even examining the raw disk sectors by conventional methods #+ will not reveal the original file data. PASSES=7 # Number of file-shredding passes. # Increasing this slows script execution, #+ especially on large target files. BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size, #+ otherwise you get weird results. E_BADARGS=70 # Various error exit codes. E_NOT_FOUND=71 E_CHANGED_MIND=72 if [ -z "$1" ] # No filename specified. then echo "Usage: `basename $0` filename" exit $E_BADARGS fi file=$1 if [ ! -e "$file" ] then echo "File \"$file\" not found." exit $E_NOT_FOUND fi echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? " read answer case "$answer" in [nN]) echo "Changed your mind, huh?" exit $E_CHANGED_MIND ;; *) echo "Blotting out file \"$file\".";; esac flength=$(ls -l "$file" | awk '{print $5}') # Field 5 is file length. pass_count=1 chmod u+w "$file" # Allow overwriting/deleting the file. echo while [ "$pass_count" -le "$PASSES" ] do echo "Pass #$pass_count" sync # Flush buffers. dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength # Fill with random bytes. sync # Flush buffers again. dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength # Fill with zeros. sync # Flush buffers yet again. let "pass_count += 1" echo done rm -f $file # Finally, delete scrambled and shredded file. sync # Flush buffers a final time. echo "File \"$file\" blotted out and deleted."; echo exit 0 # This is a fairly secure, if inefficient and slow method #+ of thoroughly "shredding" a file. # The "shred" command, part of the GNU "fileutils" package, #+ does the same thing, although more efficiently. # The file cannot not be "undeleted" or retrieved by normal methods. # However . . . #+ this simple method would *not* likely withstand #+ sophisticated forensic analysis. # This script may not play well with a journaled file system. # Exercise (difficult): Fix it so it does. # Tom Vier's "wipe" file-deletion package does a much more thorough job #+ of file shredding than this simple script. # http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2 # For an in-depth analysis on the topic of file deletion and security, #+ see Peter Gutmann's paper, #+ "Secure Deletion of Data From Magnetic and Solid-State Memory". # http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html See also the dd thread entry in the bibliography. od The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as /dev/urandom, and as a filter for binary data. head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p' # Sample output: 1324725719, 3918166450, 2989231420, etc. # From rnd.sh example script, by Stéphane Chazelas See also Example 9-31 and Example A-36. hexdump Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough equivalent of od, above, but not nearly as useful. May be used to view the contents of a binary file, in combination with dd and less. dd if=/bin/ls | hexdump -C | less # The -C option nicely formats the output in tabular form. objdump Displays information about an object file or binary executable in either hexadecimal form or as a disassembled listing (with the -d option). +---------------------------------------------------------+ |bash$ objdump -d /bin/ls | |/bin/ls: file format elf32-i386 | | | | Disassembly of section .init: | | | | 080490bc <.init>: | | 80490bc: 55 push %ebp | | 80490bd: 89 e5 mov %esp,%ebp| | . . . | | | +---------------------------------------------------------+ mcookie This command generates a "magic cookie," a 128-bit (32-character) pseudorandom hexadecimal number, normally used as an authorization "signature" by the X server. This also available for use in a script as a "quick 'n dirty" random number. random000=$(mcookie) Of course, a script could use md5sum for the same purpose. # Generate md5 checksum on the script itself. random001=`md5sum $0 | awk '{print $1}'` # Uses 'awk' to strip off the filename. The mcookie command gives yet another way to generate a "unique" filename. Example 15-61. Filename generator #!/bin/bash # tempfile-name.sh: temp filename generator BASE_STR=`mcookie` # 32-character magic cookie. POS=11 # Arbitrary position in magic cookie string. LEN=5 # Get $LEN consecutive characters. prefix=temp # This is, after all, a "temp" file. # For more "uniqueness," generate the #+ filename prefix using the same method #+ as the suffix, below. suffix=${BASE_STR:POS:LEN} # Extract a 5-character string, #+ starting at position 11. temp_filename=$prefix.$suffix # Construct the filename. echo "Temp filename = "$temp_filename"" # sh tempfile-name.sh # Temp filename = temp.e19ea # Compare this method of generating "unique" filenames #+ with the 'date' method in ex51.sh. exit 0 units This utility converts between different units of measure. While normally invoked in interactive mode, units may find use in a script. Example 15-62. Converting meters to miles #!/bin/bash # unit-conversion.sh convert_units () # Takes as arguments the units to convert. { cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}') # Strip off everything except the actual conversion factor. echo "$cf" } Unit1=miles Unit2=meters cfactor=`convert_units $Unit1 $Unit2` quantity=3.73 result=$(echo $quantity*$cfactor | bc) echo "There are $result $Unit2 in $quantity $Unit1." # What happens if you pass incompatible units, #+ such as "acres" and "miles" to the function? exit 0 m4 A hidden treasure, m4 is a powerful macro [77] processing filter, virtually a complete language. Although originally written as a pre-processor for RatFor, m4 turned out to be useful as a stand-alone utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to its extensive macro expansion facilities. The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses. Example 15-63. Using m4 #!/bin/bash # m4.sh: Using the m4 macro processor # Strings string=abcdA01 echo "len($string)" | m4 # 7 echo "substr($string,4)" | m4 # A01 echo "regexp($string,[0-1][0-1],\&Z)" | m4 # 01Z # Arithmetic echo "incr(22)" | m4 # 23 echo "eval(99 / 3)" | m4 # 33 exit xmessage This X-based variant of echo pops up a message/query window on the desktop. xmessage Left click to continue -button okay zenity The [http://freshmeat.net/projects/zenity] zenity utility is adept at displaying GTK+ dialog widgets and very suitable for scripting purposes. doexec The doexec command enables passing an arbitrary list of arguments to a binary executable. In particular, passing argv[0] (which corresponds to $0 in a script) lets the executable be invoked by various names, and it can then carry out different sets of actions, according to the name by which it was called. What this amounts to is roundabout way of passing options to an executable. For example, the /usr/local/bin directory might contain a binary called "aaa". Invoking doexec /usr/local/bin/aaa list would list all those files in the current working directory beginning with an "a", while invoking (the same executable with) doexec /usr/local/bin/aaa delete would delete those files. Note The various behaviors of the executable must be defined within the code of the executable itself, analogous to something like the following in a shell script: case `basename $0` in "name1" ) do_something;; "name2" ) do_something_else;; "name3" ) do_yet_another_thing;; * ) bail_out;; esac dialog The dialog family of tools provide a method of calling interactive "dialog" boxes from a script. The more elaborate variations of dialog -- gdialog, Xdialog, and kdialog -- actually invoke X-Windows widgets. sox The sox, or "sound exchange" command plays and performs transformations on sound files. In fact, the /usr/bin/play executable (now deprecated) is nothing but a shell wrapper for sox. For example, sox soundfile.wav soundfile.au changes a WAV sound file into a (Sun audio format) AU sound file. Shell scripts are ideally suited for batch-processing sox operations on sound files. For examples, see the Linux Radio Timeshift HOWTO and the MP3do Project. ------------------------------------------------------------------------ Chapter 16. System and Administrative Commands The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused. Users and Groups users Show all logged on users. This is the approximate equivalent of who -q. groups Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers. +-------------------------------------------------------+ |bash$ groups | |bozita cdrom cdwriter audio xgrp | | | |bash$ echo $GROUPS | |501 | +-------------------------------------------------------+ chown, chgrp The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. [78] +-------------------------------------------------------+ |root# chown bozo *.txt | | | | | +-------------------------------------------------------+ The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation. chgrp --recursive dunderheads *.data # The "dunderheads" group will now own all the "*.data" files #+ all the way down the $PWD directory tree (that's what "recursive" means). useradd, userdel The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system [79] and deletes associated files. Note The adduser command is a synonym for useradd and is usually a symbolic link to it. usermod Modify a user account. Changes may be made to the password, group membership, expiration date, and other attributes of a given user's account. With this command, a user's password may be locked, which has the effect of disabling the account. groupmod Modify a given group. The group name and/or ID number may be changed using this command. id The id command lists the real and effective user IDs and the group IDs of the user associated with the current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables. +-----------------------------------------------------------------------------+ |bash$ id | |uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)| | | |bash$ echo $UID | |501 | +-----------------------------------------------------------------------------+ Note The id command shows the effective IDs only when they differ from the real ones. Also see Example 9-5. lid The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users belonging to a given group. May be invoked only by root. +-------------------------------------------------------+ |root# lid bozo | | bozo(gid=500) | | | | | |root# lid daemon | | bin(gid=1) | | daemon(gid=2) | | adm(gid=4) | | lp(gid=7) | | | +-------------------------------------------------------+ who Show all users logged on to the system. +-------------------------------------------------------+ |bash$ who | |bozo tty1 Apr 27 17:45 | | bozo pts/0 Apr 27 17:46 | | bozo pts/1 Apr 27 17:47 | | bozo pts/2 Apr 27 17:49 | | | +-------------------------------------------------------+ The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man. +-------------------------------------------------------+ |bash$ who -m | |localhost.localdomain!bozo pts/2 Apr 27 17:49 | | | +-------------------------------------------------------+ whoami is similar to who -m, but only lists the user name. +-------------------------------------------------------+ |bash$ whoami | |bozo | | | +-------------------------------------------------------+ w Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process. +--------------------------------------------------------------------+ |bash$ w | grep startx | |bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx| +--------------------------------------------------------------------+ logname Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above. +-------------------------------------------------------+ |bash$ logname | |bozo | | | |bash$ whoami | |bozo | +-------------------------------------------------------+ However . . . +-------------------------------------------------------+ |bash$ su | |Password: ...... | | | |bash# whoami | |root | |bash# logname | |bozo | +-------------------------------------------------------+ Note While logname prints the name of the logged in user, whoami gives the name of the user attached to the current process. As we have just seen, sometimes these are not the same. su Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See Example A-14. sudo Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script. #!/bin/bash # Some commands. sudo cp /root/secretfile /home/bozo/secret # Some more commands. The file /etc/sudoers holds the names of users permitted to invoke sudo. passwd Sets, changes, or manages a user's password. The passwd command can be used in a script, but probably should not be. Example 16-1. Setting a new password #!/bin/bash # setnew-password.sh: For demonstration purposes only. # Not a good idea to actually run this script. # This script must be run as root. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=65 # Not root? E_NOSUCHUSER=70 SUCCESS=0 if [ "$UID" -ne "$ROOT_UID" ] then echo; echo "Only root can run this script."; echo exit $E_WRONG_USER else echo echo "You should know better than to run this script, root." echo "Even root users get the blues... " echo fi username=bozo NEWPASSWORD=security_violation # Check if bozo lives here. grep -q "$username" /etc/passwd if [ $? -ne $SUCCESS ] then echo "User $username does not exist." echo "No password changed." exit $E_NOSUCHUSER fi echo "$NEWPASSWORD" | passwd --stdin "$username" # The '--stdin' option to 'passwd' permits #+ getting a new password from stdin (or a pipe). echo; echo "User $username's password changed!" # Using the 'passwd' command in a script is dangerous. exit 0 The passwd command's -l, -u, and -d options permit locking, unlocking, and deleting a user's password. Only root may use these options. ac Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities. +-------------------------------------------------------+ |bash$ ac | | total 68.08 | +-------------------------------------------------------+ last List last logged in users, as read from /var/log/wtmp. This command can also show remote logins. For example, to show the last few times the system rebooted: +-----------------------------------------------------------------------------+ |bash$ last reboot | |reboot system boot 2.6.9-1.667 Fri Feb 4 18:18 (00:02) | | reboot system boot 2.6.9-1.667 Fri Feb 4 15:20 (01:27) | | reboot system boot 2.6.9-1.667 Fri Feb 4 12:56 (00:49) | | reboot system boot 2.6.9-1.667 Thu Feb 3 21:08 (02:17) | | . . . | | | | wtmp begins Tue Feb 1 12:50:09 2005 | +-----------------------------------------------------------------------------+ newgrp Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds only limited use. Note Kurt Glaesemann points out that the newgrp command could prove helpful in setting the default group permissions for files a user writes. However, the chgrp command might be more convenient for this purpose. Terminals tty Echoes the name (filename) of the current user's terminal. Note that each separate xterm window counts as a different terminal. +-------------------------------------------------------+ |bash$ tty | |/dev/pts/1 | +-------------------------------------------------------+ stty Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully. Example 16-2. Setting an erase character #!/bin/bash # erase.sh: Using "stty" to set an erase character when reading input. echo -n "What is your name? " read name # Try to backspace #+ to erase characters of input. # Problems? echo "Your name is $name." stty erase '#' # Set "hashmark" (#) as erase character. echo -n "What is your name? " read name # Use # to erase last character typed. echo "Your name is $name." exit 0 # Even after the script exits, the new key value remains set. # Exercise: How would you reset the erase character to the default value? Example 16-3. secret password: Turning off terminal echoing #!/bin/bash # secret-pw.sh: secret password echo echo -n "Enter password " read passwd echo "password is $passwd" echo -n "If someone had been looking over your shoulder, " echo "your password would have been compromised." echo && echo # Two line-feeds in an "and list." stty -echo # Turns off screen echo. echo -n "Enter password again " read passwd echo echo "password is $passwd" echo stty echo # Restores screen echo. exit 0 # Do an 'info stty' for more on this useful-but-tricky command. A creative use of stty is detecting a user keypress (without hitting ENTER). Example 16-4. Keypress detection #!/bin/bash # keypress.sh: Detect a user keypress ("hot keys"). echo old_tty_settings=$(stty -g) # Save old settings (why?). stty -icanon Keypress=$(head -c1) # or $(dd bs=1 count=1 2> /dev/null) # on non-GNU systems echo echo "Key pressed was \""$Keypress"\"." echo stty "$old_tty_settings" # Restore old settings. # Thanks, Stephane Chazelas. exit 0 Also see Example 9-3 and Example A-43. +------------------------------------------------------------------------------------------+ |terminals and modes | | | |Normally, a terminal works in the canonical mode. When a user hits a key, the resulting | |character does not immediately go to the program actually running in this terminal. A | |buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this | |sends all the stored keystrokes to the program running. There is even a basic line editor | |inside the terminal. | | | |+--------------------------------------------------------------------------------------+ | ||bash$ stty -a | | ||speed 9600 baud; rows 36; columns 96; line = 0; | | || intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = ; eol2 = ;| | || start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; | | || ... | | || isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt | | || | | |+--------------------------------------------------------------------------------------+ | | | |Using canonical mode, it is possible to redefine the special keys for the local terminal | |line editor. | | | |+-------------------------------------------------------------------------------+ | ||bash$ cat > filexxx | | ||whaIfoo barhello world | | || | | ||bash$ cat filexxx | | ||hello world | | ||bash$ wc -c < filexxx | | ||12 | | || | | |+-------------------------------------------------------------------------------+ | | | |The process controlling the terminal receives only 12 characters (11 alphabetic ones, plus| |a newline), although the user hit 26 keys. | | | |In non-canonical ("raw") mode, every key hit (including special editing keys such as | |ctl-H) sends a character immediately to the controlling process. | | | |The Bash prompt disables both icanon and echo, since it replaces the basic terminal line | |editor with its own more elaborate one. For example, when you hit ctl-A at the Bash | |prompt, there's no ^A echoed by the terminal, but Bash gets a \1 character, interprets it,| |and moves the cursor to the begining of the line. | | | |Stéphane Chazelas | +------------------------------------------------------------------------------------------+ setterm Set certain terminal attributes. This command writes to its terminal's stdout a string that changes the behavior of that terminal. +-------------------------------------------------------+ |bash$ setterm -cursor off | |bash$ | | | +-------------------------------------------------------+ The setterm command can be used within a script to change the appearance of text written to stdout, although there are certainly better tools available for this purpose. setterm -bold on echo bold hello setterm -bold off echo normal hello tset Show or initialize terminal settings. This is a less capable version of stty. +-------------------------------------------------------+ |bash$ tset -r | |Terminal type is xterm-xfree86. | | Kill is control-U (^U). | | Interrupt is control-C (^C). | | | +-------------------------------------------------------+ setserial Set or display serial port parameters. This command must be run by root and is usually found in a system setup script. # From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ getty, agetty The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty. mesg Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal. Tip It can be quite annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions. wall This is an acronym for "write all," i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 18-1). +---------------------------------------------------------------+ |bash$ wall System going down for maintenance in 5 minutes! | |Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001...| | | | System going down for maintenance in 5 minutes! | | | +---------------------------------------------------------------+ Note If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to that terminal. Information and Statistics uname Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see Example 15-5). The -s option shows only the OS type. +-----------------------------------------------------------------+ |bash$ uname | |Linux | | | |bash$ uname -s | |Linux | | | | | |bash$ uname -a | |Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006| | i686 i686 i386 GNU/Linux | +-----------------------------------------------------------------+ arch Show system architecture. Equivalent to uname -m. See Example 10-26. +-------------------------------------------------------+ |bash$ arch | |i686 | | | |bash$ uname -m | |i686 | +-------------------------------------------------------+ lastcomm Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities. lastlog List the last login time of all system users. This references the /var/log/lastlog file. +-----------------------------------------------------------------------+ |bash$ lastlog | |root tty1 Fri Dec 7 18:43:21 -0700 2001 | | bin **Never logged in** | | daemon **Never logged in** | | ... | | bozo tty1 Sat Dec 8 21:14:29 -0700 2001| | | | | | | |bash$ lastlog | grep root | |root tty1 Fri Dec 7 18:43:21 -0700 2001 | | | +-----------------------------------------------------------------------+ Caution This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file. lsof List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results. +----------------------------------------------------------------------------------+ |bash$ lsof | |COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME | | init 1 root mem REG 3,5 30748 30303 /sbin/init | | init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.so | | init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.3.so| | cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr | | ... | | | +----------------------------------------------------------------------------------+ The lsof command is a useful, if complex administrative tool. If you are unable to dismount a filesystem and get an error message that it is still in use, then running lsof helps determine which files are still open on that filesystem. The -i option lists open network socket files, and this can help trace intrusion or hack attempts. +---------------------------------------------------------------------------------------+ |bash$ lsof -an -i tcp | |COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME | | firefox 2330 bozo 32u IPv4 9956 TCP 66.0.118.137:57596->67.112.7.104:http ...| | firefox 2330 bozo 38u IPv4 10535 TCP 66.0.118.137:57708->216.79.48.24:http ...| | | +---------------------------------------------------------------------------------------+ strace System trace: diagnostic and debugging tool for tracing system calls and signals. This command and ltrace, following, are useful for diagnosing why a given program or package fails to run . . . perhaps due to missing libraries or related causes. +-------------------------------------------------------+ |bash$ strace df | |execve("/bin/df", ["df"], [/* 45 vars */]) = 0 | | uname({sys="Linux", node="bozo.localdomain", ...}) = 0| | brk(0) = 0x804f5e4 | | | | ... | | | +-------------------------------------------------------+ This is the Linux equivalent of the Solaris truss command. ltrace Library trace: diagnostic and debugging tool that traces library calls invoked by a given command. +----------------------------------------------------------------------------------+ |bash$ ltrace df | |__libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 :| | setlocale(6, "") = "en_US.UTF-8" | |bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale" | |textdomain("coreutils") = "coreutils" | |__cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0 | |getenv("DF_BLOCK_SIZE") = NULL | | | | ... | | | +----------------------------------------------------------------------------------+ nmap Network mapper and port scanner. This command scans a server to locate open ports and the services associated with those ports. It can also report information about packet filters and firewalls. This is an important security tool for locking down a network against hacking attempts. #!/bin/bash SERVER=$HOST # localhost.localdomain (127.0.0.1). PORT_NUMBER=25 # SMTP port. nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open? # grep -w matches whole words only, #+ so this wouldn't match port 1025, for example. exit 0 # 25/tcp open smtp nc The nc (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and servers. +-------------------------------------------------------+ |bash$ nc localhost.localdomain 25 | |220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;| | Thu, 31 Mar 2005 15:41:35 -0700 | +-------------------------------------------------------+ Example 16-5. Checking a remote server for identd #! /bin/sh ## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed. ## Args: target port [port port port ...] ## Hose stdout _and_ stderr together. ## ## Advantages: runs slower than ident-scan, giving remote inetd less cause ##+ for alarm, and only hits the few known daemon ports you specify. ## Disadvantages: requires numeric-only port args, the output sleazitude, ##+ and won't work for r-services when coming from high source ports. # Script author: Hobbit # Used in ABS Guide with permission. # --------------------------------------------------- E_BADARGS=65 # Need at least two args. TWO_WINKS=2 # How long to sleep. THREE_WINKS=3 IDPORT=113 # Authentication "tap ident" port. RAND1=999 RAND2=31337 TIMEOUT0=9 TIMEOUT1=8 TIMEOUT2=4 # --------------------------------------------------- case "${2}" in "" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;; esac # Ping 'em once and see if they *are* running identd. nc -z -w $TIMEOUT0 "$1" $IDPORT || \ { echo "Oops, $1 isn't running identd." ; exit 0 ; } # -z scans for listening daemons. # -w $TIMEOUT = How long to try to connect. # Generate a randomish base port. RP=`expr $$ % $RAND1 + $RAND2` TRG="$1" shift while test "$1" ; do nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null & PROC=$! sleep $THREE_WINKS echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1 sleep $TWO_WINKS # Does this look like a lamer script or what . . . ? # ABS Guide author comments: "Ain't really all that bad . . . #+ kinda clever, actually." kill -HUP $PROC RP=`expr ${RP} + 1` shift done exit $? # Notes: # ----- # Try commenting out line 30 and running this script #+ with "localhost.localdomain 25" as arguments. # For more of Hobbit's 'nc' example scripts, #+ look in the documentation: #+ the /usr/share/doc/nc-X.XX/scripts directory. And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair: echo clone | nc thunk.org 5000 > e2fsprogs.dat free Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more. +-----------------------------------------------------------------------------+ |bash$ free | | total used free shared buffers cached | | Mem: 30504 28624 1880 15820 1608 16376| | -/+ buffers/cache: 10640 19864 | | Swap: 68540 3128 65412 | +-----------------------------------------------------------------------------+ To show unused RAM memory: +-------------------------------------------------------+ |bash$ free | grep Mem | awk '{ print $4 }' | |1880 | +-------------------------------------------------------+ procinfo Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing. +--------------------------------------------------------------------------+ |bash$ procinfo | grep Bootup | |Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829| +--------------------------------------------------------------------------+ lsdev List devices, that is, show installed hardware. +-------------------------------------------------------+ |bash$ lsdev | |Device DMA IRQ I/O Ports | | ------------------------------------------------ | | cascade 4 2 | | dma 0080-008f | | dma1 0000-001f | | dma2 00c0-00df | | fpu 00f0-00ff | | ide0 14 01f0-01f7 03f6-03f6 | | ... | | | +-------------------------------------------------------+ du Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified. +-------------------------------------------------------+ |bash$ du -ach | |1.0k ./wi.sh | | 1.0k ./tst.sh | | 1.0k ./random.file | | 6.0k . | | 6.0k total | +-------------------------------------------------------+ df Shows filesystem usage in tabular form. +------------------------------------------------------------------+ |bash$ df | |Filesystem 1k-blocks Used Available Use% Mounted on| | /dev/hda5 273262 92607 166547 36% / | | /dev/hda8 222525 123951 87085 59% /home | | /dev/hda7 1408796 1075744 261488 80% /usr | +------------------------------------------------------------------+ dmesg Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script. +-------------------------------------------------------------+ |bash$ dmesg | grep hda | |Kernel command line: ro root=/dev/hda2 | | hda: IBM-DLGA-23080, ATA DISK drive | | hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63| | hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4 | | | +-------------------------------------------------------------+ stat Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files. +---------------------------------------------------------------------------+ |bash$ stat test.cru | | File: "test.cru" | | Size: 49970 Allocated Blocks: 100 Filetype: Regular File| | Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo) | | Device: 3,8 Inode: 18185 Links: 1 | | Access: Sat Jun 2 16:40:24 2001 | | Modify: Sat Jun 2 16:40:24 2001 | | Change: Sat Jun 2 16:40:24 2001 | | | +---------------------------------------------------------------------------+ If the target file does not exist, stat returns an error message. +-------------------------------------------------------+ |bash$ stat nonexistent-file | |nonexistent-file: No such file or directory | | | +-------------------------------------------------------+ In a script, you can use stat to extract information about files (and filesystems) and set variables accordingly. #!/bin/bash # fileinfo2.sh # Per suggestion of Joël Bourquard and . . . # http://www.linuxquestions.org/questions/showthread.php?t=410766 FILENAME=testfile.txt file_name=$(stat -c%n "$FILENAME") # Same as "$FILENAME" of course. file_owner=$(stat -c%U "$FILENAME") file_size=$(stat -c%s "$FILENAME") # Certainly easier than using "ls -l $FILENAME" #+ and then parsing with sed. file_inode=$(stat -c%i "$FILENAME") file_type=$(stat -c%F "$FILENAME") file_access_rights=$(stat -c%A "$FILENAME") echo "File name: $file_name" echo "File owner: $file_owner" echo "File size: $file_size" echo "File inode: $file_inode" echo "File type: $file_type" echo "File access rights: $file_access_rights" exit 0 sh fileinfo2.sh File name: testfile.txt File owner: bozo File size: 418 File inode: 1730378 File type: regular file File access rights: -rw-rw-r-- vmstat Display virtual memory statistics. +------------------------------------------------------------------------------+ |bash$ vmstat | | procs memory swap io system cpu | | r b w swpd free buff cache si so bi bo in cs us sy id| | 0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89| | | +------------------------------------------------------------------------------+ netstat Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (Chapter 27). See Example 27-4. netstat -r is equivalent to route. +--------------------------------------------------------------------------------+ |bash$ netstat | |Active Internet connections (w/o servers) | | Proto Recv-Q Send-Q Local Address Foreign Address State | | Active UNIX domain sockets (w/o servers) | | Proto RefCnt Flags Type State I-Node Path | | unix 11 [ ] DGRAM 906 /dev/log | | unix 3 [ ] STREAM CONNECTED 4514 /tmp/.X11-unix/X0 | | unix 3 [ ] STREAM CONNECTED 4513 | | . . . | +--------------------------------------------------------------------------------+ Note A netstat -lptu shows sockets that are listening to ports, and the associated processes. This can be useful for determining whether a computer has been hacked or compromised. uptime Shows how long the system has been running, along with associated statistics. +------------------------------------------------------------+ |bash$ uptime | |10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27| +------------------------------------------------------------+ Note A load average of 1 or less indicates that the system handles processes immediately. A load average greater than 1 means that processes are being queued. When the load average gets above 3, then system performance is significantly degraded. hostname Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable. +-------------------------------------------------------+ |bash$ hostname | |localhost.localdomain | | | |bash$ echo $HOSTNAME | |localhost.localdomain | +-------------------------------------------------------+ Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to hostname also perform these functions. hostid Echo a 32-bit hexadecimal numerical identifier for the host machine. +-------------------------------------------------------+ |bash$ hostid | |7f0100 | +-------------------------------------------------------+ Note This command allegedly fetches a "unique" serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed. The network address of a typical non-networked Linux machine, is found in /etc/hosts. +-------------------------------------------------------+ |bash$ cat /etc/hosts | |127.0.0.1 localhost.localdomain localhost| +-------------------------------------------------------+ As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid. sar Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation ("Old" SCO) released sar as Open Source in June, 1999. This command is not part of the base Linux distribution, but may be obtained as part of the[http://perso.wanadoo.fr/sebastien.godard/] sysstat utilities package, written by Sebastien Godard. +-----------------------------------------------------------------------+ |bash$ sar | |Linux 2.4.9 (brooks.seringas.fr) 09/26/03 | | | |10:30:00 CPU %user %nice %system %iowait %idle| |10:40:00 all 2.21 10.90 65.48 0.00 21.41| |10:50:00 all 3.36 0.00 72.36 0.00 24.28| |11:00:00 all 1.12 0.00 80.77 0.00 18.11| |Average: all 2.23 3.63 72.87 0.00 21.27| | | |14:32:30 LINUX RESTART | | | |15:00:00 CPU %user %nice %system %iowait %idle| |15:10:00 all 8.59 2.40 17.47 0.00 71.54| |15:20:00 all 4.07 1.00 11.95 0.00 82.98| |15:30:00 all 0.79 2.94 7.56 0.00 88.71| |Average: all 6.33 1.70 14.71 0.00 77.26| | | +-----------------------------------------------------------------------+ readelf Show information and statistics about a designated elf binary. This is part of the binutils package. +-------------------------------------------------------------------+ |bash$ readelf -h /bin/bash | |ELF Header: | | Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 | | Class: ELF32 | | Data: 2's complement, little endian| | Version: 1 (current) | | OS/ABI: UNIX - System V | | ABI Version: 0 | | Type: EXEC (Executable file) | | . . . | +-------------------------------------------------------------------+ size The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers. +-------------------------------------------------------+ |bash$ size /bin/bash | | text data bss dec hex filename | | 495971 22496 17392 535859 82d33 /bin/bash | | | +-------------------------------------------------------+ System Logs logger Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger. logger Experiencing instability in network connection at 23:10, 05/21. # Now, do a 'tail /var/log/messages'. By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages. logger -t $0 -i Logging at line "$LINENO". # The "-t" option specifies the tag for the logger entry. # The "-i" option records the process ID. # tail /var/log/message # ... # Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3. logrotate This utility manages the system log files, rotating, compressing, deleting, and/or e-mailing them, as appropriate. This keeps the /var/log from getting cluttered with old log files. Usually cron runs logrotate on a daily basis. Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system-wide ones. Note Stefano Falsetto has created [http://www.gnu.org/software/rottlog/] rottlog, which he considers to be an improved version of logrotate. Job Control ps Process Statistics: lists currently executing processes by owner and PID (process ID). This is usually invoked with ax or aux options, and may be piped to grep or sed to search for a specific process (see Example 14-14 and Example 27-3). +-----------------------------------------------------------------+ |bash$ ps ax | grep sendmail | |295 ? S 0:00 sendmail: accepting connections on port 25| +-----------------------------------------------------------------+ To display system processes in graphical "tree" format: ps afjx or ps ax --forest. pgrep, pkill Combining the ps command with grep or kill. +-------------------------------------------------------+ |bash$ ps a | grep mingetty | |2212 tty2 Ss+ 0:00 /sbin/mingetty tty2 | | 2213 tty3 Ss+ 0:00 /sbin/mingetty tty3 | | 2214 tty4 Ss+ 0:00 /sbin/mingetty tty4 | | 2215 tty5 Ss+ 0:00 /sbin/mingetty tty5 | | 2216 tty6 Ss+ 0:00 /sbin/mingetty tty6 | | 4849 pts/2 S+ 0:00 grep mingetty | | | | | |bash$ pgrep mingetty | |2212 mingetty | | 2213 mingetty | | 2214 mingetty | | 2215 mingetty | | 2216 mingetty | | | +-------------------------------------------------------+ Compare the action of pkill with killall. pstree Lists currently executing processes in "tree" format. The -p option shows the PIDs, as well as the process names. top Continuously updated display of most cpu-intensive processes. The -b option displays in text mode, so that the output may be parsed or accessed from a script. +--------------------------------------------------------------------------------+ |bash$ top -b | | 8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13 | | 45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped | | CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle | | Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff | | Swap: 157208K av, 0K used, 157208K free 37244K cached| | | | PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND | | 848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top | | 1 root 8 0 512 512 444 S 0.0 0.6 0:04 init | | 2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd | | ... | | | +--------------------------------------------------------------------------------+ nice Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are renice and snice, which change the priority of a running process or processes, and skill, which sends a kill signal to a process or processes. nohup Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process. pidof Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act on the PID of a process (not its name), it is sometimes necessary to identify that PID. The pidof command is the approximate counterpart to the $PPID internal variable. +-------------------------------------------------------+ |bash$ pidof xclock | |880 | | | +-------------------------------------------------------+ Example 16-6. pidof helps kill a process #!/bin/bash # kill-process.sh NOPROCESS=2 process=xxxyyyzzz # Use nonexistent process. # For demo purposes only... # ... don't want to actually kill any actual process with this script. # # If, for example, you wanted to use this script to logoff the Internet, # process=pppd t=`pidof $process` # Find pid (process id) of $process. # The pid is needed by 'kill' (can't 'kill' by program name). if [ -z "$t" ] # If process not present, 'pidof' returns null. then echo "Process $process was not running." echo "Nothing killed." exit $NOPROCESS fi kill $t # May need 'kill -9' for stubborn process. # Need a check here to see if process allowed itself to be killed. # Perhaps another " t=`pidof $process` " or ... # This entire script could be replaced by # kill $(pidof -x process_name) # or # killall process_name # but it would not be as instructive. exit 0 fuser Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be invoked with the -k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services. +--------------------------------------------------------------------+ |bash$ fuser -u /usr/bin/vim | |/usr/bin/vim: 3207e(bozo) | | | | | | | |bash$ fuser -u /dev/null | |/dev/null: 3009(bozo) 3010(bozo) 3197(bozo) 3199(bozo)| | | +--------------------------------------------------------------------+ One important application for fuser is when physically inserting or removing storage media, such as CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error message. This means that some user(s) and/or process(es) are accessing the device. An fuser -um /dev/device_name will clear up the mystery, so you can kill any relevant processes. +-------------------------------------------------------+ |bash$ umount /mnt/usbdrive | |umount: /mnt/usbdrive: device is busy | | | | | | | |bash$ fuser -um /dev/usbdrive | |/mnt/usbdrive: 1772c(bozo) | | | |bash$ kill -9 1772 | |bash$ umount /mnt/usbdrive | | | +-------------------------------------------------------+ The fuser command, invoked with the -n option identifies the processes accessing a port. This is especially useful in combination with nmap. +---------------------------------------------------------+ |root# nmap localhost.localdomain | |PORT STATE SERVICE | | 25/tcp open smtp | | | | | | | |root# fuser -un tcp 25 | |25/tcp: 2095(root) | | | |root# ps ax | grep 2095 | grep -v grep | |2095 ? Ss 0:00 sendmail: accepting connections| | | +---------------------------------------------------------+ cron Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab. Note Some flavors of Linux run crond, Matthew Dillon's version of cron. Process Control and Booting init The init command is the parent of all processes. Called in the final step of a bootup, init determines the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only. telinit Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous -- be certain you understand it well before using! runlevel Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file. halt, shutdown, reboot Command set to shut the system down, usually just prior to a power down. Warning On some Linux distros, the halt command has 755 permissions, so it can be invoked by a non-root user. A careless halt in a terminal or a script may shut down the system! service Starts or stops a system service. The startup scripts in /etc/init.d and /etc/rc.d use this command to start services at bootup. +--------------------------------------------------------------------+ |root# /sbin/service iptables stop | |Flushing firewall rules: [ OK ] | | Setting chains to policy ACCEPT: filter [ OK ]| | Unloading iptables modules: [ OK ]| | | +--------------------------------------------------------------------+ Network ifconfig Network interface configuration and tuning utility. +----------------------------------------------------------------+ |bash$ ifconfig -a | |lo Link encap:Local Loopback | | inet addr:127.0.0.1 Mask:255.0.0.0 | | UP LOOPBACK RUNNING MTU:16436 Metric:1 | | RX packets:10 errors:0 dropped:0 overruns:0 frame:0 | | TX packets:10 errors:0 dropped:0 overruns:0 carrier:0| | collisions:0 txqueuelen:0 | | RX bytes:700 (700.0 b) TX bytes:700 (700.0 b) | +----------------------------------------------------------------+ The ifconfig command is most often used at bootup to set up the interfaces, or to shut them down when rebooting. # Code snippets from /etc/rc.d/init.d/network # ... # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 0 [ -x /sbin/ifconfig ] || exit 0 # ... for i in $interfaces ; do if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then action "Shutting down interface $i: " ./ifdown $i boot fi # The GNU-specific "-q" option to "grep" means "quiet", i.e., #+ producing no output. # Redirecting output to /dev/null is therefore not strictly necessary. # ... echo "Currently active devices:" echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'` # ^^^^^ should be quoted to prevent globbing. # The following also work. # echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })' # echo $(/sbin/ifconfig | sed -e 's/ .*//') # Thanks, S.C., for additional comments. See also Example 29-6. iwconfig This is the command set for configuring a wireless network. It is the wireless equivalent of ifconfig, above. ip General purpose utility for setting up, changing, and analyzing IP (Internet Protocol) networks and attached devices. This command is part of the iproute2 package. +-------------------------------------------------------------------+ |bash$ ip link show | |1: lo: mtu 16436 qdisc noqueue | | link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 | | 2: eth0: mtu 1500 qdisc pfifo_fast qlen 1000| | link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff | | 3: sit0: mtu 1480 qdisc noop | | link/sit 0.0.0.0 brd 0.0.0.0 | | | | | |bash$ ip route list | |169.254.0.0/16 dev lo scope link | | | +-------------------------------------------------------------------+ Or, in a script: #!/bin/bash # Script by Juan Nicolas Ruiz # Used with his kind permission. # Setting up (and stopping) a GRE tunnel. # --- start-tunnel.sh --- LOCAL_IP="192.168.1.17" REMOTE_IP="10.0.5.33" OTHER_IFACE="192.168.0.100" REMOTE_NET="192.168.3.0/24" /sbin/ip tunnel add netb mode gre remote $REMOTE_IP \ local $LOCAL_IP ttl 255 /sbin/ip addr add $OTHER_IFACE dev netb /sbin/ip link set netb up /sbin/ip route add $REMOTE_NET dev netb exit 0 ############################################# # --- stop-tunnel.sh --- REMOTE_NET="192.168.3.0/24" /sbin/ip route del $REMOTE_NET dev netb /sbin/ip link set netb down /sbin/ip tunnel del netb exit 0 route Show info about or make changes to the kernel routing table. +------------------------------------------------------------------------------+ |bash$ route | |Destination Gateway Genmask Flags MSS Window irtt Iface| | pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0| | 127.0.0.0 * 255.0.0.0 U 40 0 0 lo | | default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0| | | +------------------------------------------------------------------------------+ chkconfig Check network and system configuration. This command lists and manages the network and system services started at bootup in the /etc/rc?.d directory. Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of some Linux flavors. +----------------------------------------------------------------------+ |bash$ chkconfig --list | |atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off | | rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off| | ... | | | +----------------------------------------------------------------------+ tcpdump Network packet "sniffer." This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria. Dump ip packet traffic between hosts bozoville and caduceus: +-------------------------------------------------------+ |bash$ tcpdump ip host bozoville and caduceus | | | +-------------------------------------------------------+ Of course, the output of tcpdump can be parsed with certain of the previously discussed text processing utilities. Filesystem mount Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file /etc/fstab provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file /etc/mtab shows the currently mounted filesystems and partitions (including the virtual ones, such as /proc). mount -a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this to get everything mounted. mount -t iso9660 /dev/cdrom /mnt/cdrom # Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem. mount /mnt/cdrom # Shortcut, if /mnt/cdrom listed in /etc/fstab The versatile mount command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 filesystem image before burning it onto a CDR. [80] Example 16-7. Checking a CD image # As root... mkdir /mnt/cdtest # Prepare a mount point, if not already there. mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image. # "-o loop" option equivalent to "losetup /dev/loop0" cd /mnt/cdtest # Now, check the image. ls -alR # List the files in the directory tree there. # And so forth. umount Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be umounted, else filesystem corruption may result. umount /mnt/cdrom # You may now press the eject button and safely remove the disk. Note The automount utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On "multispindle" laptops with swappable floppy and optical drives, this can cause problems, however. gnome-mount The newer Linux distros have deprecated mount and umount. The successor, for command-line mounting of removable storage devices, is gnome-mount. It can take the -d option to mount a device file by its listing in /dev. For example, to mount a USB flash drive: +--------------------------------------------------------------------+ |bash$ gnome-mount -d /dev/sda1 | |gnome-mount 0.4 | | | | | |bash$ df | |. . . | | /dev/sda1 63584 12034 51550 19% /media/disk| | | +--------------------------------------------------------------------+ sync Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot. At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see Example 15-60) or when the lights begin to flicker. losetup Sets up and configures loopback devices. Example 16-8. Creating a filesystem in a file SIZE=1000000 # 1 meg head -c $SIZE < /dev/zero > file # Set up file of designated size. losetup /dev/loop0 file # Set it up as loopback device. mke2fs /dev/loop0 # Create filesystem. mount -o loop /dev/loop0 /mnt # Mount it. # Thanks, S.C. mkswap Creates a swap partition or file. The swap area must subsequently be enabled with swapon. swapon, swapoff Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown. mke2fs Create a Linux ext2 filesystem. This command must be invoked as root. Example 16-9. Adding a new hard drive #!/bin/bash # Adding a second hard drive to system. # Software configuration. Assumes hardware already mounted. # From an article by the author of the ABS Guide. # In issue #38 of _Linux Gazette_, http://www.linuxgazette.com. ROOT_UID=0 # This script must be run as root. E_NOTROOT=67 # Non-root exit error. if [ "$UID" -ne "$ROOT_UID" ] then echo "Must be root to run this script." exit $E_NOTROOT fi # Use with extreme caution! # If something goes wrong, you may wipe out your current filesystem. NEWDISK=/dev/hdb # Assumes /dev/hdb vacant. Check! MOUNTPOINT=/mnt/newdisk # Or choose another mount point. fdisk $NEWDISK mke2fs -cv $NEWDISK1 # Check for bad blocks (verbose output). # Note: ^ /dev/hdb1, *not* /dev/hdb! mkdir $MOUNTPOINT chmod 777 $MOUNTPOINT # Makes new drive accessible to all users. # Now, test ... # mount -t ext2 /dev/hdb1 /mnt/newdisk # Try creating a directory. # If it works, umount it, and proceed. # Final step: # Add the following line to /etc/fstab. # /dev/hdb1 /mnt/newdisk ext2 defaults 1 1 exit See also Example 16-8 and Example 28-3. tune2fs Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root. Warning This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem. dumpe2fs Dump (list to stdout) very verbose filesystem info. This must be invoked as root. +-------------------------------------------------------+ |root# dumpe2fs /dev/hda7 | grep 'ount count' | |dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09 | | Mount count: 6 | | Maximum mount count: 20 | +-------------------------------------------------------+ hdparm List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused. fdisk Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root. Warning Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem. fsck, e2fsck, debugfs Filesystem check, repair, and debug command set. fsck: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2. e2fsck: ext2 filesystem checker. debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only! Caution All of these should be invoked as root, and they can damage or destroy a filesystem if misused. badblocks Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. [81] As an example, badblocks /dev/fd0 tests a floppy disk. The badblocks command may be invoked destructively (overwrite all data) or in non-destructive read-only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command. lsusb, usbmodules The lsusb command lists all USB (Universal Serial Bus) buses and the devices hooked up to them. The usbmodules command outputs information about the driver modules for connected USB devices. +-------------------------------------------------------+ |bash$ lsusb | |Bus 001 Device 001: ID 0000:0000 | | Device Descriptor: | | bLength 18 | | bDescriptorType 1 | | bcdUSB 1.00 | | bDeviceClass 9 Hub | | bDeviceSubClass 0 | | bDeviceProtocol 0 | | bMaxPacketSize0 8 | | idVendor 0x0000 | | idProduct 0x0000 | | | | . . . | | | +-------------------------------------------------------+ lspci Lists pci busses present. +----------------------------------------------------------------------------+ |bash$ lspci | |00:00.0 Host bridge: Intel Corporation 82845 845 | | (Brookdale) Chipset Host Bridge (rev 04) | | 00:01.0 PCI bridge: Intel Corporation 82845 845 | | (Brookdale) Chipset AGP Bridge (rev 04) | | 00:1d.0 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #1) (rev 02)| | 00:1d.1 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #2) (rev 02)| | 00:1d.2 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #3) (rev 02)| | 00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 42) | | | | . . . | | | +----------------------------------------------------------------------------+ mkbootdisk Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. Of special interest is the --iso option, which uses mkisofs to create a bootable ISO9660 filesystem image suitable for burning a bootable CDR. The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory. mkisofs Creates an ISO9660 filesystem suitable for a CDR image. chroot CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a "chroot jail"). Note that after a chroot, the execution path for system binaries is no longer valid. A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin. Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as the base directory, rather than / as is normally the case. An alias XX 'chroot /aaa/bbb ls' in a user's ~/.bashrc effectively restricts which portion of the filesystem she may run command "XX" on. The chroot command is also handy when running from an emergency boot floppy (chroot to /dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care. Caution It might be necessary to copy certain system files to a chrooted directory, since the normal $PATH can no longer be relied upon. lockfile This utility is part of the procmail package ([http://www.procmail.org] www.procmail.org). It creates a lock file, a semaphore that controls access to a file, device, or resource. +--------------------------------------------------------------+ | Definition: A semaphore is a flag or signal. (The usage | | originated in railroading, where a colored flag, lantern, or | | striped movable arm semaphore indicated whether a particular | | track was in use and therefore unavailable for another | | train.) A UNIX process can check the appropriate semaphore | | to determine whether a particular resource is | | available/accessible. | +--------------------------------------------------------------+ The lock file serves as a flag that this particular file, device, or resource is in use by a process (and is therefore "busy"). The presence of a lock file permits only restricted access (or no access) to other processes. lockfile /home/bozo/lockfiles/$0.lock # Creates a write-protected lockfile prefixed with the name of the script. lockfile /home/bozo/lockfiles/${0##*/}.lock # A safer version of the above, as pointed out by E. Choroba. Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Firefox is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts to create a lock file that already exists, the script will likely hang. Normally, applications create and check for lock files in the /var/lock directory. [82] A script can test for the presence of a lock file by something like the following. appname=xyzip # Application "xyzip" created lock file "/var/lock/xyzip.lock". if [ -e "/var/lock/$appname.lock" ] then #+ Prevent other programs & scripts # from accessing files/resources used by xyzip. ... flock Much less useful than the lockfile command is flock. It sets an "advisory" lock on a file and then executes a command while the lock is on. This is to prevent any other process from setting a lock on that file until completion of the specified command. flock $0 cat $0 > lockfile__$0 # Set a lock on the script the above line appears in, #+ while listing the script to stdout. Note Unlike lockfile, flock does not automatically create a lock file. mknod Creates block or character device files (may be necessary when installing new hardware on the system). The MAKEDEV utility has virtually all of the functionality of mknod, and is easier to use. MAKEDEV Utility for creating device files. It must be run as root, and in the /dev directory. It is a sort of advanced version of mknod. tmpwatch Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by cron to remove stale log files. Backup dump, restore The dump command is an elaborate filesystem backup utility, generally used on larger installations and networks. [83] It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The restore command restores backups made with dump. fdformat Perform a low-level format on a floppy disk (/dev/fd0*). System Resources ulimit Sets an upper limit on use of system resources. Usually invoked with the -f option, which sets a limit on file size (ulimit -f 1000 limits files to 1 meg maximum). [84] The -t option limits the coredump size (ulimit -c 0 eliminates coredumps). Normally, the value of ulimit would be set in /etc/profile and/or ~/.bash_profile (see Appendix G). Important Judicious use of ulimit can protect a system against the dreaded fork bomb. #!/bin/bash # This script is for illustrative purposes only. # Run it at your own peril -- it WILL freeze your system. while true # Endless loop. do $0 & # This script invokes itself . . . #+ forks an infinite number of times . . . #+ until the system freezes up because all resources exhausted. done # This is the notorious "sorcerer's appentice" scenario. exit 0 # Will not exit here, because this script will never terminate. A ulimit -Hu XX (where XX is the user process limit) in /etc/profile would abort this script when it exceeded the preset limit. quota Display user or group disk quotas. setquota Set user or group disk quotas from the command-line. umask User file creation permissions mask. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by umask. The (octal) value passed to umask defines the file permissions disabled. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022). [85] Of course, the user may later change the attributes of particular files with chmod. The usual practice is to set the value of umask in /etc/profile and/or ~/.bash_profile (see Appendix G). Example 16-10. Using umask to hide an output file from prying eyes #!/bin/bash # rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file. # Usage: ./rot13a.sh filename # or ./rot13a.sh $OUTFILE # ^^ Input from stdin or a file. ^^^^^^^^^^ Output redirected to file. exit 0 rdev Get info about or make changes to root device, swap space, or video mode. The functionality of rdev has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is a dangerous command, if misused. Modules lsmod List installed kernel modules. +-------------------------------------------------------+ |bash$ lsmod | |Module Size Used by | | autofs 9456 2 (autoclean) | | opl3 11376 0 | | serial_cs 5456 0 (unused) | | sb 34752 0 | | uart401 6384 0 [sb] | | sound 58368 0 [opl3 sb uart401] | | soundlow 464 0 [sound] | | soundcore 2800 6 [sb sound] | | ds 6448 2 [serial_cs] | | i82365 22928 2 | | pcmcia_core 45984 0 [serial_cs ds i82365]| | | +-------------------------------------------------------+ Note Doing a cat /proc/modules gives the same information. insmod Force installation of a kernel module (use modprobe instead, when possible). Must be invoked as root. rmmod Force unloading of a kernel module. Must be invoked as root. modprobe Module loader that is normally invoked automatically in a startup script. Must be invoked as root. depmod Creates module dependency file. Usually invoked from a startup script. modinfo Output information about a loadable module. +-------------------------------------------------------------+ |bash$ modinfo hid | |filename: /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o | | description: "USB HID support drivers" | | author: "Andreas Gal, Vojtech Pavlik "| | license: "GPL" | | | +-------------------------------------------------------------+ Miscellaneous env Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The [varname=xxx] permits changing the environmental variable varname for the duration of the script. With no options specified, this command lists all the environmental variable settings. [86] Note The first line of a script (the "sha-bang" line) may use env when the path to the shell or interpreter is unknown. #! /usr/bin/env perl print "This Perl script will run,\n"; print "even when I don't know where to find Perl.\n"; # Good for portable cross-platform scripts, # where the Perl binaries may not be in the expected place. # Thanks, S.C. Or even ... #!/bin/env bash # Queries the $PATH enviromental variable for the location of bash. # Therefore ... # This script will run where Bash is not in its usual place, in /bin. ... ldd Show shared lib dependencies for an executable file. +-------------------------------------------------------+ |bash$ ldd /bin/ls | |libc.so.6 => /lib/libc.so.6 (0x4000c000) | |/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000) | +-------------------------------------------------------+ watch Run a command repeatedly, at specified time intervals. The default is two-second intervals, but this may be changed with the -n option. watch -n 5 tail /var/log/messages # Shows tail end of system log, /var/log/messages, every five seconds. Note Unfortunately, piping the output of watch command to grep does not work. strip Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging it impossible. This command often occurs in a Makefile, but rarely in a shell script. nm List symbols in an unstripped compiled binary. rdist Remote distribution client: synchronizes, clones, or backs up a file system on a remote server. ------------------------------------------------------------------------ 16.1. Analyzing a System Script Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is "killall," [87] used to suspend running processes at system shutdown. Example 16-11. killall, from /etc/rc.d/init.d #!/bin/sh # --> Comments added by the author of this document marked by "# -->". # --> This is part of the 'rc' script package # --> by Miquel van Smoorenburg, . # --> This particular script seems to be Red Hat / FC specific # --> (may not be present in other distributions). # Bring down all unneeded services that are still running #+ (there shouldn't be any, so this is just a sanity check) for i in /var/lock/subsys/*; do # --> Standard for/in loop, but since "do" is on same line, # --> it is necessary to add ";". # Check if the script is there. [ ! -f $i ] && continue # --> This is a clever use of an "and list", equivalent to: # --> if [ ! -f "$i" ]; then continue # Get the subsystem name. subsys=${i#/var/lock/subsys/} # --> Match variable name, which, in this case, is the file name. # --> This is the exact equivalent of subsys=`basename $i`. # --> It gets it from the lock file name # -->+ (if there is a lock file, # -->+ that's proof the process has been running). # --> See the "lockfile" entry, above. # Bring the subsystem down. if [ -f /etc/rc.d/init.d/$subsys.init ]; then /etc/rc.d/init.d/$subsys.init stop else /etc/rc.d/init.d/$subsys stop # --> Suspend running jobs and daemons. # --> Note that "stop" is a positional parameter, # -->+ not a shell builtin. fi done That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there. Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the -vn flags (sh -vn scriptname). Add extensive comments. Change the "action" commands to "echos". Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand parts of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the "initscripts" documentation. Part 5. Advanced Topics At this point, we are ready to delve into certain of the difficult and unusual aspects of scripting. Along the way, we will attempt to "push the envelope" in various ways and examine boundary conditions (what happens when we move into uncharted territory?). Table of Contents 17. Regular Expressions 17.1. A Brief Introduction to Regular Expressions 17.2. Globbing 18. Here Documents 18.1. Here Strings 19. I/O Redirection 19.1. Using exec 19.2. Redirecting Code Blocks 19.3. Applications 20. Subshells 21. Restricted Shells 22. Process Substitution 23. Functions 23.1. Complex Functions and Function Complexities 23.2. Local Variables 23.3. Recursion Without Local Variables 24. Aliases 25. List Constructs 26. Arrays 27. /dev and /proc 27.1. /dev 27.2. /proc 28. Of Zeros and Nulls 29. Debugging 30. Options 31. Gotchas 32. Scripting With Style 32.1. Unofficial Shell Scripting Stylesheet 33. Miscellany 33.1. Interactive and non-interactive shells and scripts 33.2. Operator Precedence 33.3. Shell Wrappers 33.4. Tests and Comparisons: Alternatives 33.5. A script calling itself (recursion) 33.6. "Colorizing" Scripts 33.7. Optimizations 33.8. Assorted Tips 33.9. Security Issues 33.10. Portability Issues 33.11. Shell Scripting Under Windows 34. Bash, versions 2, 3, and 4 34.1. Bash, version 2 34.2. Bash, version 3 34.3. Bash, version 4 ------------------------------------------------------------------------ Chapter 17. Regular Expressions . . . the intellectual activity associated with software development is largely one of gaining insight. --Stowe Boyd To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and utilities commonly used in scripts, such as grep, expr, sed and awk, interpret and use REs. As of version 3, Bash has acquired its own RE-match operator: =~. ------------------------------------------------------------------------ 17.1. A Brief Introduction to Regular Expressions An expression is a string of characters. Those characters having an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning [88] for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that match (or specify) patterns. A Regular Expression contains one or more of the following: * A character set. These are the characters retaining their literal meaning. The simplest type of Regular Expression consists only of a character set, with no metacharacters. * An anchor. These designate (anchor) the position in the line of text that the RE is to match. For example, ^, and $ are anchors. * Modifiers. These expand or narrow (modify) the range of text the RE is to match. Modifiers include the asterisk, brackets, and the backslash. The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters -- a string or a part of a string. * The asterisk -- * -- matches any number of repeats of the character string or RE preceding it, including zero instances. "1133*" matches 11 + one or more 3's: 113, 1133, 1133333, and so forth. * The dot -- . -- matches any one character, except a newline. [89] "13." matches 13 + at least one of any character (including a space): 1133, 11333, but not 13 (additional character missing). See Example 15-18 for a demonstration of dot single-character matching. * The caret -- ^ -- matches the beginning of a line, but sometimes, depending on context, negates the meaning of a set of characters in an RE. * The dollar sign -- $ -- at the end of an RE matches the end of a line. "XXX$" matches XXX at the end of a line. "^$" matches blank lines. * Brackets -- [...] -- enclose a set of characters to match in a single RE. "[xyz]" matches any one of the characters x, y, or z. "[c-n]" matches any one of the characters in the range c to n. "[B-Pk-y]" matches any one of the characters in the ranges B to P and k to y. "[a-z0-9]" matches any single lowercase letter or any digit. "[^b-d]" matches any character except those in the range b to d. This is an instance of ^ negating or inverting the meaning of the following RE (taking on a role similar to ! in a different context). Combined sequences of bracketed characters match common word patterns. "[Yy][Ee][Ss]" matches yes, Yes, YES, yEs, and so forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches any Social Security number. * The backslash -- \ -- escapes a special character, which means that character gets interpreted literally (and is therefore no longer special). A "\$" reverts back to its literal meaning of "$", rather than its RE meaning of end-of-line. Likewise a "\\" has the literal meaning of "\". * Escaped "angle brackets" -- \<...\> -- mark word boundaries. The angle brackets must be escaped, since otherwise they have only their literal character meaning. "\" matches the word "the," but not the words "them," "there," "other," etc. +-----------------------------------------------------------+ |bash$ cat textfile | |This is line 1, of which there is only one instance. | | This is the only instance of line 2. | | This is line 3, another line. | | This is line 4. | | | | | |bash$ grep 'the' textfile | |This is line 1, of which there is only one instance. | | This is the only instance of line 2. | | This is line 3, another line. | | | | | |bash$ grep '\' textfile | |This is the only instance of line 2. | | | +-----------------------------------------------------------+ +----------------------------------------------------------------------+ | The only way to be certain that a particular RE works is to test it. | | | | TEST FILE: tstfile # No match. | | # No match. | | Run grep "1133*" on this file. # Match. | | # No match. | | # No match. | | This line contains the number 113. # Match. | | This line contains the number 13. # No match. | | This line contains the number 133. # No match. | | This line contains the number 1133. # Match. | | This line contains the number 113312. # Match. | | This line contains the number 1112. # No match. | | This line contains the number 113312312. # Match. | | This line contains no numbers at all. # No match. | | | | +------------------------------------------------------------------+ | | |bash$ grep "1133*" tstfile | | | |Run grep "1133*" on this file. # Match. | | | | This line contains the number 113. # Match. | | | | This line contains the number 1133. # Match. | | | | This line contains the number 113312. # Match. | | | | This line contains the number 113312312. # Match. | | | | | | | +------------------------------------------------------------------+ | +----------------------------------------------------------------------+ * Extended REs. Additional metacharacters added to the basic set. Used in egrep, awk, and Perl. * The question mark -- ? -- matches zero or one of the previous RE. It is generally used for matching single characters. * The plus -- + -- matches one or more of the previous RE. It serves a role similar to the *, but does not match zero occurrences. # GNU versions of sed and awk can use "+", # but it needs to be escaped. echo a111b | sed -ne '/a1\+b/p' echo a111b | grep 'a1\+b' echo a111b | gawk '/a1+b/' # All of above are equivalent. # Thanks, S.C. * Escaped "curly brackets" -- \{ \} -- indicate the number of occurrences of a preceding RE to match. It is necessary to escape the curly brackets since they have only their literal character meaning otherwise. This usage is technically not part of the basic RE set. "[0-9]\{5\}" matches exactly five digits (characters in the range of 0 to 9). Note Curly brackets are not available as an RE in the "classic" (non-POSIX compliant) version of awk. However, the GNU extended version of awk, gawk, has the --re-interval option that permits them (without being escaped). +------------------------------------------------+ |bash$ echo 2222 | gawk --re-interval '/2{3}/' | |2222 | | | +------------------------------------------------+ Perl and some egrep versions do not require escaping the curly brackets. * Parentheses -- ( ) -- enclose a group of REs. They are useful with the following "|" operator and in substring extraction using expr. * The -- | -- "or" RE operator matches any of a set of alternate characters. +-----------------------------------------------------------------+ |bash$ egrep 're(a|e)d' misc.txt | |People who read seem to be better informed than those who do not.| | The clarinet produces sound by the vibration of its reed. | | | +-----------------------------------------------------------------+ Note Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions described above, as do the GNU utilities. * POSIX Character Classes. [:class:] This is an alternate method of specifying a range of characters to match. * [:alnum:] matches alphabetic or numeric characters. This is equivalent to A-Za-z0-9. * [:alpha:] matches alphabetic characters. This is equivalent to A-Za-z. * [:blank:] matches a space or a tab. * [:cntrl:] matches control characters. * [:digit:] matches (decimal) digits. This is equivalent to 0-9. * [:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This is the same as [:print:], below, but excluding the space character. * [:lower:] matches lowercase alphabetic characters. This is equivalent to a-z. * [:print:] (printable characters). Matches characters in the range of ASCII 32 - 126. This is the same as [:graph:], above, but adding the space character. * [:space:] matches whitespace characters (space and horizontal tab). * [:upper:] matches uppercase alphabetic characters. This is equivalent to A-Z. * [:xdigit:] matches hexadecimal digits. This is equivalent to 0-9A-Fa-f. Important POSIX character classes generally require quoting or double brackets ([[ ]]). +-----------------------------------------------------------+ |bash$ grep [[:digit:]] test.file | |abc=723 | | | +-----------------------------------------------------------+ # ... if [[ $arow =~ [[:digit:]] ]] # Numerical input? then # POSIX char class if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal! # ... # From ktour.sh example script. These character classes may even be used with globbing, to a limited extent. +-----------------------------------------------------------+ |bash$ ls -l ?[[:digit:]][[:digit:]]? | |-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b | | | +-----------------------------------------------------------+ POSIX character classes are used in Example 15-21 and Example 15-22. Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See Example A-12 and Example A-16 for illustrations of this. The standard reference on this complex topic is Friedl's Mastering Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives a very lucid treatment of REs. See the Bibliography for more information on these books. ------------------------------------------------------------------------ 17.2. Globbing Bash itself cannot recognize Regular Expressions. Inside scripts, it is commands and utilities -- such as sed and awk -- that interpret RE's. Bash does carry out filename expansion [90] -- a process known as globbing -- but this does not use the standard RE set. Instead, globbing recognizes and expands wild cards. Globbing interprets the standard wild card characters [91] -- * and ?, character lists in square brackets, and certain other special characters (such as ^ for negating the sense of a match). There are important limitations on wild card characters in globbing, however. Strings containing * will not match filenames that start with a dot, as, for example, .bashrc. [92] Likewise, the ? has a different meaning in globbing than as part of an RE. +----------------------------------------------------------------------+ |bash$ ls -l | |total 2 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 | | -rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh | | -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt | | | |bash$ ls -l t?.sh | |-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh | | | |bash$ ls -l [ab]* | |-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 | | | |bash$ ls -l [a-c]* | |-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 | | | |bash$ ls -l [^ab]* | |-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 | | -rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh | | -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt | | | |bash$ ls -l {b*,c*,*est*} | |-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1 | | -rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1 | | -rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt | | | +----------------------------------------------------------------------+ Bash performs filename expansion on unquoted command-line arguments. The echo command demonstrates this. +----------------------------------------------------------------------+ |bash$ echo * | |a.1 b.1 c.1 t2.sh test1.txt | | | |bash$ echo t* | |t2.sh test1.txt | | | |bash$ echo t?.sh | |t2.sh | | | +----------------------------------------------------------------------+ Note It is possible to modify the way Bash interprets special characters in globbing. A set -f command disables globbing, and the nocaseglob and nullglob options to shopt change globbing behavior. See also Example 10-4. ------------------------------------------------------------------------ Chapter 18. Here Documents Here and now, boys. --Aldous Huxley, Island A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor. COMMAND <. # Bram Moolenaar points out that this may not work with 'vim' #+ because of possible problems with terminal interaction. exit The above script could just as effectively have been implemented with ex, rather than vi. Here documents containing a list of ex commands are common enough to form their own category, known as ex scripts. #!/bin/bash # Replace all instances of "Smith" with "Jones" #+ in files with a ".txt" filename suffix. ORIGINAL=Smith REPLACEMENT=Jones for word in $(fgrep -l $ORIGINAL *.txt) do # ------------------------------------- ex $word < $Newfile < $OUTFILE # ----------------------------------------------------------- # Quoting the 'limit string' prevents variable expansion #+ within the body of the above 'here document.' # This permits outputting literal strings in the output file. if [ -f "$OUTFILE" ] then chmod 755 $OUTFILE # Make the generated file executable. else echo "Problem in creating file: \"$OUTFILE\"" fi # This method can also be used for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like. exit 0 It is possible to set a variable from the output of a here document. This is actually a devious form of command substitution. variable=$(cat < EOF | |lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted)| | | +-------------------------------------------------------------------------+ Caution Some utilities will not work inside a here document. Warning The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized. #!/bin/bash echo "----------------------------------------------------------------------" cat < $file.new echo "Modified file is $file.new" exit # Ends script execution. from 'man bash': Here Strings A variant of here documents, the format is: << # Redirect stdout to a file. # Creates the file if not present, otherwise overwrites it. ls -lR > dir-tree.list # Creates a file containing a listing of the directory tree. : > filename # The > truncates file "filename" to zero length. # If file not present, creates zero-length file (same effect as 'touch'). # The : serves as a dummy placeholder, producing no output. > filename # The > truncates file "filename" to zero length. # If file not present, creates zero-length file (same effect as 'touch'). # (Same result as ": >", above, but this does not work with some shells.) COMMAND_OUTPUT >> # Redirect stdout to a file. # Creates the file if not present, otherwise appends to it. # Single-line redirection commands (affect only the line they are on): # -------------------------------------------------------------------- 1>filename # Redirect stdout to file "filename." 1>>filename # Redirect and append stdout to file "filename." 2>filename # Redirect stderr to file "filename." 2>>filename # Redirect and append stderr to file "filename." &>filename # Redirect both stdout and stderr to file "filename." # # Note that &>>filename #+ -- attempting to redirect and *append* #+ stdout and stderr to file "filename" -- #+ fails with the error message, #+ syntax error near unexpected token `>'. # The &>> operator is supposed to be functional in Bash 4, #+ but as of version 4.0 still is not. M>N # "M" is a file descriptor, which defaults to 1, if not explicitly set. # "N" is a filename. # File descriptor "M" is redirect to file "N." M>&N # "M" is a file descriptor, which defaults to 1, if not set. # "N" is another file descriptor. #============================================================================== # Redirecting stdout, one line at a time. LOGFILE=script.log echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"." # These redirection commands automatically "reset" after each line. # Redirecting stderr, one line at a time. ERRORFILE=script.errors bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE. bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE. bad_command3 # Error message echoed to stderr, #+ and does not appear in $ERRORFILE. # These redirection commands also automatically "reset" after each line. #======================================================================= 2>&1 # Redirects stderr to stdout. # Error messages get sent to same place as standard output. >>filename 2>&1 bad_command >>filename 2>&1 # Appends both stdout and stderr to the file "filename" ... 2>&1 | [command(s)] bad_command 2>&1 | awk '{print $5}' # found # Sends stderr through a pipe. # |& was added to Bash 4 as an abbreviation for 2>& #+ but as of version 4.0 this still does not work. i>&j # Redirects file descriptor i to j. # All output of file pointed to by i gets sent to file pointed to by j. >&j # Redirects, by default, file descriptor 1 (stdout) to j. # All stdout gets sent to file pointed to by j. 0< FILENAME < FILENAME # Accept input from a file. # Companion command to ">", and often used in combination with it. # # grep search-word filename # Open file "filename" for reading and writing, #+ and assign file descriptor "j" to it. # If "filename" does not exist, create it. # If file descriptor "j" is not specified, default to fd 0, stdin. # # An application of this is writing at a specified place in a file. echo 1234567890 > File # Write string to "File". exec 3<> File # Open "File" and assign fd 3 to it. read -n 4 <&3 # Read only 4 characters. echo -n . >&3 # Write a decimal point there. exec 3>&- # Close fd 3. cat File # ==> 1234.67890 # Random access, by golly. | # Pipe. # General purpose process and command chaining tool. # Similar to ">", but more general in effect. # Useful for chaining commands, scripts, files, and programs together. cat *.txt | sort | uniq > result-file # Sorts the output of all the .txt files and deletes duplicate lines, # finally saves results to "result-file". Multiple instances of input and output redirection and/or pipes can be combined in a single command line. command < input-file > output-file command1 | command2 | command3 > output-file See Example 15-31 and Example A-14. Multiple output streams may be redirected to one file. ls -yz >> command.log 2>&1 # Capture result of illegal options "yz" in file "command.log." # Because stderr is redirected to the file, #+ any error messages will also be there. # Note, however, that the following does *not* give the same result. ls -yz 2>&1 >> command.log # Outputs an error message and does not write to file. # If redirecting both stdout and stderr, #+ the order of the commands makes a difference. Closing File Descriptors n<&- Close input file descriptor n. 0<&-, <&- Close stdin. n>&- Close output file descriptor n. 1>&-, >&- Close stdout. Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it. # Redirecting only stderr to a pipe. exec 3>&1 # Save current "value" of stdout. ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls'). # ^^^^ ^^^^ exec 3>&- # Now close it for the remainder of the script. # Thanks, S.C. For a more detailed introduction to I/O redirection see Appendix E. ------------------------------------------------------------------------ 19.1. Using exec An exec filename command redirects stdout to a designated file. This sends all command output that would normally go to stdout to that file. Important exec N > filename affects the entire script or current shell. Redirection in the PID of the script or shell from that point on has changed. However . . . N > filename affects only the newly-forked process, not the entire script or shell. Thank you, Ahmed Darwish, for pointing this out. Example 19-2. Redirecting stdout using exec #!/bin/bash # reassign-stdout.sh LOGFILE=logfile.txt exec 6>&1 # Link file descriptor #6 with stdout. # Saves stdout. exec > $LOGFILE # stdout replaced with file "logfile.txt". # ----------------------------------------------------------- # # All output from commands in this block sent to file $LOGFILE. echo -n "Logfile: " date echo "-------------------------------------" echo echo "Output of \"ls -al\" command" echo ls -al echo; echo echo "Output of \"df\" command" echo df # ----------------------------------------------------------- # exec 1>&6 6>&- # Restore stdout and close file descriptor #6. echo echo "== stdout now restored to default == " echo ls -al echo exit 0 Example 19-3. Redirecting both stdin and stdout in the same script with exec #!/bin/bash # upperconv.sh # Converts a specified input file to uppercase. E_FILE_ACCESS=70 E_WRONG_ARGS=71 if [ ! -r "$1" ] # Is specified input file readable? then echo "Can't read from input file!" echo "Usage: $0 input-file output-file" exit $E_FILE_ACCESS fi # Will exit with same error #+ even if input file ($1) not specified (why?). if [ -z "$2" ] then echo "Need to specify output file." echo "Usage: $0 input-file output-file" exit $E_WRONG_ARGS fi exec 4<&0 exec < $1 # Will read from input file. exec 7>&1 exec > $2 # Will write to output file. # Assumes output file writable (add check?). # ----------------------------------------------- cat - | tr a-z A-Z # Uppercase conversion. # ^^^^^ # Reads from stdin. # ^^^^^^^^^^ # Writes to stdout. # However, both stdin and stdout were redirected. # Note that the 'cat' can be omitted. # ----------------------------------------------- exec 1>&7 7>&- # Restore stout. exec 0<&4 4<&- # Restore stdin. # After restoration, the following line prints to stdout as expected. echo "File \"$1\" written to \"$2\" as uppercase conversion." exit 0 I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem. Example 19-4. Avoiding a subshell #!/bin/bash # avoid-subshell.sh # Suggested by Matthew Walker. Lines=0 echo cat myfile.txt | while read line; do { echo $line (( Lines++ )); # Incremented values of this variable #+ inaccessible outside loop. # Subshell problem. } done echo "Number of lines read = $Lines" # 0 # Wrong! echo "------------------------" exec 3<> myfile.txt while read line <&3 do { echo "$line" (( Lines++ )); # Incremented values of this variable #+ accessible outside loop. # No subshell, no problem. } done exec 3>&- echo "Number of lines read = $Lines" # 8 echo exit 0 # Lines below not seen by script. $ cat myfile.txt Line 1. Line 2. Line 3. Line 4. Line 5. Line 6. Line 7. Line 8. ------------------------------------------------------------------------ 19.2. Redirecting Code Blocks Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of stdin. Even a function may use this form of redirection (see Example 23-11). The < operator at the end of the code block accomplishes this. Example 19-5. Redirected while loop #!/bin/bash # redir2.sh if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi #+ Filename=${1:-names.data} # can replace the above test (parameter substitution). count=0 echo while [ "$name" != Smith ] # Why is variable $name in quotes? do read name # Reads from $Filename, rather than stdin. echo $name let "count += 1" done <"$Filename" # Redirects stdin to file $Filename. # ^^^^^^^^^^^^ echo; echo "$count names read"; echo exit 0 # Note that in some older shell scripting languages, #+ the redirected loop would run as a subshell. # Therefore, $count would return 0, the initialized value outside the loop. # Bash and ksh avoid starting a subshell *whenever possible*, #+ so that this script, for example, runs correctly. # (Thanks to Heiner Steven for pointing this out.) # However . . . # Bash *can* sometimes start a subshell in a PIPED "while-read" loop, #+ as distinct from a REDIRECTED "while" loop. abc=hi echo -e "1\n2\n3" | while read l do abc="$l" echo $abc done echo $abc # Thanks, Bruno de Oliveira Schneider, for demonstrating this #+ with the above snippet of code. # And, thanks, Brian Onn, for correcting an annotation error. Example 19-6. Alternate form of redirected while loop #!/bin/bash # This is an alternate form of the preceding script. # Suggested by Heiner Steven #+ as a workaround in those situations when a redirect loop #+ runs as a subshell, and therefore variables inside the loop # +do not keep their values upon loop termination. if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi exec 3<&0 # Save stdin to file descriptor 3. exec 0<"$Filename" # Redirect standard input. count=0 echo while [ "$name" != Smith ] do read name # Reads from redirected stdin ($Filename). echo $name let "count += 1" done # Loop reads from file $Filename #+ because of line 20. # The original version of this script terminated the "while" loop with #+ done <"$Filename" # Exercise: # Why is this unnecessary? exec 0<&3 # Restore old stdin. exec 3<&- # Close temporary fd 3. echo; echo "$count names read"; echo exit 0 Example 19-7. Redirected until loop #!/bin/bash # Same as previous example, but with "until" loop. if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi # while [ "$name" != Smith ] until [ "$name" = Smith ] # Change != to =. do read name # Reads from $Filename, rather than stdin. echo $name done <"$Filename" # Redirects stdin to file $Filename. # ^^^^^^^^^^^^ # Same results as with "while" loop in previous example. exit 0 Example 19-8. Redirected for loop #!/bin/bash if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target file. # # Very contrived and kludgy, nevertheless shows that #+ it's possible to redirect stdin within a "for" loop... #+ if you're clever enough. # # More concise is line_count=$(wc -l < "$Filename") for name in `seq $line_count` # Recall that "seq" prints sequence of numbers. # while [ "$name" != Smith ] -- more complicated than a "while" loop -- do read name # Reads from $Filename, rather than stdin. echo $name if [ "$name" = Smith ] # Need all this extra baggage here. then break fi done <"$Filename" # Redirects stdin to file $Filename. # ^^^^^^^^^^^^ exit 0 We can modify the previous example to also redirect the output of the loop. Example 19-9. Redirected for loop (both stdin and stdout redirected) #!/bin/bash if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi Savefile=$Filename.new # Filename to save results in. FinalName=Jonah # Name to terminate "read" on. line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target file. for name in `seq $line_count` do read name echo "$name" if [ "$name" = "$FinalName" ] then break fi done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename, # ^^^^^^^^^^^^^^^^^^^^^^^^^^^ and saves it to backup file. exit 0 Example 19-10. Redirected if/then test #!/bin/bash if [ -z "$1" ] then Filename=names.data # Default, if no filename specified. else Filename=$1 fi TRUE=1 if [ "$TRUE" ] # if true and if : also work. then read name echo $name fi <"$Filename" # ^^^^^^^^^^^^ # Reads only first line of file. # An "if/then" test has no way of iterating unless embedded in a loop. exit 0 Example 19-11. Data file names.data for above examples Aristotle Belisarius Capablanca Euler Goethe Hamurabi Jonah Laplace Maroczy Purcell Schmidt Semmelweiss Smith Turing Venn Wilkinson Znosko-Borowski # This is a data file for #+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh". Redirecting the stdout of a code block has the effect of saving its output to a file. See Example 3-2. Here documents are a special case of redirected code blocks. That being the case, it should be possible to feed the output of a here document into the stdin for a while loop. # This example by Albert Siersema # Used with permission (thanks!). function doesOutput() # Could be an external command too, of course. # Here we show you can use a function as well. { ls -al *.jpg | awk '{print $5,$9}' } nr=0 # We want the while loop to be able to manipulate these and totalSize=0 #+ to be able to see the changes after the while finished. while read fileSize fileName ; do echo "$fileName is $fileSize bytes" let nr++ totalSize=$((totalSize+fileSize)) # Or: "let totalSize+=fileSize" done<&7 # This *appends* the date to the file. # ^^^^^^^ command substitution # See below. } case $LOG_LEVEL in 1) exec 3>&2 4> /dev/null 5> /dev/null;; 2) exec 3>&2 4>&2 5> /dev/null;; 3) exec 3>&2 4>&2 5>&2;; *) exec 3> /dev/null 4> /dev/null 5> /dev/null;; esac FD_LOGVARS=6 if [[ $LOG_VARS ]] then exec 6>> /var/log/vars.log else exec 6> /dev/null # Bury output. fi FD_LOGEVENTS=7 if [[ $LOG_EVENTS ]] then # exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log) # Above line fails in versions of Bash more recent than 2.04. Why? exec 7>> /var/log/event.log # Append to "event.log". log # Write time and date. else exec 7> /dev/null # Bury output. fi echo "DEBUG3: beginning" >&${FD_DEBUG3} ls -l >&5 2>&4 # command1 >&5 2>&4 echo "Done" # command2 echo "sending mail" >&${FD_LOGEVENTS} # Writes "sending mail" to file descriptor #7. exit 0 ------------------------------------------------------------------------ Chapter 20. Subshells Running a shell script launches a new process, a subshell. +----------------------------------------------------------------------+ | Definition: A subshell is a child process launched by a shell (or | | shell script). | +----------------------------------------------------------------------+ A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child process) of the parent shell. A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously. #!/bin/bash # subshell-test.sh ( # Inside parentheses, and therefore a subshell . . . while [ 1 ] # Endless loop. do echo "Subshell running . . ." done ) # Script will run forever, #+ or at least until terminated by a Ctl-C. exit $? # End of script (but will never get here). Now, run the script: sh subshell-test.sh And, while the script is running, from a different xterm: ps -ef | grep subshell-test.sh UID PID PPID C STIME TTY TIME CMD 500 2698 2502 0 14:26 pts/4 00:00:00 sh subshell-test.sh 500 2699 2698 21 14:26 pts/4 00:00:24 sh subshell-test.sh ^^^^ Analysis: PID 2698, the script, launched PID 2699, the subshell. Note: The "UID ..." line would be filtered out by the "grep" command, but is shown here for illustrative purposes. In general, an external command in a script forks off a subprocess, [96] whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents. Command List within Parentheses ( command1; command2; command3; ... ) A command list embedded between parentheses runs as a subshell. Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, variables local to the child process. Example 20-1. Variable scope in a subshell #!/bin/bash # subshell.sh echo echo "We are outside the subshell." echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL" # Bash, version 3, adds the new $BASH_SUBSHELL variable. echo; echo outer_variable=Outer global_variable= # Define global variable for "storage" of #+ value of subshell variable. ( echo "We are inside the subshell." echo "Subshell level INSIDE subshell = $BASH_SUBSHELL" inner_variable=Inner echo "From inside subshell, \"inner_variable\" = $inner_variable" echo "From inside subshell, \"outer\" = $outer_variable" global_variable="$inner_variable" # Will this allow "exporting" #+ a subshell variable? ) echo; echo echo "We are outside the subshell." echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL" echo if [ -z "$inner_variable" ] then echo "inner_variable undefined in main body of shell" else echo "inner_variable defined in main body of shell" fi echo "From main body of shell, \"inner_variable\" = $inner_variable" # $inner_variable will show as blank (uninitialized) #+ because variables defined in a subshell are "local variables". # Is there a remedy for this? echo "global_variable = "$global_variable"" # Why doesn't this work? echo # ======================================================================= # Additionally ... echo "-----------------"; echo var=41 # Global variable. ( let "var+=1"; echo "\$var INSIDE subshell = $var" ) # 42 echo "\$var OUTSIDE subshell = $var" # 41 # Variable operations inside a subshell, even to a GLOBAL variable #+ do not affect the value of the variable outside the subshell! exit 0 # Question: # -------- # Once having exited a subshell, #+ is there any way to reenter that very same subshell #+ to modify or access the subshell variables? See also $BASHPID and Example 31-2. +----------------------------------------------------------------------+ | Definition: The scope of a variable is the context in which it has | | meaning, in which it has a value that can be referenced. For | | example, the scope of a local variable lies only within the | | function, block of code, or subshell within which it is defined, | | while the scope of a global variable is the entire script in which | | it appears. | +----------------------------------------------------------------------+ Note While the $BASH_SUBSHELL internal variable indicates the nesting level of a subshell, the $SHLVL variable shows no change within a subshell. echo " \$BASH_SUBSHELL outside subshell = $BASH_SUBSHELL" # 0 ( echo " \$BASH_SUBSHELL inside subshell = $BASH_SUBSHELL" ) # 1 ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2 # ^ ^ *** nested *** ^ ^ echo echo " \$SHLVL outside subshell = $SHLVL" # 3 ( echo " \$SHLVL inside subshell = $SHLVL" ) # 3 (No change!) Directory changes made in a subshell do not carry over to the parent shell. Example 20-2. List User Profiles #!/bin/bash # allprofs.sh: Print all user profiles. # This script written by Heiner Steven, and modified by the document author. FILE=.bashrc # File containing user profile, #+ was ".profile" in original script. for home in `awk -F: '{print $6}' /etc/passwd` do [ -d "$home" ] || continue # If no home directory, go to next. [ -r "$home" ] || continue # If not readable, go to next. (cd $home; [ -e $FILE ] && less $FILE) done # When script terminates, there is no need to 'cd' back to original directory, #+ because 'cd $home' takes place in a subshell. exit 0 A subshell may be used to set up a "dedicated environment" for a command group. COMMAND1 COMMAND2 COMMAND3 ( IFS=: PATH=/bin unset TERMINFO set -C shift 5 COMMAND4 COMMAND5 exit 3 # Only exits the subshell! ) # The parent shell has not been affected, and the environment is preserved. COMMAND6 COMMAND7 As seen here, the exit command only terminates the subshell in which it is running, not the parent shell or script. One application of such a "dedicated environment" is testing whether a variable is defined. if (set -u; : $variable) 2> /dev/null then echo "Variable is set." fi # Variable has been set in current script, #+ or is an an internal Bash variable, #+ or is present in environment (has been exported). # Could also be written [[ ${variable-x} != x || ${variable-y} != y ]] # or [[ ${variable-x} != x$variable ]] # or [[ ${variable+x} = x ]] # or [[ ${variable-x} != x ]] Another application is checking for a lock file: if (set -C; : > lock_file) 2> /dev/null then : # lock_file didn't exist: no user running the script else echo "Another user is already running that script." exit 65 fi # Code snippet by Stéphane Chazelas, #+ with modifications by Paulo Marcel Coelho Aragao. + Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently. Example 20-3. Running parallel processes in subshells (cat list1 list2 list3 | sort | uniq > list123) & (cat list4 list5 list6 | sort | uniq > list456) & # Merges and sorts both sets of lists simultaneously. # Running in background ensures parallel execution. # # Same effect as # cat list1 list2 list3 | sort | uniq > list123 & # cat list4 list5 list6 | sort | uniq > list456 & wait # Don't execute the next command until subshells finish. diff list123 list456 Redirecting I/O to a subshell uses the "|" pipe operator, as in ls -al | (command). Note A code block between curly brackets does not launch a subshell. { command1; command2; command3; . . . commandN; } var1=23 echo "$var1" # 23 { var1=76; } echo "$var1" # 76 ------------------------------------------------------------------------ Chapter 21. Restricted Shells Disabled commands in restricted shells . Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script. The following commands and actions are disabled: * Using cd to change the working directory. * Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV environmental variables. * Reading or changing the $SHELLOPTS, shell environmental options. * Output redirection. * Invoking commands containing one or more /'s. * Invoking exec to substitute a different process for the shell. * Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose. * Getting out of restricted mode within the script. Example 21-1. Running a script in restricted mode #!/bin/bash # Starting the script with "#!/bin/bash -r" #+ runs entire script in restricted mode. echo echo "Changing directory." cd /usr/local echo "Now in `pwd`" echo "Coming back home." cd echo "Now in `pwd`" echo # Everything up to here in normal, unrestricted mode. set -r # set --restricted has same effect. echo "==> Now in restricted mode. <==" echo echo echo "Attempting directory change in restricted mode." cd .. echo "Still in `pwd`" echo echo echo "\$SHELL = $SHELL" echo "Attempting to change shell in restricted mode." SHELL="/bin/ash" echo echo "\$SHELL= $SHELL" echo echo echo "Attempting to redirect output in restricted mode." ls -l /usr/bin > bin.files ls -l bin.files # Try to list attempted file creation effort. echo exit 0 ------------------------------------------------------------------------ Chapter 22. Process Substitution Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in. Process substitution feeds the output of a process (or processes) into the stdin of another process. Template Command list enclosed within parentheses >(command_list) <(command_list) Process substitution uses /dev/fd/ files to send the results of the process(es) within parentheses to another process. [97] Caution There is no space between the the "<" or ">" and the parentheses. Space there would give an error message. +----------------------------------------------------------------------+ |bash$ echo >(true) | |/dev/fd/63 | | | |bash$ echo <(true) | |/dev/fd/63 | | | |bash$ echo >(true) <(true) | |/dev/fd/63 /dev/fd/62 | | | | | | | |bash$ wc <(cat /usr/share/dict/linux.words) | | 483523 483523 4992010 /dev/fd/63 | | | |bash$ grep script /usr/share/dict/linux.words | wc | | 262 262 3601 | | | |bash$ wc <(grep script /usr/share/dict/linux.words) | | 262 262 3601 /dev/fd/63 | | | +----------------------------------------------------------------------+ Note Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking /dev/fd/ files, Bash may use temporary files. (Thanks, S.C.) Process substitution can compare the output of two different commands, or even the output of different options to the same command. +----------------------------------------------------------------------+ |bash$ comm <(ls -l) <(ls -al) | |total 12 | |-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0 | |-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2 | |-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh | | total 20 | | drwxrwxrwx 2 bozo bozo 4096 Mar 10 18:10 . | | drwx------ 72 bozo bozo 4096 Mar 10 17:58 .. | | -rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0 | | -rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2 | | -rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh | +----------------------------------------------------------------------+ Process substitution can compare the contents of two directories -- to see which filenames are in one, but not the other. diff <(ls $first_directory) <(ls $second_directory) Some other usages and uses of process substitution: read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) # Read a list of random numbers from /dev/urandom, #+ process with "od" #+ and feed into stdin of "read" . . . # From "insertion-sort.bash" example script. # Courtesy of JuanJo Ciarlante. cat <(ls -l) # Same as ls -l | cat sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin) # Lists all the files in the 3 main 'bin' directories, and sorts by filename. # Note that three (count 'em) distinct commands are fed to 'sort'. diff <(command1) <(command2) # Gives difference in command output. tar cf >(bzip2 -c > file.tar.bz2) $directory_name # Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2". # # Because of the /dev/fd/ system feature, # the pipe between both commands does not need to be named. # # This can be emulated. # bzip2 -c < pipe > file.tar.bz2& tar cf pipe $directory_name rm pipe # or exec 3>&1 tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&- exec 3>&- # Thanks, Stéphane Chazelas Here is a method of circumventing the problem of an echo piped to a while-read loop running in a subshell. Example 22-1. Code block redirection without forking #!/bin/bash # wr-ps.bash: while-read loop with process substitution. # This example contributed by Tomas Pospisek. # (Heavily edited by the ABS Guide author.) echo echo "random input" | while read i do global=3D": Not available outside the loop." # ... because it runs in a subshell. done echo "\$global (from outside the subprocess) = $global" # $global (from outside the subprocess) = echo; echo "--"; echo while read i do echo $i global=3D": Available outside the loop." # ... because it does *not* run in a subshell. done < <( echo "random input" ) # ^ ^ echo "\$global (using process substitution) = $global" # Random input # $global (using process substitution) = 3D: Available outside the loop. echo; echo "##########"; echo # And likewise . . . declare -a inloop index=0 cat $0 | while read line do inloop[$index]="$line" ((index++)) # It runs in a subshell, so ... done echo "OUTPUT = " echo ${inloop[*]} # ... nothing echoes. echo; echo "--"; echo declare -a outloop index=0 while read line do outloop[$index]="$line" ((index++)) # It does *not* run in a subshell, so ... done < <( cat $0 ) echo "OUTPUT = " echo ${outloop[*]} # ... the entire script echoes. exit $? A reader sent in the following interesting example of process substitution. # Script fragment taken from SuSE distribution: # --------------------------------------------------------------# while read des what mask iface; do # Some commands ... done < <(route -n) # ^ ^ First < is redirection, second is process substitution. # To test it, let's make it do something. while read des what mask iface; do echo $des $what $mask $iface done < <(route -n) # Output: # Kernel IP routing table # Destination Gateway Genmask Flags Metric Ref Use Iface # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo # --------------------------------------------------------------# # As Stéphane Chazelas points out, #+ an easier-to-understand equivalent is: route -n | while read des what mask iface; do # Variables set from output of pipe. echo $des $what $mask $iface done # This yields the same output as above. # However, as Ulrich Gayer points out . . . #+ this simplified equivalent uses a subshell for the while loop, #+ and therefore the variables disappear when the pipe terminates. # --------------------------------------------------------------# # However, Filip Moritz comments that there is a subtle difference #+ between the above two examples, as the following shows. ( route -n | while read x; do ((y++)); done echo $y # $y is still unset while read x; do ((y++)); done < <(route -n) echo $y # $y has the number of lines of output of route -n ) More generally spoken ( : | x=x # seems to start a subshell like : | ( x=x ) # while x=x < <(:) # does not ) # This is useful, when parsing csv and the like. # That is, in effect, what the original SuSE code fragment does. ------------------------------------------------------------------------ Chapter 23. Functions Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function. function function_name { command... } or function_name () { command... } This second form will cheer the hearts of C programmers (and is more portable). As in C, the function's opening bracket may optionally appear on the second line. function_name () { command... } Note A function may be "compacted" into a single line. fun () { echo "This is a function"; echo; } # ^ ^ In this case, however, a semicolon must follow the final command in the function. fun () { echo "This is a function"; echo } # Error! # ^ Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command. Example 23-1. Simple functions #!/bin/bash JUST_A_SECOND=1 funky () { # This is about as simple as functions get. echo "This is a funky function." echo "Now exiting funky function." } # Function declaration must precede call. fun () { # A somewhat more complex function. i=0 REPEATS=30 echo echo "And now the fun really begins." echo sleep $JUST_A_SECOND # Hey, wait a second! while [ $i -lt $REPEATS ] do echo "----------FUNCTIONS---------->" echo "<------------ARE-------------" echo "<------------FUN------------>" echo let "i+=1" done } # Now, call the functions. funky fun exit 0 The function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C. f1 # Will give an error message, since function "f1" not yet defined. declare -f f1 # This doesn't help either. f1 # Still an error message. # However... f1 () { echo "Calling function \"f2\" from within function \"f1\"." f2 } f2 () { echo "Function \"f2\"." } f1 # Function "f2" is not actually called until this point, #+ although it is referenced before its definition. # This is permissible. # Thanks, S.C. Note Functions may not be empty! #!/bin/bash # empty-function.sh empty () { } exit 0 # Will not exit here! # $ sh empty-function.sh # empty-function.sh: line 6: syntax error near unexpected token `}' # empty-function.sh: line 6: `}' # $ echo $? # 2 # However ... not_quite_empty () { illegal_command } # A script containing this function will *not* bomb #+ as long as the function is not called. # Thank you, Thiemo Kellner, for pointing this out. It is even possible to nest a function within another function, although this is not very useful. f1 () { f2 () # nested { echo "Function \"f2\", inside \"f1\"." } } f2 # Gives an error message. # Even a preceding "declare -f f2" wouldn't help. echo f1 # Does nothing, since calling "f1" does not automatically call "f2". f2 # Now, it's all right to call "f2", #+ since its definition has been made visible by calling "f1". # Thanks, S.C. Function declarations can appear in unlikely places, even where a command would otherwise go. ls -l | foo() { echo "foo"; } # Permissible, but useless. if [ "$USER" = bozo ] then bozo_greet () # Function definition embedded in an if/then construct. { echo "Hello, Bozo." } fi bozo_greet # Works only for Bozo, and other users get an error. # Something like this might be useful in some contexts. NO_EXIT=1 # Will enable function definition below. [[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-list". # If $NO_EXIT is 1, declares "exit ()". # This disables the "exit" builtin by aliasing it to "true". exit # Invokes "exit ()" function, not "exit" builtin. # Or, similarly: filename=file1 [ -f "$filename" ] && foo () { rm -f "$filename"; echo "File "$filename" deleted."; } || foo () { echo "File "$filename" not found."; touch bar; } foo # Thanks, S.C. and Christopher Head Functions can take strange forms. _(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; } # ^^^ No space between function name and parentheses. # This doesn't always work. Why not? # Now, let's invoke the function. _ # __________ # ^^^^^^^^^^ 10 underscores (10 x function name)! # A "naked" underscore is an acceptable function name. Note What happens when different versions of the same function appear in a script? # As Yan Chen points out, # when a function is defined multiple times, # the final version is what is invoked. # This is not, however, particularly useful. func () { echo "First version of func ()." } func () { echo "Second version of func ()." } func # Second version of func (). exit $? # It is even possible to use functions to override #+ or preempt system commands. # Of course, this is *not* advisable. ------------------------------------------------------------------------ 23.1. Complex Functions and Function Complexities Functions may process arguments passed to them and return an exit status to the script for further processing. function_name $arg1 $arg2 The function refers to the passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth. Example 23-2. Function Taking Parameters #!/bin/bash # Functions and parameters DEFAULT=default # Default param value. func2 () { if [ -z "$1" ] # Is parameter #1 zero length? then echo "-Parameter #1 is zero length.-" # Or no parameter passed. else echo "-Param #1 is \"$1\".-" fi variable=${1-$DEFAULT} # What does echo "variable = $variable" #+ parameter substitution show? # --------------------------- # It distinguishes between #+ no param and a null param. if [ "$2" ] then echo "-Parameter #2 is \"$2\".-" fi return 0 } echo echo "Nothing passed." func2 # Called with no params echo echo "Zero-length parameter passed." func2 "" # Called with zero-length param echo echo "Null parameter passed." func2 "$uninitialized_param" # Called with uninitialized param echo echo "One parameter passed." func2 first # Called with one param echo echo "Two parameters passed." func2 first second # Called with two params echo echo "\"\" \"second\" passed." func2 "" second # Called with zero-length first parameter echo # and ASCII string as a second one. exit 0 Important The shift command works on arguments passed to functions (see Example 33-16). But, what about command-line arguments passed to the script? Does a function see them? Well, let's clear up the confusion. Example 23-3. Functions and command-line args passed to the script #!/bin/bash # func-cmdlinearg.sh # Call this script with a command-line argument, #+ something like $0 arg1. func () { echo "$1" } echo "First call to function: no arg passed." echo "See if command-line arg is seen." func # No! Command-line arg not seen. echo "============================================================" echo echo "Second call to function: command-line arg passed explicitly." func $1 # Now it's seen! exit 0 In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals. Functions interpret their arguments literally. Indirect variable references (see Example 34-2) provide a clumsy sort of mechanism for passing variable pointers to functions. Example 23-4. Passing an indirect reference to a function #!/bin/bash # ind-func.sh: Passing an indirect reference to a function. echo_var () { echo "$1" } message=Hello Hello=Goodbye echo_var "$message" # Hello # Now, let's pass an indirect reference to the function. echo_var "${!message}" # Goodbye echo "-------------" # What happens if we change the contents of "hello" variable? Hello="Hello, again!" echo_var "$message" # Hello echo_var "${!message}" # Hello, again! exit 0 The next logical question is whether parameters can be dereferenced after being passed to a function. Example 23-5. Dereferencing a parameter passed to a function #!/bin/bash # dereference.sh # Dereferencing parameter passed to a function. # Script by Bruce W. Clare. dereference () { y=\$"$1" # Name of variable. echo $y # $Junk x=`eval "expr \"$y\" "` echo $1=$x eval "$1=\"Some Different Text \"" # Assign new value. } Junk="Some Text" echo $Junk "before" # Some Text before dereference Junk echo $Junk "after" # Some Different Text after exit 0 Example 23-6. Again, dereferencing a parameter passed to a function #!/bin/bash # ref-params.sh: Dereferencing a parameter passed to a function. # (Complex Example) ITERATIONS=3 # How many times to get input. icount=1 my_read () { # Called with my_read varname, #+ outputs the previous value between brackets as the default value, #+ then asks for a new value. local local_var echo -n "Enter a value " eval 'echo -n "[$'$1'] "' # Previous value. # eval echo -n "[\$$1] " # Easier to understand, #+ but loses trailing space in user prompt. read local_var [ -n "$local_var" ] && eval $1=\$local_var # "And-list": if "local_var" then set "$1" to its value. } echo while [ "$icount" -le "$ITERATIONS" ] do my_read var echo "Entry #$icount = $var" let "icount += 1" echo done # Thanks to Stephane Chazelas for providing this instructive example. exit 0 Exit and Return exit status Functions return a value, called an exit status. This is analogous to the exit status returned by a command. The exit status may be explicitly specified by a return statement, otherwise it is the exit status of the last command in the function (0 if successful, and a non-zero error code if not). This exit status may be used in the script by referencing it as $?. This mechanism effectively permits script functions to have a "return value" similar to C functions. return Terminates a function. A return command [98] optionally takes an integer argument, which is returned to the calling script as the "exit status" of the function, and this exit status is assigned to the variable $?. Example 23-7. Maximum of two numbers #!/bin/bash # max.sh: Maximum of two integers. E_PARAM_ERR=250 # If less than 2 params passed to function. EQUAL=251 # Return value if both params equal. # Error values out of range of any #+ params that might be fed to the function. max2 () # Returns larger of two numbers. { # Note: numbers compared must be less than 257. if [ -z "$2" ] then return $E_PARAM_ERR fi if [ "$1" -eq "$2" ] then return $EQUAL else if [ "$1" -gt "$2" ] then return $1 else return $2 fi fi } max2 33 34 return_val=$? if [ "$return_val" -eq $E_PARAM_ERR ] then echo "Need to pass two parameters to the function." elif [ "$return_val" -eq $EQUAL ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi exit 0 # Exercise (easy): # --------------- # Convert this to an interactive script, #+ that is, have the script ask for input (two numbers). Tip For a function to return a string or array, use a dedicated variable. count_lines_in_etc_passwd() { [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd)) # If /etc/passwd is readable, set REPLY to line count. # Returns both a parameter value and status information. # The 'echo' seems unnecessary, but . . . #+ it removes excess whitespace from the output. } if count_lines_in_etc_passwd then echo "There are $REPLY lines in /etc/passwd." else echo "Cannot count lines in /etc/passwd." fi # Thanks, S.C. Example 23-8. Converting numbers to Roman numerals #!/bin/bash # Arabic number to Roman numeral conversion # Range: 0 - 200 # It's crude, but it works. # Extending the range and otherwise improving the script is left as an exercise. # Usage: roman number-to-convert LIMIT=200 E_ARG_ERR=65 E_OUT_OF_RANGE=66 if [ -z "$1" ] then echo "Usage: `basename $0` number-to-convert" exit $E_ARG_ERR fi num=$1 if [ "$num" -gt $LIMIT ] then echo "Out of range!" exit $E_OUT_OF_RANGE fi to_roman () # Must declare function before first call to it. { number=$1 factor=$2 rchar=$3 let "remainder = number - factor" while [ "$remainder" -ge 0 ] do echo -n $rchar let "number -= factor" let "remainder = number - factor" done return $number # Exercises: # --------- # 1) Explain how this function works. # Hint: division by successive subtraction. # 2) Extend to range of the function. # Hint: use "echo" and command-substitution capture. } to_roman $num 100 C num=$? to_roman $num 90 LXXXX num=$? to_roman $num 50 L num=$? to_roman $num 40 XL num=$? to_roman $num 10 X num=$? to_roman $num 9 IX num=$? to_roman $num 5 V num=$? to_roman $num 4 IV num=$? to_roman $num 1 I # Successive calls to conversion function! # Is this really necessary??? Can it be simplified? echo exit See also Example 10-28. Important The largest positive integer a function can return is 255. The return command is closely tied to the concept of exit status, which accounts for this particular limitation. Fortunately, there are various workarounds for those situations requiring a large integer return value from a function. Example 23-9. Testing large return values in a function #!/bin/bash # return-test.sh # The largest positive value a function can return is 255. return_test () # Returns whatever passed to it. { return $1 } return_test 27 # o.k. echo $? # Returns 27. return_test 255 # Still o.k. echo $? # Returns 255. return_test 257 # Error! echo $? # Returns 1 (return code for miscellaneous error). # ====================================================== return_test -151896 # Do large negative numbers work? echo $? # Will this return -151896? # No! It returns 168. # Version of Bash before 2.05b permitted #+ large negative integer return values. # Newer versions of Bash plug this loophole. # This may break older scripts. # Caution! # ====================================================== exit 0 A workaround for obtaining large integer "return values" is to simply assign the "return value" to a global variable. Return_Val= # Global variable to hold oversize return value of function. alt_return_test () { fvar=$1 Return_Val=$fvar return # Returns 0 (success). } alt_return_test 1 echo $? # 0 echo "return value = $Return_Val" # 1 alt_return_test 256 echo "return value = $Return_Val" # 256 alt_return_test 257 echo "return value = $Return_Val" # 257 alt_return_test 25701 echo "return value = $Return_Val" #25701 A more elegant method is to have the function echo its "return value to stdout," and then capture it by command substitution. See the discussion of this in Section 33.8. Example 23-10. Comparing two large integers #!/bin/bash # max2.sh: Maximum of two LARGE integers. # This is the previous "max.sh" example, #+ modified to permit comparing large integers. EQUAL=0 # Return value if both params equal. E_PARAM_ERR=-99999 # Not enough params passed to function. # ^^^^^^ Out of range of any params that might be passed. max2 () # "Returns" larger of two numbers. { if [ -z "$2" ] then echo $E_PARAM_ERR return fi if [ "$1" -eq "$2" ] then echo $EQUAL return else if [ "$1" -gt "$2" ] then retval=$1 else retval=$2 fi fi echo $retval # Echoes (to stdout), rather than returning value. # Why? } return_val=$(max2 33001 33997) # ^^^^ Function name # ^^^^^ ^^^^^ Params passed # This is actually a form of command substitution: #+ treating a function as if it were a command, #+ and assigning the stdout of the function to the variable "return_val." # ========================= OUTPUT ======================== if [ "$return_val" -eq "$E_PARAM_ERR" ] then echo "Error in parameters passed to comparison function!" elif [ "$return_val" -eq "$EQUAL" ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi # ========================================================= exit 0 # Exercises: # --------- # 1) Find a more elegant way of testing #+ the parameters passed to the function. # 2) Simplify the if/then structure at "OUTPUT." # 3) Rewrite the script to take input from command-line parameters. Here is another example of capturing a function "return value." Understanding it requires some knowledge of awk. month_length () # Takes month number as an argument. { # Returns number of days in month. monthD="31 28 31 30 31 30 31 31 30 31 30 31" # Declare as local? echo "$monthD" | awk '{ print $'"${1}"' }' # Tricky. # ^^^^^^^^^ # Parameter passed to function ($1 -- month number), then to awk. # Awk sees this as "print $1 . . . print $12" (depending on month number) # Template for passing a parameter to embedded awk script: # $'"${script_parameter}"' # Needs error checking for correct parameter range (1-12) #+ and for February in leap year. } # ---------------------------------------------- # Usage example: month=4 # April, for example (4th month). days_in=$(month_length $month) echo $days_in # 30 # ---------------------------------------------- See also Example A-7 and Example A-37. Exercise: Using what we have just learned, extend the previous Roman numerals example to accept arbitrarily large input. Redirection Redirecting the stdin of a function A function is essentially a code block, which means its stdin can be redirected (as in Example 3-1). Example 23-11. Real name from username #!/bin/bash # realname.sh # # From username, gets "real name" from /etc/passwd. ARGCOUNT=1 # Expect one arg. E_WRONGARGS=65 file=/etc/passwd pattern=$1 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` USERNAME" exit $E_WRONGARGS fi file_excerpt () # Scan file for pattern, { #+ then print relevant portion of line. while read line # "while" does not necessarily need [ condition ] do echo "$line" | grep $1 | awk -F":" '{ print $5 }' # Have awk use ":" delimiter. done } <$file # Redirect into function's stdin. file_excerpt $pattern # Yes, this entire script could be reduced to # grep PATTERN /etc/passwd | awk -F":" '{ print $5 }' # or # awk -F: '/PATTERN/ {print $5}' # or # awk -F: '($1 == "username") { print $5 }' # real name from username # However, it might not be as instructive. exit 0 There is an alternate, and perhaps less confusing method of redirecting a function's stdin. This involves redirecting the stdin to an embedded bracketed code block within the function. # Instead of: Function () { ... } < file # Try this: Function () { { ... } < file } # Similarly, Function () # This works. { { echo $* } | tr a b } Function () # This doesn't work. { echo $* } | tr a b # A nested code block is mandatory here. # Thanks, S.C. Note Emmanuel Rouat's sample bashrc file contains some instructive examples of functions. ------------------------------------------------------------------------ 23.2. Local Variables What makes a variable local? local variables A variable declared as local is one that is visible only within the block of code in which it appears. It has local scope. In a function, a local variable has meaning only within that function block. Example 23-12. Local variable visibility #!/bin/bash # Global and local variables inside a function. func () { local loc_var=23 # Declared as local variable. echo # Uses the 'local' builtin. echo "\"loc_var\" in function = $loc_var" global_var=999 # Not declared as local. # Defaults to global. echo "\"global_var\" in function = $global_var" } func # Now, to see if local variable "loc_var" exists outside function. echo echo "\"loc_var\" outside function = $loc_var" # $loc_var outside function = # No, $loc_var not visible globally. echo "\"global_var\" outside function = $global_var" # $global_var outside function = 999 # $global_var is visible globally. echo exit 0 # In contrast to C, a Bash variable declared inside a function #+ is local *only* if declared as such. Caution Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local. #!/bin/bash func () { global_var=37 # Visible only within the function block #+ before the function has been called. } # END OF FUNCTION echo "global_var = $global_var" # global_var = # Function "func" has not yet been called, #+ so $global_var is not visible here. func echo "global_var = $global_var" # global_var = 37 # Has been set by function call. ------------------------------------------------------------------------ 23.2.1. Local variables and recursion. +---------------------------------------------------------------------------+ |Recursion is an interesting and sometimes useful form of self-reference. | |Herbert Mayer defines it as ". . . expressing an algorithm by using a | |simpler version of that same algorithm . . ." | | | |Consider a definition defined in terms of itself, [99] an expression | |implicit in its own expression, [100] a snake swallowing its own tail, | |[101] or . . . a function that calls itself. [102] | | | |Example 23-13. Demonstration of a simple recursive function | | | |#!/bin/bash | |# recursion-demo.sh | |# Demonstration of recursion. | | | |RECURSIONS=9 # How many times to recurse. | |r_count=0 # Must be global. Why? | | | |recurse () | |{ | | var="$1" | | | | while [ "$var" -ge 0 ] | | do | | echo "Recursion count = "$r_count" +-+ \$var = "$var"" | | (( var-- )); (( r_count++ )) | | recurse "$var" # Function calls itself (recurses) | | done #+ until what condition is met? | |} | | | |recurse $RECURSIONS | | | |exit $? | | | |Example 23-14. Another simple demonstration | | | |#!/bin/bash | |# recursion-def.sh | |# A script that defines "recursion" in a rather graphic way. | | | |RECURSIONS=10 | |r_count=0 | |sp=" " | | | |define_recursion () | |{ | | ((r_count++)) | | sp="$sp"" " | | echo -n "$sp" | | echo "\"The act of recurring ... \"" # Per 1913 Webster's dictionary. | | | | while [ $r_count -le $RECURSIONS ] | | do | | define_recursion | | done | |} | | | |echo | |echo "Recursion: " | |define_recursion | |echo | | | |exit $? | +---------------------------------------------------------------------------+ Local variables are a useful tool for writing recursive code, but this practice generally involves a great deal of computational overhead and is definitely not recommended in a shell script. [103] Example 23-15. Recursion, using a local variable #!/bin/bash # factorial # --------- # Does bash permit recursion? # Well, yes, but... # It's so slow that you gotta have rocks in your head to try it. MAX_ARG=5 E_WRONG_ARGS=85 E_RANGE_ERR=86 if [ -z "$1" ] then echo "Usage: `basename $0` number" exit $E_WRONG_ARGS fi if [ "$1" -gt $MAX_ARG ] then echo "Out of range ($MAX_ARG is maximum)." # Let's get real now. # If you want greater range than this, #+ rewrite it in a Real Programming Language. exit $E_RANGE_ERR fi fact () { local number=$1 # Variable "number" must be declared as local, #+ otherwise this doesn't work. if [ "$number" -eq 0 ] then factorial=1 # Factorial of 0 = 1. else let "decrnum = number - 1" fact $decrnum # Recursive function call (the function calls itself). let "factorial = $number * $?" fi return $factorial } fact $1 echo "Factorial of $1 is $?." exit 0 Also see Example A-15 for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate in a script. ------------------------------------------------------------------------ 23.3. Recursion Without Local Variables A function may recursively call itself even without use of local variables. Example 23-16. The Fibonacci Sequence #!/bin/bash # fibo.sh : Fibonacci sequence (recursive) # Author: M. Cooper # License: GPL3 # --------------------------------- # Fibo(0) = 0 # Fibo(1) = 1 # else # Fibo(j) = Fibo(j-1) + Fibo(j-2) # --------------------------------- MAXTERM=15 # Number of terms (+1) to generate. MINIDX=2 # If idx is less than 2, then Fibo(idx) = idx. Fibonacci () { idx=$1 # Doesn't need to be local. Why not? if [ "$idx" -lt "$MINIDX" ] then echo "$idx" # First two terms are 0 1 ... see above. else (( --idx )) # j-1 term1=$( Fibonacci $idx ) # Fibo(j-1) (( --idx )) # j-2 term2=$( Fibonacci $idx ) # Fibo(j-2) echo $(( term1 + term2 )) fi # An ugly, ugly kludge. # The more elegant implementation of recursive fibo in C #+ is a straightforward translation of the algorithm in lines 7 - 10. } for i in $(seq 0 $MAXTERM) do # Calculate $MAXTERM+1 terms. FIBO=$(Fibonacci $i) echo -n "$FIBO " done # 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 # Takes a while, doesn't it? Recursion in a script is slow. echo exit 0 Example 23-17. The Towers of Hanoi #! /bin/bash # # The Towers Of Hanoi # Bash script # Copyright (C) 2000 Amit Singh. All Rights Reserved. # http://hanoi.kernelthread.com # # Tested under Bash version 2.05b.0(13)-release. # Also works under Bash version 3.x. # # Used in "Advanced Bash Scripting Guide" #+ with permission of script author. # Slightly modified and commented by ABS author. #=================================================================# # The Tower of Hanoi is a mathematical puzzle attributed to #+ Edouard Lucas, a nineteenth-century French mathematician. # # There are three vertical posts set in a base. # The first post has a set of annular rings stacked on it. # These rings are disks with a hole drilled out of the center, #+ so they can slip over the posts and rest flat. # The rings have different diameters, and they stack in ascending #+ order, according to size. # The smallest ring is on top, and the largest on the bottom. # # The task is to transfer the stack of rings #+ to one of the other posts. # You can move only one ring at a time to another post. # You are permitted to move rings back to the original post. # You may place a smaller ring atop a larger one, #+ but *not* vice versa. # Again, it is forbidden to place a larger ring atop a smaller one. # # For a small number of rings, only a few moves are required. #+ For each additional ring, #+ the required number of moves approximately doubles, #+ and the "strategy" becomes increasingly complicated. # # For more information, see http://hanoi.kernelthread.com #+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney. # # # ... ... ... # | | | | | | # _|_|_ | | | | # |_____| | | | | # |_______| | | | | # |_________| | | | | # |___________| | | | | # | | | | | | # .--------------------------------------------------------------. # |**************************************************************| # #1 #2 #3 # #=================================================================# E_NOPARAM=66 # No parameter passed to script. E_BADPARAM=67 # Illegal number of disks passed to script. Moves= # Global variable holding number of moves. # Modification to original script. dohanoi() { # Recursive function. case $1 in 0) ;; *) dohanoi "$(($1-1))" $2 $4 $3 echo move $2 "-->" $3 ((Moves++)) # Modification to original script. dohanoi "$(($1-1))" $4 $3 $2 ;; esac } case $# in 1) case $(($1>0)) in # Must have at least one disk. 1) # Nested case statement. dohanoi $1 1 3 2 echo "Total moves = $Moves" # 2^n - 1, where n = # of disks. exit 0; ;; *) echo "$0: illegal value for number of disks"; exit $E_BADPARAM; ;; esac ;; *) echo "usage: $0 N" echo " Where \"N\" is the number of disks." exit $E_NOPARAM; ;; esac # Exercises: # --------- # 1) Would commands beyond this point ever be executed? # Why not? (Easy) # 2) Explain the workings of the workings of the "dohanoi" function. # (Difficult -- see the Dewdney reference, above.) ------------------------------------------------------------------------ Chapter 24. Aliases A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence. If, for example, we include alias lm="ls -l | more" in the ~/.bashrc file, then each lm [104] typed at the command-line will automatically be replaced by a ls -l | more. This can save a great deal of typing at the command-line and avoid having to remember complex combinations of commands and options. Setting alias rm="rm -i" (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently deleting important files. In a script, aliases have very limited usefulness. It would be nice if aliases could assume some of the functionality of the C preprocessor, such as macro expansion, but unfortunately Bash does not expand arguments within the alias body. [105] Moreover, a script fails to expand an alias itself within "compound constructs," such as if/then statements, loops, and functions. An added limitation is that an alias will not expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function. Example 24-1. Aliases within a script #!/bin/bash # alias.sh shopt -s expand_aliases # Must set this option, else script will not expand aliases. # First, some fun. alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."' Jesse_James echo; echo; echo; alias ll="ls -l" # May use either single (') or double (") quotes to define an alias. echo "Trying aliased \"ll\":" ll /usr/X11R6/bin/mk* #* Alias works. echo directory=/usr/X11R6/bin/ prefix=mk* # See if wild card causes problems. echo "Variables \"directory\" + \"prefix\" = $directory$prefix" echo alias lll="ls -l $directory$prefix" echo "Trying aliased \"lll\":" lll # Long listing of all files in /usr/X11R6/bin stating with mk. # An alias can handle concatenated variables -- including wild card -- o.k. TRUE=1 echo if [ TRUE ] then alias rr="ls -l" echo "Trying aliased \"rr\" within if/then statement:" rr /usr/X11R6/bin/mk* #* Error message results! # Aliases not expanded within compound statements. echo "However, previously expanded alias still recognized:" ll /usr/X11R6/bin/mk* fi echo count=0 while [ $count -lt 3 ] do alias rrr="ls -l" echo "Trying aliased \"rrr\" within \"while\" loop:" rrr /usr/X11R6/bin/mk* #* Alias will not expand here either. # alias.sh: line 57: rrr: command not found let count+=1 done echo; echo alias xyz='cat $0' # Script lists itself. # Note strong quotes. xyz # This seems to work, #+ although the Bash documentation suggests that it shouldn't. # # However, as Steve Jacobson points out, #+ the "$0" parameter expands immediately upon declaration of the alias. exit 0 The unalias command removes a previously set alias. Example 24-2. unalias: Setting and unsetting an alias #!/bin/bash # unalias.sh shopt -s expand_aliases # Enables alias expansion. alias llm='ls -al | more' llm echo unalias llm # Unset alias. llm # Error message results, since 'llm' no longer recognized. exit 0 +----------------------------------------------------------------------+ |bash$ ./unalias.sh | |total 6 | |drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 . | |drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 .. | |-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh | | | |./unalias.sh: llm: command not found | +----------------------------------------------------------------------+ ------------------------------------------------------------------------ Chapter 25. List Constructs The and list and or list constructs provide a means of processing a number of commands consecutively. These can effectively replace complex nested if/then or even case statements. Chaining together commands and list command-1 && command-2 && command-3 && ... command-n Each command executes in turn, provided that the previous command has given a return value of true (zero). At the first false (non-zero) return, the command chain terminates (the first command returning false is the last one to execute). Example 25-1. Using an and list to test for command-line arguments #!/bin/bash # and list if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \ # ^^ ^^ ^^ echo "Argument #2 = $2" then echo "At least 2 arguments passed to script." # All the chained commands return true. else echo "Fewer than 2 arguments passed to script." # At least one of the chained commands returns false. fi # Note that "if [ ! -z $1 ]" works, but its alleged equivalent, # "if [ -n $1 ]" does not. # However, quoting fixes this. # if "[ -n "$1" ]" works. # ^ ^ Careful! # It is always best to QUOTE the variables being tested. # This accomplishes the same thing, using "pure" if/then statements. if [ ! -z "$1" ] then echo "Argument #1 = $1" fi if [ ! -z "$2" ] then echo "Argument #2 = $2" echo "At least 2 arguments passed to script." else echo "Fewer than 2 arguments passed to script." fi # It's longer and more ponderous than using an "and list". exit $? Example 25-2. Another command-line arg test using an and list #!/bin/bash ARGS=1 # Number of arguments expected. E_BADARGS=85 # Exit value if incorrect number of args passed. test $# -ne $ARGS && \ # ^^^^^^^^^^^^ condition #1 echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS # ^^ # If condition #1 tests true (wrong number of args passed to script), #+ then the rest of the line executes, and script terminates. # Line below executes only if the above test fails. echo "Correct number of arguments passed to this script." exit 0 # To check exit value, do a "echo $?" after script termination. Of course, an and list can also set variables to a default value. arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT # Set $arg1 to command-line arguments, if any. # But . . . set to DEFAULT if not specified on command-line. or list command-1 || command-2 || command-3 || ... command-n Each command executes in turn for as long as the previous command returns false. At the first true return, the command chain terminates (the first command returning true is the last one to execute). This is obviously the inverse of the "and list". Example 25-3. Using or lists in combination with an and list #!/bin/bash # delete.sh, a not-so-cunning file deletion utility. # Usage: delete filename E_BADARGS=85 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS # No arg? Bail out. else file=$1 # Set filename. fi [ ! -f "$file" ] && echo "File \"$file\" not found. \ Cowardly refusing to delete a nonexistent file." # AND LIST, to give error message if file not present. # Note echo message continuing on to a second line after an escape. [ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.") # OR LIST, to delete file if present. # Note logic inversion above. # AND LIST executes on true, OR LIST on false. exit $? Caution If the first command in an or list returns true, it will execute. # ==> The following snippets from the /etc/rc.d/init.d/single #+==> script by Miquel van Smoorenburg #+==> illustrate use of "and" and "or" lists. # ==> "Arrowed" comments added by document author. [ -x /usr/bin/clear ] && /usr/bin/clear # ==> If /usr/bin/clear exists, then invoke it. # ==> Checking for the existence of a command before calling it #+==> avoids error messages and other awkward consequences. # ==> . . . # If they want to run something in single user mode, might as well run it... for i in /etc/rc1.d/S[0-9][0-9]* ; do # Check if the script is there. [ -x "$i" ] || continue # ==> If corresponding file in $PWD *not* found, #+==> then "continue" by jumping to the top of the loop. # Reject backup files and files generated by rpm. case "$1" in *.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig) continue;; esac [ "$i" = "/etc/rc1.d/S00single" ] && continue # ==> Set script name, but don't execute it yet. $i start done # ==> . . . Important The exit status of an and list or an or list is the exit status of the last command executed. Clever combinations of and and or lists are possible, but the logic may easily become convoluted and require close attention to operator precedence rules, and possibly extensive debugging. false && true || echo false # false # Same result as ( false && true ) || echo false # false # But NOT false && ( true || echo false ) # (nothing echoed) # Note left-to-right grouping and evaluation of statements, #+ since the logic operators "&&" and "||" have equal precedence. # It's usually best to avoid such complexities. # Thanks, S.C. See Example A-7 and Example 7-4 for illustrations of using and / or list constructs to test variables. ------------------------------------------------------------------------ Chapter 26. Arrays Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the variable[xx] notation. Alternatively, a script may introduce the entire array by an explicit declare -a variable statement. To dereference (retrieve the contents of) an array element, use curly bracket notation, that is, ${element[xx]}. Example 26-1. Simple array usage #!/bin/bash area[11]=23 area[13]=37 area[51]=UFOs # Array members need not be consecutive or contiguous. # Some members of the array can be left uninitialized. # Gaps in the array are okay. # In fact, arrays with sparse data ("sparse arrays") #+ are useful in spreadsheet-processing software. echo -n "area[11] = " echo ${area[11]} # {curly brackets} needed. echo -n "area[13] = " echo ${area[13]} echo "Contents of area[51] are ${area[51]}." # Contents of uninitialized array variable print blank (null variable). echo -n "area[43] = " echo ${area[43]} echo "(area[43] unassigned)" echo # Sum of two array variables assigned to third area[5]=`expr ${area[11]} + ${area[13]}` echo "area[5] = area[11] + area[13]" echo -n "area[5] = " echo ${area[5]} area[6]=`expr ${area[11]} + ${area[51]}` echo "area[6] = area[11] + area[51]" echo -n "area[6] = " echo ${area[6]} # This fails because adding an integer to a string is not permitted. echo; echo; echo # ----------------------------------------------------------------- # Another array, "area2". # Another way of assigning array variables... # array_name=( XXX YYY ZZZ ... ) area2=( zero one two three four ) echo -n "area2[0] = " echo ${area2[0]} # Aha, zero-based indexing (first element of array is [0], not [1]). echo -n "area2[1] = " echo ${area2[1]} # [1] is second element of array. # ----------------------------------------------------------------- echo; echo; echo # ----------------------------------------------- # Yet another array, "area3". # Yet another way of assigning array variables... # array_name=([xx]=XXX [yy]=YYY ...) area3=([17]=seventeen [24]=twenty-four) echo -n "area3[17] = " echo ${area3[17]} echo -n "area3[24] = " echo ${area3[24]} # ----------------------------------------------- exit 0 As we have seen, a convenient way of initializing an entire array is the array=( element1 element2 ... elementN ) notation. +----------------------------------------------------------------------+ | Bash permits array operations on variables, even if the variables | | are not explicitly declared as arrays. | | | | string=abcABC123ABCabc | | echo ${string[@]} # abcABC123ABCabc | | echo ${string[*]} # abcABC123ABCabc | | echo ${string[0]} # abcABC123ABCabc | | echo ${string[1]} # No output! | | # Why? | | echo ${#string[@]} # 1 | | # One element in the array. | | # The string itself. | | | | # Thank you, Michael Zick, for pointing this out. | | | | Once again this demonstrates that Bash variables are untyped. | +----------------------------------------------------------------------+ Example 26-2. Formatting a poem #!/bin/bash # poem.sh: Pretty-prints one of the ABS Guide author's favorite poems. # Lines of the poem (single stanza). Line[1]="I do not know which to prefer," Line[2]="The beauty of inflections" Line[3]="Or the beauty of innuendoes," Line[4]="The blackbird whistling" Line[5]="Or just after." # Note that quoting permits embedding whitespace. # Attribution. Attrib[1]=" Wallace Stevens" Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\"" # This poem is in the Public Domain (copyright expired). echo tput bold # Bold print. for index in 1 2 3 4 5 # Five lines. do printf " %s\n" "${Line[index]}" done for index in 1 2 # Two attribution lines. do printf " %s\n" "${Attrib[index]}" done tput sgr0 # Reset terminal. # See 'tput' docs. echo exit 0 # Exercise: # -------- # Modify this script to pretty-print a poem from a text data file. Array variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use. Example 26-3. Various array operations #!/bin/bash # array-ops.sh: More fun with arrays. array=( zero one two three four five ) # Element 0 1 2 3 4 5 echo ${array[0]} # zero echo ${array:0} # zero # Parameter expansion of first element, #+ starting at position # 0 (1st character). echo ${array:1} # ero # Parameter expansion of first element, #+ starting at position # 1 (2nd character). echo "--------------" echo ${#array[0]} # 4 # Length of first element of array. echo ${#array} # 4 # Length of first element of array. # (Alternate notation) echo ${#array[1]} # 3 # Length of second element of array. # Arrays in Bash have zero-based indexing. echo ${#array[*]} # 6 # Number of elements in array. echo ${#array[@]} # 6 # Number of elements in array. echo "--------------" array2=( [0]="first element" [1]="second element" [3]="fourth element" ) # ^ ^ ^ ^ ^ ^ ^ ^ ^ # Quoting permits embedding whitespace within individual array elements. echo ${array2[0]} # first element echo ${array2[1]} # second element echo ${array2[2]} # # Skipped in initialization, and therefore null. echo ${array2[3]} # fourth element echo ${#array2[0]} # 13 (length of first element) echo ${#array2[*]} # 3 (number of elements in array) exit Many of the standard string operations work on arrays. Example 26-4. String operations on arrays #!/bin/bash # array-strops.sh: String operations on arrays. # Script by Michael Zick. # Used in ABS Guide with permission. # Fixups: 05 May 08, 04 Aug 08. # In general, any string operation using the ${name ... } notation #+ can be applied to all string elements in an array, #+ with the ${name[@] ... } or ${name[*] ...} notation. arrayZ=( one two three four five five ) echo # Trailing Substring Extraction echo ${arrayZ[@]:0} # one two three four five five # ^ All elements. echo ${arrayZ[@]:1} # two three four five five # ^ All elements following element[0]. echo ${arrayZ[@]:1:2} # two three # ^ Only the two elements after element[0]. echo "---------" # Substring Removal # Removes shortest match from front of string(s). echo ${arrayZ[@]#f*r} # one two three five five # ^ # Applied to all elements of the array. # Matches "four" and removes it. # Longest match from front of string(s) echo ${arrayZ[@]##t*e} # one two four five five # ^^ # Applied to all elements of the array. # Matches "three" and removes it. # Shortest match from back of string(s) echo ${arrayZ[@]%h*e} # one two t four five five # ^ # Applied to all elements of the array. # Matches "hree" and removes it. # Longest match from back of string(s) echo ${arrayZ[@]%%t*e} # one two four five five # ^^ # Applied to all elements of the array. # Matches "three" and removes it. echo "----------------------" # Substring Replacement # Replace first occurrence of substring with replacement. echo ${arrayZ[@]/fiv/XYZ} # one two three four XYZe XYZe # ^ # Applied to all elements of the array. # Replace all occurrences of substring. echo ${arrayZ[@]//iv/YY} # one two three four fYYe fYYe # Applied to all elements of the array. # Delete all occurrences of substring. # Not specifing a replacement defaults to 'delete' ... echo ${arrayZ[@]//fi/} # one two three four ve ve # ^^ # Applied to all elements of the array. # Replace front-end occurrences of substring. echo ${arrayZ[@]/#fi/XY} # one two three four XYve XYve # ^ # Applied to all elements of the array. # Replace back-end occurrences of substring. echo ${arrayZ[@]/%ve/ZZ} # one two three four fiZZ fiZZ # ^ # Applied to all elements of the array. echo ${arrayZ[@]/%o/XX} # one twXX three four five five # ^ # Why? echo "-----------------------------" replacement() { echo -n "!!!" } echo ${arrayZ[@]/%e/$(replacement)} # ^ ^^^^^^^^^^^^^^ # on!!! two thre!!! four fiv!!! fiv!!! # The stdout of replacement() is the replacement string. # Q.E.D: The replacement action is, in effect, an 'assignment.' echo "------------------------------------" # Accessing the "for-each": echo ${arrayZ[@]//*/$(replacement optional_arguments)} # ^^ ^^^^^^^^^^^^^ # !!! !!! !!! !!! !!! !!! # Now, if Bash would only pass the matched string #+ to the function being called . . . echo exit 0 # Before reaching for a Big Hammer -- Perl, Python, or all the rest -- # recall: # $( ... ) is command substitution. # A function runs as a sub-process. # A function writes its output (if echo-ed) to stdout. # Assignment, in conjunction with "echo" and command substitution, #+ can read a function's stdout. # The name[@] notation specifies (the equivalent of) a "for-each" #+ operation. # Bash is more powerful than you think! Command substitution can construct the individual elements of an array. Example 26-5. Loading the contents of a script into an array #!/bin/bash # script-array.sh: Loads this script into an array. # Inspired by an e-mail from Chris Martin (thanks!). script_contents=( $(cat "$0") ) # Stores contents of this script ($0) #+ in an array. for element in $(seq 0 $((${#script_contents[@]} - 1))) do # ${#script_contents[@]} #+ gives number of elements in the array. # # Question: # Why is seq 0 necessary? # Try changing it to seq 1. echo -n "${script_contents[$element]}" # List each field of this script on a single line. # echo -n "${script_contents[element]}" also works because of ${ ... }. echo -n " -- " # Use " -- " as a field separator. done echo exit 0 # Exercise: # -------- # Modify this script so it lists itself #+ in its original format, #+ complete with whitespace, line breaks, etc. In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array elements, or even an entire array. Example 26-6. Some special properties of arrays #!/bin/bash declare -a colors # All subsequent commands in this script will treat #+ the variable "colors" as an array. echo "Enter your favorite colors (separated from each other by a space)." read -a colors # Enter at least 3 colors to demonstrate features below. # Special option to 'read' command, #+ allowing assignment of elements in an array. echo element_count=${#colors[@]} # Special syntax to extract number of elements in array. # element_count=${#colors[*]} works also. # # The "@" variable allows word splitting within quotes #+ (extracts variables separated by whitespace). # # This corresponds to the behavior of "$@" and "$*" #+ in positional parameters. index=0 while [ "$index" -lt "$element_count" ] do # List all the elements in the array. echo ${colors[$index]} # ${colors[index]} also works because it's within ${ ... } brackets. let "index = $index + 1" # Or: # ((index++)) done # Each array element listed on a separate line. # If this is not desired, use echo -n "${colors[$index]} " # # Doing it with a "for" loop instead: # for i in "${colors[@]}" # do # echo "$i" # done # (Thanks, S.C.) echo # Again, list all the elements in the array, but using a more elegant method. echo ${colors[@]} # echo ${colors[*]} also works. echo # The "unset" command deletes elements of an array, or entire array. unset colors[1] # Remove 2nd element of array. # Same effect as colors[1]= echo ${colors[@]} # List array again, missing 2nd element. unset colors # Delete entire array. # unset colors[*] and #+ unset colors[@] also work. echo; echo -n "Colors gone." echo ${colors[@]} # List array again, now empty. exit 0 As seen in the previous example, either ${array_name[@]} or ${array_name[*]} refers to all the elements of the array. Similarly, to get a count of the number of elements in an array, use either ${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length (number of characters) of ${array_name[0]}, the first element of the array. Example 26-7. Of empty arrays and empty elements #!/bin/bash # empty-array.sh # Thanks to Stephane Chazelas for the original example, #+ and to Michael Zick and Omair Eshkenazi, for extending it. # And to Nathan Coulter for clarifications and corrections. # An empty array is not the same as an array with empty elements. array0=( first second third ) array1=( '' ) # "array1" consists of one empty element. array2=( ) # No elements . . . "array2" is empty. array3=( ) # What about this array? echo ListArray() { echo echo "Elements in array0: ${array0[@]}" echo "Elements in array1: ${array1[@]}" echo "Elements in array2: ${array2[@]}" echo "Elements in array3: ${array3[@]}" echo echo "Length of first element in array0 = ${#array0}" echo "Length of first element in array1 = ${#array1}" echo "Length of first element in array2 = ${#array2}" echo "Length of first element in array3 = ${#array3}" echo echo "Number of elements in array0 = ${#array0[*]}" # 3 echo "Number of elements in array1 = ${#array1[*]}" # 1 (Surprise!) echo "Number of elements in array2 = ${#array2[*]}" # 0 echo "Number of elements in array3 = ${#array3[*]}" # 0 } # =================================================================== ListArray # Try extending those arrays. # Adding an element to an array. array0=( "${array0[@]}" "new1" ) array1=( "${array1[@]}" "new1" ) array2=( "${array2[@]}" "new1" ) array3=( "${array3[@]}" "new1" ) ListArray # or array0[${#array0[*]}]="new2" array1[${#array1[*]}]="new2" array2[${#array2[*]}]="new2" array3[${#array3[*]}]="new2" ListArray # When extended as above, arrays are 'stacks' ... # Above is the 'push' ... # The stack 'height' is: height=${#array2[@]} echo echo "Stack height for array2 = $height" # The 'pop' is: unset array2[${#array2[@]}-1] # Arrays are zero-based, height=${#array2[@]} #+ which means first element has index 0. echo echo "POP" echo "New stack height for array2 = $height" ListArray # List only 2nd and 3rd elements of array0. from=1 # Zero-based numbering. to=2 array3=( ${array0[@]:1:2} ) echo echo "Elements in array3: ${array3[@]}" # Works like a string (array of characters). # Try some other "string" forms. # Replacement: array4=( ${array0[@]/second/2nd} ) echo echo "Elements in array4: ${array4[@]}" # Replace all matching wildcarded string. array5=( ${array0[@]//new?/old} ) echo echo "Elements in array5: ${array5[@]}" # Just when you are getting the feel for this . . . array6=( ${array0[@]#*new} ) echo # This one might surprise you. echo "Elements in array6: ${array6[@]}" array7=( ${array0[@]#new1} ) echo # After array6 this should not be a surprise. echo "Elements in array7: ${array7[@]}" # Which looks a lot like . . . array8=( ${array0[@]/new1/} ) echo echo "Elements in array8: ${array8[@]}" # So what can one say about this? # The string operations are performed on #+ each of the elements in var[@] in succession. # Therefore : Bash supports string vector operations. # If the result is a zero length string, #+ that element disappears in the resulting assignment. # However, if the expansion is in quotes, the null elements remain. # Michael Zick: Question, are those strings hard or soft quotes? # Nathan Coulter: There is no such thing as "soft quotes." #! What's really happening is that #!+ the pattern matching happens after #!+ all the other expansions of [word] #!+ in cases like ${parameter#word}. zap='new*' array9=( ${array0[@]/$zap/} ) echo echo "Number of elements in array9: ${#array9[@]}" array9=( "${array0[@]/$zap/}" ) echo "Elements in array9: ${array9[@]}" # This time the null elements remain. echo "Number of elements in array9: ${#array9[@]}" # Just when you thought you were still in Kansas . . . array10=( ${array0[@]#$zap} ) echo echo "Elements in array10: ${array10[@]}" # But, the asterisk in zap won't be interpreted if quoted. array10=( ${array0[@]#"$zap"} ) echo echo "Elements in array10: ${array10[@]}" # Well, maybe we _are_ still in Kansas . . . # (Revisions to above code block by Nathan Coulter.) # Compare array7 with array10. # Compare array8 with array9. # Reiterating: No such thing as soft quotes! # Nathan Coulter explains: # Pattern matching of 'word' in ${parameter#word} is done after #+ parameter expansion and *before* quote removal. # In the normal case, pattern matching is done *after* quote removal. exit The relationship of ${array_name[@]} and ${array_name[*]} is analogous to that between $@ and $*. This powerful array notation has a number of uses. # Copying an array. array2=( "${array1[@]}" ) # or array2="${array1[@]}" # # However, this fails with "sparse" arrays, #+ arrays with holes (missing elements) in them, #+ as Jochen DeSmet points out. # ------------------------------------------ array1[0]=0 # array1[1] not assigned array1[2]=2 array2=( "${array1[@]}" ) # Copy it? echo ${array2[0]} # 0 echo ${array2[2]} # (null), should be 2 # ------------------------------------------ # Adding an element to an array. array=( "${array[@]}" "new element" ) # or array[${#array[*]}]="new element" # Thanks, S.C. Tip The array=( element1 element2 ... elementN ) initialization operation, with the help of command substitution, makes it possible to load the contents of a text file into an array. #!/bin/bash filename=sample_file # cat sample_file # # 1 a b c # 2 d e fg declare -a array1 array1=( `cat "$filename"`) # Loads contents # List file to stdout #+ of $filename into array1. # # array1=( `cat "$filename" | tr '\n' ' '`) # change linefeeds in file to spaces. # Not necessary because Bash does word splitting, #+ changing linefeeds to spaces. echo ${array1[@]} # List the array. # 1 a b c 2 d e fg # # Each whitespace-separated "word" in the file #+ has been assigned to an element of the array. element_count=${#array1[*]} echo $element_count # 8 Clever scripting makes it possible to add array operations. Example 26-8. Initializing arrays #! /bin/bash # array-assign.bash # Array operations are Bash-specific, #+ hence the ".bash" in the script name. # Copyright (c) Michael S. Zick, 2003, All rights reserved. # License: Unrestricted reuse in any form, for any purpose. # Version: $ID$ # # Clarification and additional comments by William Park. # Based on an example provided by Stephane Chazelas #+ which appeared in an earlier version of the #+ Advanced Bash Scripting Guide. # Output format of the 'times' command: # User CPU System CPU # User CPU of dead children System CPU of dead children # Bash has two versions of assigning all elements of an array #+ to a new array variable. # Both drop 'null reference' elements #+ in Bash versions 2.04 and later. # An additional array assignment that maintains the relationship of #+ [subscript]=value for arrays may be added to newer versions. # Constructs a large array using an internal command, #+ but anything creating an array of several thousand elements #+ will do just fine. declare -a bigOne=( /dev/* ) # All the files in /dev . . . echo echo 'Conditions: Unquoted, default IFS, All-Elements-Of' echo "Number of elements in array is ${#bigOne[@]}" # set -vx echo echo '- - testing: =( ${array[@]} ) - -' times declare -a bigTwo=( ${bigOne[@]} ) # Note parens: ^ ^ times echo echo '- - testing: =${array[@]} - -' times declare -a bigThree=${bigOne[@]} # No parentheses this time. times # Comparing the numbers shows that the second form, pointed out #+ by Stephane Chazelas, is faster. # # As William Park explains: #+ The bigTwo array assigned element by element (because of parentheses), #+ whereas bigThree assigned as a single string. # So, in essence, you have: # bigTwo=( [0]="..." [1]="..." [2]="..." ... ) # bigThree=( [0]="... ... ..." ) # # Verify this by: echo ${bigTwo[0]} # echo ${bigThree[0]} # I will continue to use the first form in my example descriptions #+ because I think it is a better illustration of what is happening. # The reusable portions of my examples will actual contain #+ the second form where appropriate because of the speedup. # MSZ: Sorry about that earlier oversight folks. # Note: # ---- # The "declare -a" statements in lines 32 and 44 #+ are not strictly necessary, since it is implicit #+ in the Array=( ... ) assignment form. # However, eliminating these declarations slows down #+ the execution of the following sections of the script. # Try it, and see. exit 0 Note Adding a superfluous declare -a statement to an array declaration may speed up execution of subsequent operations on the array. Example 26-9. Copying and concatenating arrays #! /bin/bash # CopyArray.sh # # This script written by Michael Zick. # Used here with permission. # How-To "Pass by Name & Return by Name" #+ or "Building your own assignment statement". CpArray_Mac() { # Assignment Command Statement Builder echo -n 'eval ' echo -n "$2" # Destination name echo -n '=( ${' echo -n "$1" # Source name echo -n '[@]} )' # That could all be a single command. # Matter of style only. } declare -f CopyArray # Function "Pointer" CopyArray=CpArray_Mac # Statement Builder Hype() { # Hype the array named $1. # (Splice it together with array containing "Really Rocks".) # Return in array named $2. local -a TMP local -a hype=( Really Rocks ) $($CopyArray $1 TMP) TMP=( ${TMP[@]} ${hype[@]} ) $($CopyArray TMP $2) } declare -a before=( Advanced Bash Scripting ) declare -a after echo "Array Before = ${before[@]}" Hype before after echo "Array After = ${after[@]}" # Too much hype? echo "What ${after[@]:3:2}?" declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} ) # ---- substring extraction ---- echo "Array Modest = ${modest[@]}" # What happened to 'before' ? echo "Array Before = ${before[@]}" exit 0 Example 26-10. More on concatenating arrays #! /bin/bash # array-append.bash # Copyright (c) Michael S. Zick, 2003, All rights reserved. # License: Unrestricted reuse in any form, for any purpose. # Version: $ID$ # # Slightly modified in formatting by M.C. # Array operations are Bash-specific. # Legacy UNIX /bin/sh lacks equivalents. # Pipe the output of this script to 'more' #+ so it doesn't scroll off the terminal. # Or, redirect output to a file. declare -a array1=( zero1 one1 two1 ) # Subscript packed. declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 ) # Subscript sparse -- [1] is not defined. echo echo '- Confirm that the array is really subscript sparse. -' echo "Number of elements: 4" # Hard-coded for illustration. for (( i = 0 ; i < 4 ; i++ )) do echo "Element [$i]: ${array2[$i]}" done # See also the more general code example in basics-reviewed.bash. declare -a dest # Combine (append) two arrays into a third array. echo echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator' echo '- Undefined elements not present, subscripts not maintained. -' # # The undefined elements do not exist; they are not being dropped. dest=( ${array1[@]} ${array2[@]} ) # dest=${array1[@]}${array2[@]} # Strange results, possibly a bug. # Now, list the result. echo echo '- - Testing Array Append - -' cnt=${#dest[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${dest[$i]}" done # Assign an array to a single array element (twice). dest[0]=${array1[@]} dest[1]=${array2[@]} # List the result. echo echo '- - Testing modified array - -' cnt=${#dest[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${dest[$i]}" done # Examine the modified second element. echo echo '- - Reassign and list second element - -' declare -a subArray=${dest[1]} cnt=${#subArray[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${subArray[$i]}" done # The assignment of an entire array to a single element #+ of another array using the '=${ ... }' array assignment #+ has converted the array being assigned into a string, #+ with the elements separated by a space (the first character of IFS). # If the original elements didn't contain whitespace . . . # If the original array isn't subscript sparse . . . # Then we could get the original array structure back again. # Restore from the modified second element. echo echo '- - Listing restored element - -' declare -a subArray=( ${dest[1]} ) cnt=${#subArray[@]} echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do echo "Element [$i]: ${subArray[$i]}" done echo '- - Do not depend on this behavior. - -' echo '- - This behavior is subject to change - -' echo '- - in versions of Bash newer than version 2.05b - -' # MSZ: Sorry about any earlier confusion folks. exit 0 -- Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left for the reader to decide. Example 26-11. The Bubble Sort #!/bin/bash # bubble.sh: Bubble sort, of sorts. # Recall the algorithm for a bubble sort. In this particular version... # With each successive pass through the array to be sorted, #+ compare two adjacent elements, and swap them if out of order. # At the end of the first pass, the "heaviest" element has sunk to bottom. # At the end of the second pass, the next "heaviest" one has sunk next to bottom. # And so forth. # This means that each successive pass needs to traverse less of the array. # You will therefore notice a speeding up in the printing of the later passes. exchange() { # Swaps two members of the array. local temp=${Countries[$1]} # Temporary storage #+ for element getting swapped out. Countries[$1]=${Countries[$2]} Countries[$2]=$temp return } declare -a Countries # Declare array, #+ optional here since it's initialized below. # Is it permissable to split an array variable over multiple lines #+ using an escape (\)? # Yes. Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \ Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \ Israel Peru Canada Oman Denmark Wales France Kenya \ Xanadu Qatar Liechtenstein Hungary) # "Xanadu" is the mythical place where, according to Coleridge, #+ Kubla Khan did a pleasure dome decree. clear # Clear the screen to start with. echo "0: ${Countries[*]}" # List entire array at pass 0. number_of_elements=${#Countries[@]} let "comparisons = $number_of_elements - 1" count=1 # Pass number. while [ "$comparisons" -gt 0 ] # Beginning of outer loop do index=0 # Reset index to start of array after each pass. while [ "$index" -lt "$comparisons" ] # Beginning of inner loop do if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ] # If out of order... # Recalling that \> is ASCII comparison operator #+ within single brackets. # if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]] #+ also works. then exchange $index `expr $index + 1` # Swap. fi let "index += 1" # Or, index+=1 on Bash, ver. 3.1 or newer. done # End of inner loop # ---------------------------------------------------------------------- # Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative. # # for (( last = $number_of_elements - 1 ; last > 0 ; last-- )) ## Fix by C.Y. Hunt ^ (Thanks!) # do # for (( i = 0 ; i < last ; i++ )) # do # [[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \ # && exchange $i $((i+1)) # done # done # ---------------------------------------------------------------------- let "comparisons -= 1" # Since "heaviest" element bubbles to bottom, #+ we need do one less comparison each pass. echo echo "$count: ${Countries[@]}" # Print resultant array at end of each pass. echo let "count += 1" # Increment pass count. done # End of outer loop # All done. exit 0 -- Is it possible to nest arrays within arrays? #!/bin/bash # "Nested" array. # Michael Zick provided this example, #+ with corrections and clarifications by William Park. AnArray=( $(ls --inode --ignore-backups --almost-all \ --directory --full-time --color=none --time=status \ --sort=time -l ${PWD} ) ) # Commands and options. # Spaces are significant . . . and don't quote anything in the above. SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} ) # This array has six elements: #+ SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]} # [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} ) # # Arrays in Bash are (circularly) linked lists #+ of type string (char *). # So, this isn't actually a nested array, #+ but it's functionally similar. echo "Current directory and date of last status change:" echo "${SubArray[@]}" exit 0 -- Embedded arrays in combination with indirect references create some fascinating possibilities Example 26-12. Embedded arrays and indirect references #!/bin/bash # embedded-arrays.sh # Embedded arrays and indirect references. # This script by Dennis Leeuw. # Used with permission. # Modified by document author. ARRAY1=( VAR1_1=value11 VAR1_2=value12 VAR1_3=value13 ) ARRAY2=( VARIABLE="test" STRING="VAR1=value1 VAR2=value2 VAR3=value3" ARRAY21=${ARRAY1[*]} ) # Embed ARRAY1 within this second array. function print () { OLD_IFS="$IFS" IFS=$'\n' # To print each array element #+ on a separate line. TEST1="ARRAY2[*]" local ${!TEST1} # See what happens if you delete this line. # Indirect reference. # This makes the components of $TEST1 #+ accessible to this function. # Let's see what we've got so far. echo echo "\$TEST1 = $TEST1" # Just the name of the variable. echo; echo echo "{\$TEST1} = ${!TEST1}" # Contents of the variable. # That's what an indirect #+ reference does. echo echo "-------------------------------------------"; echo echo # Print variable echo "Variable VARIABLE: $VARIABLE" # Print a string element IFS="$OLD_IFS" TEST2="STRING[*]" local ${!TEST2} # Indirect reference (as above). echo "String element VAR2: $VAR2 from STRING" # Print an array element TEST2="ARRAY21[*]" local ${!TEST2} # Indirect reference (as above). echo "Array element VAR1_1: $VAR1_1 from ARRAY21" } print echo exit 0 # As the author of the script notes, #+ "you can easily expand it to create named-hashes in bash." # (Difficult) exercise for the reader: implement this. -- Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a resource-intensive application of this nature should really be written in a compiled language, such as C. It runs excruciatingly slowly as a script. Example 26-13. The Sieve of Eratosthenes #!/bin/bash # sieve.sh (ex68.sh) # Sieve of Eratosthenes # Ancient algorithm for finding prime numbers. # This runs a couple of orders of magnitude slower #+ than the equivalent program written in C. LOWER_LIMIT=1 # Starting with 1. UPPER_LIMIT=1000 # Up to 1000. # (You may set this higher . . . if you have time on your hands.) PRIME=1 NON_PRIME=0 let SPLIT=UPPER_LIMIT/2 # Optimization: # Need to test numbers only halfway to upper limit. Why? declare -a Primes # Primes[] is an array. initialize () { # Initialize the array. i=$LOWER_LIMIT until [ "$i" -gt "$UPPER_LIMIT" ] do Primes[i]=$PRIME let "i += 1" done # Assume all array members guilty (prime) #+ until proven innocent. } print_primes () { # Print out the members of the Primes[] array tagged as prime. i=$LOWER_LIMIT until [ "$i" -gt "$UPPER_LIMIT" ] do if [ "${Primes[i]}" -eq "$PRIME" ] then printf "%8d" $i # 8 spaces per number gives nice, even columns. fi let "i += 1" done } sift () # Sift out the non-primes. { let i=$LOWER_LIMIT+1 # Let's start with 2. until [ "$i" -gt "$UPPER_LIMIT" ] do if [ "${Primes[i]}" -eq "$PRIME" ] # Don't bother sieving numbers already sieved (tagged as non-prime). then t=$i while [ "$t" -le "$UPPER_LIMIT" ] do let "t += $i " Primes[t]=$NON_PRIME # Tag as non-prime all multiples. done fi let "i += 1" done } # ============================================== # main () # Invoke the functions sequentially. initialize sift print_primes # This is what they call structured programming. # ============================================== echo exit 0 # -------------------------------------------------------- # # Code below line will not execute, because of 'exit.' # This improved version of the Sieve, by Stephane Chazelas, #+ executes somewhat faster. # Must invoke with command-line argument (limit of primes). UPPER_LIMIT=$1 # From command-line. let SPLIT=UPPER_LIMIT/2 # Halfway to max number. Primes=( '' $(seq $UPPER_LIMIT) ) i=1 until (( ( i += 1 ) > SPLIT )) # Need check only halfway. do if [[ -n $Primes[i] ]] then t=$i until (( ( t += i ) > UPPER_LIMIT )) do Primes[t]= done fi done echo ${Primes[*]} exit $? Example 26-14. The Sieve of Eratosthenes, Optimized #!/bin/bash # Optimized Sieve of Eratosthenes # Script by Jared Martin, with very minor changes by ABS Guide author. # Used in ABS Guide with permission (thanks!). # Based on script in Advanced Bash Scripting Guide. # http://tldp.org/LDP/abs/html/arrays.html#PRIMES0 (ex68.sh). # http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf (reference) # Check results against http://primes.utm.edu/lists/small/1000.txt # Necessary but not sufficient would be, e.g., # (($(sieve 7919 | wc -w) == 1000)) && echo "7919 is the 1000th prime" UPPER_LIMIT=${1:?"Need an upper limit of primes to search."} Primes=( '' $(seq ${UPPER_LIMIT}) ) typeset -i i t Primes[i=1]='' # 1 is not a prime. until (( ( i += 1 ) > (${UPPER_LIMIT}/i) )) # Need check only ith-way. do # Why? if ((${Primes[t=i*(i-1), i]})) # Obscure, but instructive, use of arithmetic expansion in subscript. then until (( ( t += i ) > ${UPPER_LIMIT} )) do Primes[t]=; done fi done # echo ${Primes[*]} echo # Change to original script for pretty-printing (80-col. display). printf "%8d" ${Primes[*]} echo; echo exit $? Compare these array-based prime number generators with alternatives that do not use arrays, Example A-15, and Example 15-46. -- Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support. Example 26-15. Emulating a push-down stack #!/bin/bash # stack.sh: push-down stack simulation # Similar to the CPU stack, a push-down stack stores data items #+ sequentially, but releases them in reverse order, last-in first-out. BP=100 # Base Pointer of stack array. # Begin at element 100. SP=$BP # Stack Pointer. # Initialize it to "base" (bottom) of stack. Data= # Contents of stack location. # Must use global variable, #+ because of limitation on function return range. # 100 Base pointer <-- Base Pointer # 99 First data item # 98 Second data item # ... More data # Last data item <-- Stack pointer declare -a stack push() # Push item on stack. { if [ -z "$1" ] # Nothing to push? then return fi let "SP -= 1" # Bump stack pointer. stack[$SP]=$1 return } pop() # Pop item off stack. { Data= # Empty out data item. if [ "$SP" -eq "$BP" ] # Stack empty? then return fi # This also keeps SP from getting past 100, #+ i.e., prevents a runaway stack. Data=${stack[$SP]} let "SP += 1" # Bump stack pointer. return } status_report() # Find out what's happening. { echo "-------------------------------------" echo "REPORT" echo "Stack Pointer = $SP" echo "Just popped \""$Data"\" off the stack." echo "-------------------------------------" echo } # ======================================================= # Now, for some fun. echo # See if you can pop anything off empty stack. pop status_report echo push garbage pop status_report # Garbage in, garbage out. value1=23; push $value1 value2=skidoo; push $value2 value3=LAST; push $value3 pop # LAST status_report pop # skidoo status_report pop # 23 status_report # Last-in, first-out! # Notice how the stack pointer decrements with each push, #+ and increments with each pop. echo exit 0 # ======================================================= # Exercises: # --------- # 1) Modify the "push()" function to permit pushing # + multiple element on the stack with a single function call. # 2) Modify the "pop()" function to permit popping # + multiple element from the stack with a single function call. # 3) Add error checking to the critical functions. # That is, return an error code, depending on # + successful or unsuccessful completion of the operation, # + and take appropriate action. # 4) Using this script as a starting point, # + write a stack-based 4-function calculator. -- Fancy manipulation of array "subscripts" may require intermediate variables. For projects involving this, again consider using a more powerful programming language, such as Perl or C. Example 26-16. Complex array application: Exploring a weird mathematical series #!/bin/bash # Douglas Hofstadter's notorious "Q-series": # Q(1) = Q(2) = 1 # Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2 # This is a "chaotic" integer series with strange #+ and unpredictable behavior. # The first 20 terms of the series are: # 1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12 # See Hofstadter's book, _Goedel, Escher, Bach: An Eternal Golden Braid_, #+ p. 137, ff. LIMIT=100 # Number of terms to calculate. LINEWIDTH=20 # Number of terms printed per line. Q[1]=1 # First two terms of series are 1. Q[2]=1 echo echo "Q-series [$LIMIT terms]:" echo -n "${Q[1]} " # Output first two terms. echo -n "${Q[2]} " for ((n=3; n <= $LIMIT; n++)) # C-like loop expression. do # Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]] for n>2 # Need to break the expression into intermediate terms, #+ since Bash doesn't handle complex array arithmetic very well. let "n1 = $n - 1" # n-1 let "n2 = $n - 2" # n-2 t0=`expr $n - ${Q[n1]}` # n - Q[n-1] t1=`expr $n - ${Q[n2]}` # n - Q[n-2] T0=${Q[t0]} # Q[n - Q[n-1]] T1=${Q[t1]} # Q[n - Q[n-2]] Q[n]=`expr $T0 + $T1` # Q[n - Q[n-1]] + Q[n - Q[n-2]] echo -n "${Q[n]} " if [ `expr $n % $LINEWIDTH` -eq 0 ] # Format output. then # ^ modulo echo # Break lines into neat chunks. fi done echo exit 0 # This is an iterative implementation of the Q-series. # The more intuitive recursive implementation is left as an exercise. # Warning: calculating this series recursively takes a VERY long time #+ via a script. C/C++ would be orders of magnitude faster. -- Bash supports only one-dimensional arrays, though a little trickery permits simulating multi-dimensional ones. Example 26-17. Simulating a two-dimensional array, then tilting it #!/bin/bash # twodim.sh: Simulating a two-dimensional array. # A one-dimensional array consists of a single row. # A two-dimensional array stores rows sequentially. Rows=5 Columns=5 # 5 X 5 Array. declare -a alpha # char alpha [Rows] [Columns]; # Unnecessary declaration. Why? load_alpha () { local rc=0 local index for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y do # Use different symbols if you like. local row=`expr $rc / $Columns` local column=`expr $rc % $Rows` let "index = $row * $Rows + $column" alpha[$index]=$i # alpha[$row][$column] let "rc += 1" done # Simpler would be #+ declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y ) #+ but this somehow lacks the "flavor" of a two-dimensional array. } print_alpha () { local row=0 local index echo while [ "$row" -lt "$Rows" ] # Print out in "row major" order: do #+ columns vary, #+ while row (outer loop) remains the same. local column=0 echo -n " " # Lines up "square" array with rotated one. while [ "$column" -lt "$Columns" ] do let "index = $row * $Rows + $column" echo -n "${alpha[index]} " # alpha[$row][$column] let "column += 1" done let "row += 1" echo done # The simpler equivalent is # echo ${alpha[*]} | xargs -n $Columns echo } filter () # Filter out negative array indices. { echo -n " " # Provides the tilt. # Explain how. if [[ "$1" -ge 0 && "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]] then let "index = $1 * $Rows + $2" # Now, print it rotated. echo -n " ${alpha[index]}" # alpha[$row][$column] fi } rotate () # Rotate the array 45 degrees -- { #+ "balance" it on its lower lefthand corner. local row local column for (( row = Rows; row > -Rows; row-- )) do # Step through the array backwards. Why? for (( column = 0; column < Columns; column++ )) do if [ "$row" -ge 0 ] then let "t1 = $column - $row" let "t2 = $column" else let "t1 = $column" let "t2 = $column + $row" fi filter $t1 $t2 # Filter out negative array indices. # What happens if you don't do this? done echo; echo done # Array rotation inspired by examples (pp. 143-146) in #+ "Advanced C Programming on the IBM PC," by Herbert Mayer #+ (see bibliography). # This just goes to show that much of what can be done in C #+ can also be done in shell scripting. } #--------------- Now, let the show begin. ------------# load_alpha # Load the array. print_alpha # Print it out. rotate # Rotate it 45 degrees counterclockwise. #-----------------------------------------------------# exit 0 # This is a rather contrived, not to mention inelegant simulation. # Exercises: # --------- # 1) Rewrite the array loading and printing functions # in a more intuitive and less kludgy fashion. # # 2) Figure out how the array rotation functions work. # Hint: think about the implications of backwards-indexing an array. # # 3) Rewrite this script to handle a non-square array, # such as a 6 X 4 one. # Try to minimize "distortion" when the array is rotated. A two-dimensional array is essentially equivalent to a one-dimensional one, but with additional addressing modes for referencing and manipulating the individual elements by row and column position. For an even more elaborate example of simulating a two-dimensional array, see Example A-10. -- For more interesting scripts using arrays, see: * Example 11-3 * Example 15-46 * Example A-22 * Example A-44 * Example A-41 * Example A-42 ------------------------------------------------------------------------ Chapter 27. /dev and /proc A Linux or UNIX filesystem typically has the /dev and /proc special-purpose directories. ------------------------------------------------------------------------ 27.1. /dev The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [106] Appropriately enough, these are called device files. As an example, the hard drive partitions containing the mounted filesystem(s) have entries in /dev, as df shows. +----------------------------------------------------------------------+ |bash$ df | |Filesystem 1k-blocks Used Available Use% | | Mounted on | | /dev/hda6 495876 222748 247527 48% / | | /dev/hda1 50755 3887 44248 9% /boot | | /dev/hda8 367013 13262 334803 4% /home | | /dev/hda5 1714416 1123624 503704 70% /usr | | | +----------------------------------------------------------------------+ Among other things, the /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. [107] This permits mounting an entire filesystem within a single large file. See Example 16-8 and Example 16-7. A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive partition), /dev/udp (User Datagram Packet port), and /dev/tcp. For instance: To manually mount a USB flash drive, append the following line to /etc/fstab. [108] /dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0 (See also Example A-23.) Checking whether a disk is in the CD-burner (soft-linked to /dev/hdc): head -1 /dev/hdc # head: cannot open '/dev/hdc' for reading: No medium found # (No disc in the drive.) # head: error reading '/dev/hdc': Input/output error # (There is a disk in the drive, but it can't be read; #+ possibly it's an unrecorded CDR blank.) # Stream of characters and assorted gibberish # (There is a pre-recorded disk in the drive, #+ and this is raw output -- a stream of ASCII and binary data.) # Here we see the wisdom of using 'head' to limit the output #+ to manageable proportions, rather than 'cat' or something similar. # Now, it's just a matter of checking/parsing the output and taking #+ appropriate action. When executing a command on a /dev/tcp/$host/$port pseudo-device file, Bash opens a TCP connection to the associated socket. +----------------------------------------------------------------------+ | A socket is a communications node associated with a specific I/O | | port. (This is analogous to a hardware socket, or receptacle, for a | | connecting cable.) It permits data transfer between hardware devices | | on the same machine, between machines on the same network, between | | machines across different networks, and, of course, between machines | | at different locations on the Internet. | +----------------------------------------------------------------------+ The following examples assume an active Internet connection. Getting the time from nist.gov: +----------------------------------------------------------------------+ |bash$ cat /dev/tcp/www.net.cn/80 | |bash$ echo -e "GET / HTTP/1.0\n" >&5 | |bash$ cat <&5 | | | +----------------------------------------------------------------------+ [Thanks, Mark and Mihai Maties.] Example 27-1. Using /dev/tcp for troubleshooting #!/bin/bash # dev-tcp.sh: /dev/tcp redirection to check Internet connection. # Script by Troy Engel. # Used with permission. TCP_HOST=www.dns-diy.com # A known spam-friendly ISP. TCP_PORT=80 # Port 80 is http. # Try to connect. (Somewhat similar to a 'ping' . . .) echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT} MYEXIT=$? : <From the bash reference: /dev/tcp/host/port If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. EXPLANATION if [ "X$MYEXIT" = "X0" ]; then echo "Connection successful. Exit code: $MYEXIT" else echo "Connection unsuccessful. Exit code: $MYEXIT" fi exit $MYEXIT Example 27-2. Playing music #!/bin/bash # music.sh # MUSIC WITHOUT EXTERNAL FILES # Author: Antonio Macchi # Used in ABS Guide with permission # /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte), #+ 1 channel (mono) duration=2000 # If 8000 bytes = 1 second, then 2000 = 1/4 second. volume=$'\xc0' # Max volume = \xff (or \x00). mute=$'\x80' # No volume = \x80 (the middle). function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz :: { #+ 8000 fps / 440 = 16 :: A = 16 bytes per second) for t in `seq 0 $duration` do test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute done } e=`mknote 49` g=`mknote 41` a=`mknote 36` b=`mknote 32` c=`mknote 30` cis=`mknote 29` d=`mknote 27` e2=`mknote 24` n=`mknote 32767` # European notation. echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \ $n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp # dsp = Digital Signal Processor exit # A "bonny" example of a shell script! ------------------------------------------------------------------------ 27.2. /proc The /proc directory is actually a pseudo-filesystem. The files in /proc mirror currently running system and kernel processes and contain information and statistics about them. +-------------------------------------------------------------------------------------------+ |bash$ cat /proc/devices | |Character devices: | | 1 mem | | 2 pty | | 3 ttyp | | 4 ttyS | | 5 cua | | 7 vcs | | 10 misc | | 14 sound | | 29 fb | | 36 netlink | | 128 ptm | | 136 pts | | 162 raw | | 254 pcmcia | | | | Block devices: | | 1 ramdisk | | 2 fd | | 3 ide0 | | 9 md | | | | | | | |bash$ cat /proc/interrupts | | CPU0 | | 0: 84505 XT-PIC timer | | 1: 3375 XT-PIC keyboard | | 2: 0 XT-PIC cascade | | 5: 1 XT-PIC soundblaster | | 8: 1 XT-PIC rtc | | 12: 4231 XT-PIC PS/2 Mouse | | 14: 109373 XT-PIC ide0 | | NMI: 0 | | ERR: 0 | | | | | |bash$ cat /proc/partitions | |major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse running use aveq| | | | 3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030 | | 3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140 | | 3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0 | | 3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210 | | ... | | | | | | | |bash$ cat /proc/loadavg | |0.13 0.42 0.27 2/44 1119 | | | | | | | |bash$ cat /proc/apm | |1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ? | | | | | | | |bash$ cat /proc/acpi/battery/BAT0/info | |present: yes | | design capacity: 43200 mWh | | last full capacity: 36640 mWh | | battery technology: rechargeable | | design voltage: 10800 mV | | design capacity warning: 1832 mWh | | design capacity low: 200 mWh | | capacity granularity 1: 1 mWh | | capacity granularity 2: 1 mWh | | model number: IBM-02K6897 | | serial number: 1133 | | battery type: LION | | OEM info: Panasonic | | | | | | | |bash$ fgrep Mem /proc/meminfo | |MemTotal: 515216 kB | | MemFree: 266248 kB | | | +-------------------------------------------------------------------------------------------+ Shell scripts may extract data from certain of the files in /proc. [109] FS=iso # ISO filesystem support in kernel? grep $FS /proc/filesystems # iso9660 kernel_version=$( awk '{ print $3 }' /proc/version ) CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo ) if [ "$CPU" = "Pentium(R)" ] then run_some_commands ... else run_other_commands ... fi cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' ) # Current operating speed (in MHz) of the cpu on your machine. # On a laptop this may vary, depending on use of battery #+ or AC power. #!/bin/bash # get-commandline.sh # Get the command-line parameters of a process. OPTION=cmdline # Identify PID. pid=$( echo $(pidof "$1") | awk '{ print $1 }' ) # Get only first ^^^^^^^^^^^^^^^^^^ of multiple instances. echo echo "Process ID of (first instance of) "$1" = $pid" echo -n "Command-line arguments: " cat /proc/"$pid"/"$OPTION" | xargs -0 echo # Formats output: ^^^^^^^^^^^^^^^ # (Thanks, Han Holl, for the fixup!) echo; echo # For example: # sh get-commandline.sh xterm + devfile="/proc/bus/usb/devices" text="Spd" USB1="Spd=12" USB2="Spd=480" bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}') # ^^^^ Stop after first match. if [ "$bus_speed" = "$USB1" ] then echo "USB 1.1 port found." # Do something appropriate for USB 1.1. fi Note It is even possible to control certain peripherals with commands sent to the /proc directory. +-----------------------------------------------------------------+ | root# echo on > /proc/acpi/ibm/light | | | +-----------------------------------------------------------------+ This turns on the Thinklight in certain models of IBM/Lenovo Thinkpads. (May not work on all Linux distros.) Of course, caution is advised when writing to /proc. The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The stat and status files keep running statistics on the process, the cmdline file holds the command-line arguments the process was invoked with, and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint. Example 27-3. Finding the process associated with a PID #!/bin/bash # pid-identifier.sh: # Gives complete path name to process associated with pid. ARGNO=1 # Number of arguments the script expects. E_WRONGARGS=65 E_BADPID=66 E_NOSUCHPROCESS=67 E_NOPERMISSION=68 PROCFILE=exe if [ $# -ne $ARGNO ] then echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr. exit $E_WRONGARGS fi pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 ) # Checks for pid in "ps" listing, field #1. # Then makes sure it is the actual process, not the process invoked by this script. # The last "grep $1" filters out this possibility. # # pidno=$( ps ax | awk '{ print $1 }' | grep $1 ) # also works, as Teemu Huovila, points out. if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string, then #+ no running process corresponds to the pid given. echo "No such process running." exit $E_NOSUCHPROCESS fi # Alternatively: # if ! ps $1 > /dev/null 2>&1 # then # no running process corresponds to the pid given. # echo "No such process running." # exit $E_NOSUCHPROCESS # fi # To simplify the entire process, use "pidof". if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission. then echo "Process $1 running, but..." echo "Can't get read permission on /proc/$1/$PROCFILE." exit $E_NOPERMISSION # Ordinary user can't access some files in /proc. fi # The last two tests may be replaced by: # if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but # this will test whether it is possible # to send a signal to the process. # then echo "PID doesn't exist or you're not its owner" >&2 # exit $E_BADPID # fi exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' ) # Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' ) # # /proc/pid-number/exe is a symbolic link #+ to the complete path name of the invoking process. if [ -e "$exe_file" ] # If /proc/pid-number/exe exists, then #+ then the corresponding process exists. echo "Process #$1 invoked by $exe_file." else echo "No such process running." fi # This elaborate script can *almost* be replaced by # ps ax | grep $1 | awk '{ print $5 }' # However, this will not work... #+ because the fifth field of 'ps' is argv[0] of the process, #+ not the executable file path. # # However, either of the following would work. # find /proc/$1/exe -printf '%l\n' # lsof -aFn -p $1 -d txt | sed -ne 's/^n//p' # Additional commentary by Stephane Chazelas. exit 0 Example 27-4. On-line connect status #!/bin/bash PROCNAME=pppd # ppp daemon PROCFILENAME=status # Where to look. NOTCONNECTED=65 INTERVAL=2 # Update every 2 seconds. pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' ) # Finding the process number of 'pppd', the 'ppp daemon'. # Have to filter out the process lines generated by the search itself. # # However, as Oleg Philon points out, #+ this could have been considerably simplified by using "pidof". # pidno=$( pidof $PROCNAME ) # # Moral of the story: #+ When a command sequence gets too complex, look for a shortcut. if [ -z "$pidno" ] # If no pid, then process is not running. then echo "Not connected." exit $NOTCONNECTED else echo "Connected."; echo fi while [ true ] # Endless loop, script can be improved here. do if [ ! -e "/proc/$pidno/$PROCFILENAME" ] # While process running, then "status" file exists. then echo "Disconnected." exit $NOTCONNECTED fi netstat -s | grep "packets received" # Get some connect statistics. netstat -s | grep "packets delivered" sleep $INTERVAL echo; echo done exit 0 # As it stands, this script must be terminated with a Control-C. # Exercises: # --------- # Improve the script so it exits on a "q" keystroke. # Make the script more user-friendly in other ways. Warning In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the machine. ------------------------------------------------------------------------ Chapter 28. Of Zeros and Nulls Faultily faultless, icily regular, splendidly null Dead perfection; no more. --Alfred Lord Tennyson /dev/zero ... /dev/null Uses of /dev/null Think of /dev/null as a black hole. It is essentially the equivalent of a write-only file. Everything written to it disappears. Attempts to read or output from it result in nothing. All the same, /dev/null can be quite useful from both the command-line and in scripts. Suppressing stdout. cat $filename >/dev/null # Contents of the file will not list to stdout. Suppressing stderr (from Example 15-3). rm $badname 2>/dev/null # So error messages [stderr] deep-sixed. Suppressing output from both stdout and stderr. cat $filename 2>/dev/null >/dev/null # If "$filename" does not exist, there will be no error message output. # If "$filename" does exist, the contents of the file will not list to stdout. # Therefore, no output at all will result from the above line of code. # # This can be useful in situations where the return code from a command #+ needs to be tested, but no output is desired. # # cat $filename &>/dev/null # also works, as Baris Cicek points out. Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example 2-1 and Example 2-3): cat /dev/null > /var/log/messages # : > /var/log/messages has same effect, but does not spawn a new process. cat /dev/null > /var/log/wtmp Automatically emptying the contents of a logfile (especially good for dealing with those nasty "cookies" sent by commercial Web sites): Example 28-1. Hiding the cookie jar # Obsolete Netscape browser. # Same principle applies to newer browsers. if [ -f ~/.netscape/cookies ] # Remove, if exists. then rm -f ~/.netscape/cookies fi ln -s /dev/null ~/.netscape/cookies # All cookies now get sent to a black hole, rather than saved to disk. Uses of /dev/zero Like /dev/null, /dev/zero is a pseudo-device file, but it actually produces a stream of nulls (binary zeros, not the ASCII kind). Output written to /dev/zero disappears, and it is fairly difficult to actually read the nulls emitted there, though it can be done with od or a hex editor. The chief use of /dev/zero is creating an initialized dummy file of predetermined length intended as a temporary swap file. Example 28-2. Setting up a swapfile using /dev/zero #!/bin/bash # Creating a swap file. # A swap file provides a temporary storage cache #+ which helps speed up certain filesystem operations. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=85 # Not root? FILE=/swap BLOCKSIZE=1024 MINBLOCKS=40 SUCCESS=0 # This script must be run as root. if [ "$UID" -ne "$ROOT_UID" ] then echo; echo "You must be root to run this script."; echo exit $E_WRONG_USER fi blocks=${1:-$MINBLOCKS} # Set to default of 40 blocks, #+ if nothing specified on command-line. # This is the equivalent of the command block below. # -------------------------------------------------- # if [ -n "$1" ] # then # blocks=$1 # else # blocks=$MINBLOCKS # fi # -------------------------------------------------- if [ "$blocks" -lt $MINBLOCKS ] then blocks=$MINBLOCKS # Must be at least 40 blocks long. fi ###################################################################### echo "Creating swap file of size $blocks blocks (KB)." dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file. mkswap $FILE $blocks # Designate it a swap file. swapon $FILE # Activate swap file. retcode=$? # Everything worked? # Note that if one or more of these commands fails, #+ then it could cause nasty problems. ###################################################################### # Exercise: # Rewrite the above block of code so that if it does not execute #+ successfully, then: # 1) an error message is echoed to stderr, # 2) all temporary files are cleaned up, and # 3) the script exits in an orderly fashion with an #+ appropriate error code. echo "Swap file created and activated." exit $retcode Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 16-8) or "securely" deleting a file (see Example 15-60). Example 28-3. Creating a ramdisk #!/bin/bash # ramdisk.sh # A "ramdisk" is a segment of system RAM memory #+ which acts as if it were a filesystem. # Its advantage is very fast access (read/write time). # Disadvantages: volatility, loss of data on reboot or powerdown. #+ less RAM available to system. # # Of what use is a ramdisk? # Keeping a large dataset, such as a table or dictionary on ramdisk, #+ speeds up data lookup, since memory access is much faster than disk access. E_NON_ROOT_USER=70 # Must run as root. ROOTUSER_NAME=root MOUNTPT=/mnt/ramdisk SIZE=2000 # 2K blocks (change as appropriate) BLOCKSIZE=1024 # 1K (1024 byte) block size DEVICE=/dev/ram0 # First ram device username=`id -nu` if [ "$username" != "$ROOTUSER_NAME" ] then echo "Must be root to run \"`basename $0`\"." exit $E_NON_ROOT_USER fi if [ ! -d "$MOUNTPT" ] # Test whether mount point already there, then #+ so no error if this script is run mkdir $MOUNTPT #+ multiple times. fi ############################################################################## dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device. # Why is this necessary? mke2fs $DEVICE # Create an ext2 filesystem on it. mount $DEVICE $MOUNTPT # Mount it. chmod 777 $MOUNTPT # Enables ordinary user to access ramdisk. # However, must be root to unmount it. ############################################################################## # Need to test whether above commands succeed. Could cause problems otherwise. # Exercise: modify this script to make it safer. echo "\"$MOUNTPT\" now available for use." # The ramdisk is now accessible for storing files, even by an ordinary user. # Caution, the ramdisk is volatile, and its contents will disappear #+ on reboot or power loss. # Copy anything you want saved to a regular directory. # After reboot, run this script to again set up ramdisk. # Remounting /mnt/ramdisk without the other steps will not work. # Suitably modified, this script can by invoked in /etc/rc.d/rc.local, #+ to set up ramdisk automatically at bootup. # That may be appropriate on, for example, a database server. exit 0 In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format) UNIX/Linux binaries. ------------------------------------------------------------------------ Chapter 29. Debugging Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. --Brian Kernighan The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script. Example 29-1. A buggy script #!/bin/bash # ex74.sh # This is a buggy script. # Where, oh where is the error? a=37 if [$a -gt 27 ] then echo $a fi exit 0 Output from script: +----------------------------------------------------------------------+ |./ex74.sh: [37: command not found | +----------------------------------------------------------------------+ What's wrong with the above script? Hint: after the if. Example 29-2. Missing keyword #!/bin/bash # missing-keyword.sh: What error message will this generate? for a in 1 2 3 do echo "$a" # done # Required keyword 'done' commented out in line 7. exit 0 Output from script: +----------------------------------------------------------------------+ |missing-keyword.sh: line 10: syntax error: unexpected end of file | | | +----------------------------------------------------------------------+ Note that the error message does not necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error. Error messages may disregard comment lines in a script when reporting the line number of a syntax error. What if the script executes, but does not work as expected? This is the all too familiar logic error. Example 29-3. test24: another buggy script #!/bin/bash # This script is supposed to delete all filenames in current directory #+ containing embedded spaces. # It doesn't work. # Why not? badname=`ls | grep ' '` # Try this: # echo "$badname" rm "$badname" exit 0 Try to find out what's wrong with Example 29-3 by uncommenting the echo "$badname" line. Echo statements are useful for seeing whether what you expect is actually what you get. In this particular case, rm "$badname" will not give the desired results because $badname should not be quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n'. However, there are simpler ways of going about it. # Correct methods of deleting filenames containing spaces. rm *\ * rm *" "* rm *' '* # Thank you. S.C. Summarizing the symptoms of a buggy script, 1. It bombs with a "syntax error" message, or 2. It runs, but does not work as expected (logic error). 3. It runs, works as expected, but has nasty side effects (logic bomb). Tools for debugging non-working scripts include 1. Inserting echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on. Tip Even better is an echo that echoes only when debug is on. ### debecho (debug-echo), by Stefano Falsetto ### ### Will echo passed parameters only if DEBUG is set to a value. ### debecho () { if [ ! -z "$DEBUG" ]; then echo "$1" >&2 # ^^^ to stderr fi } DEBUG=on Whatever=whatnot debecho $Whatever # whatnot DEBUG= Whatever=notwhat debecho $Whatever # (Will not echo.) 2. Using the tee filter to check processes or data flows at critical points. 3. Setting option flags -n -v -x sh -n scriptname checks for syntax errors without actually running the script. This is the equivalent of inserting set -n or set -o noexec into the script. Note that certain types of syntax errors can slip past this check. sh -v scriptname echoes each command before executing it. This is the equivalent of inserting set -v or set -o verbose in the script. The -n and -v flags work well together. sh -nv scriptname gives a verbose syntax check. sh -x scriptname echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting set -x or set -o xtrace in the script. Inserting set -u or set -o nounset in the script runs it, but gives an unbound variable error message at each attempt to use an undeclared variable. 4. Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.) Example 29-4. Testing a condition with an assert #!/bin/bash # assert.sh ####################################################################### assert () # If condition false, { #+ exit from script #+ with appropriate error message. E_PARAM_ERR=98 E_ASSERT_FAILED=99 if [ -z "$2" ] # Not enough parameters passed then #+ to assert() function. return $E_PARAM_ERR # No damage done. fi lineno=$2 if [ ! $1 ] then echo "Assertion failed: \"$1\"" echo "File \"$0\", line $lineno" # Give name of file and line number. exit $E_ASSERT_FAILED # else # return # and continue executing the script. fi } # Insert a similar assert() function into a script you need to debug. ####################################################################### a=5 b=4 condition="$a -lt $b" # Error message and exit from script. # Try setting "condition" to something else #+ and see what happens. assert "$condition" $LINENO # The remainder of the script executes only if the "assert" does not fail. # Some commands. # Some more commands . . . echo "This statement echoes only if the \"assert\" does not fail." # . . . # More commands . . . exit $? 5. Using the $LINENO variable and the caller builtin. 6. Trapping at exit. The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [110] It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the first command in the script. Trapping signals trap Specifies an action on receipt of a signal; also useful for debugging. +--------------------------------------------------------------+ | A signal is a message sent to a process, either by the | | kernel or another process, telling it to take some specified | | action (usually to terminate). For example, hitting a | | Control-C sends a user interrupt, an INT signal, to a | | running program. | +--------------------------------------------------------------+ A simple instance: trap '' 2 # Ignore interrupt 2 (Control-C), with no action specified. trap 'echo "Control-C disabled."' 2 # Message when Control-C pressed. Example 29-5. Trapping at exit #!/bin/bash # Hunting variables with a trap. trap 'echo Variable Listing --- a = $a b = $b' EXIT # EXIT is the name of the signal generated upon exit from a script. # # The command specified by the "trap" doesn't execute until #+ the appropriate signal is sent. echo "This prints before the \"trap\" --" echo "even though the script sees the \"trap\" first." echo a=39 b=36 exit 0 # Note that commenting out the 'exit' command makes no difference, #+ since the script exits in any case after running out of commands. Example 29-6. Cleaning up after Control-C #!/bin/bash # logon.sh: A quick 'n dirty script to check whether you are on-line yet. umask 177 # Make sure temp files are not world readable. TRUE=1 LOGFILE=/var/log/messages # Note that $LOGFILE must be readable #+ (as root, chmod 644 /var/log/messages). TEMPFILE=temp.$$ # Create a "unique" temp file name, using process id of the script. # Using 'mktemp' is an alternative. # For example: # TEMPFILE=`mktemp temp.XXXXXX` KEYWORD=address # At logon, the line "remote IP address xxx.xxx.xxx.xxx" # appended to /var/log/messages. ONLINE=22 USER_INTERRUPT=13 CHECK_LINES=100 # How many lines in log file to check. trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT # Cleans up the temp file if script interrupted by control-c. echo while [ $TRUE ] #Endless loop. do tail -n $CHECK_LINES $LOGFILE> $TEMPFILE # Saves last 100 lines of system log file as temp file. # Necessary, since newer kernels generate many log messages at log on. search=`grep $KEYWORD $TEMPFILE` # Checks for presence of the "IP address" phrase, #+ indicating a successful logon. if [ ! -z "$search" ] # Quotes necessary because of possible spaces. then echo "On-line" rm -f $TEMPFILE # Clean up temp file. exit $ONLINE else echo -n "." # The -n option to echo suppresses newline, #+ so you get continuous rows of dots. fi sleep 1 done # Note: if you change the KEYWORD variable to "Exit", #+ this script can be used while on-line #+ to check for an unexpected logoff. # Exercise: Change the script, per the above note, # and prettify it. exit 0 # Nick Drage suggests an alternate method: while true do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0 echo -n "." # Prints dots (.....) until connected. sleep 2 done # Problem: Hitting Control-C to terminate this process may be insufficient. #+ (Dots may keep on echoing.) # Exercise: Fix this. # Stephane Chazelas has yet another alternative: CHECK_INTERVAL=1 while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD" do echo -n . sleep $CHECK_INTERVAL done echo "On-line" # Exercise: Discuss the relative strengths and weaknesses # of each of these various approaches. Note The DEBUG argument to trap causes a specified action to execute after every command in a script. This permits tracing variables, for example. Example 29-7. Tracing a variable #!/bin/bash trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG # Echoes the value of $variable after every command. variable=29 echo " Just initialized \$variable to $variable." let "variable *= 3" echo " Just multiplied \$variable by 3." exit # The "trap 'command1 . . . command2 . . .' DEBUG" construct is #+ more appropriate in the context of a complex script, #+ where inserting multiple "echo $variable" statements might be #+ awkward and time-consuming. # Thanks, Stephane Chazelas for the pointer. Output of script: VARIABLE-TRACE> $variable = "" VARIABLE-TRACE> $variable = "29" Just initialized $variable to 29. VARIABLE-TRACE> $variable = "29" VARIABLE-TRACE> $variable = "87" Just multiplied $variable by 3. VARIABLE-TRACE> $variable = "87" Of course, the trap command has other uses aside from debugging, such as disabling certain keystrokes within a script (see Example A-43). Example 29-8. Running multiple processes (on an SMP box) #!/bin/bash # parent.sh # Running multiple processes on an SMP box. # Author: Tedman Eng # This is the first of two scripts, #+ both of which must be present in the current working directory. LIMIT=$1 # Total number of process to start NUMPROC=4 # Number of concurrent threads (forks?) PROCID=1 # Starting Process ID echo "My PID is $$" function start_thread() { if [ $PROCID -le $LIMIT ] ; then ./child.sh $PROCID& let "PROCID++" else echo "Limit reached." wait exit fi } while [ "$NUMPROC" -gt 0 ]; do start_thread; let "NUMPROC--" done while true do trap "start_thread" SIGRTMIN done exit 0 # ======== Second script follows ======== #!/bin/bash # child.sh # Running multiple processes on an SMP box. # This script is called by parent.sh. # Author: Tedman Eng temp=$RANDOM index=$1 shift let "temp %= 5" let "temp += 4" echo "Starting $index Time:$temp" "$@" sleep ${temp} echo "Ending $index" kill -s SIGRTMIN $PPID exit 0 # ======================= SCRIPT AUTHOR'S NOTES ======================= # # It's not completely bug free. # I ran it with limit = 500 and after the first few hundred iterations, #+ one of the concurrent threads disappeared! # Not sure if this is collisions from trap signals or something else. # Once the trap is received, there's a brief moment while executing the #+ trap handler but before the next trap is set. During this time, it may #+ be possible to miss a trap signal, thus miss spawning a child process. # No doubt someone may spot the bug and will be writing #+ . . . in the future. # ===================================================================== # # ----------------------------------------------------------------------# ################################################################# # The following is the original script written by Vernia Damiano. # Unfortunately, it doesn't work properly. ################################################################# #!/bin/bash # Must call script with at least one integer parameter #+ (number of concurrent processes). # All other parameters are passed through to the processes started. INDICE=8 # Total number of process to start TEMPO=5 # Maximum sleep time per process E_BADARGS=65 # No arg(s) passed to script. if [ $# -eq 0 ] # Check for at least one argument passed to script. then echo "Usage: `basename $0` number_of_processes [passed params]" exit $E_BADARGS fi NUMPROC=$1 # Number of concurrent process shift PARAMETRI=( "$@" ) # Parameters of each process function avvia() { local temp local index temp=$RANDOM index=$1 shift let "temp %= $TEMPO" let "temp += 1" echo "Starting $index Time:$temp" "$@" sleep ${temp} echo "Ending $index" kill -s SIGRTMIN $$ } function parti() { if [ $INDICE -gt 0 ] ; then avvia $INDICE "${PARAMETRI[@]}" & let "INDICE--" else trap : SIGRTMIN fi } trap parti SIGRTMIN while [ "$NUMPROC" -gt 0 ]; do parti; let "NUMPROC--" done wait trap - SIGRTMIN exit $? : <|) | |----------------+-----------------+-----------------------------------| | -D | (none) | List double-quoted strings | | | | prefixed by $, but do not execute | | | | commands in script | |----------------+-----------------+-----------------------------------| | -a | allexport | Export all defined variables | |----------------+-----------------+-----------------------------------| | -b | notify | Notify when jobs running in | | | | background terminate (not of much | | | | use in a script) | |----------------+-----------------+-----------------------------------| | -c ... | (none) | Read commands from ... | |----------------+-----------------+-----------------------------------| | checkjobs | | Informs user of any open jobs | | | | upon shell exit. Introduced in | | | | version 4 of Bash, and still | | | | "experimental." Usage: shopt -s | | | | checkjobs (Caution: may hang!) | |----------------+-----------------+-----------------------------------| | -e | errexit | Abort script at first error, when | | | | a command exits with non-zero | | | | status (except in until or while | | | | loops, if-tests, list constructs) | |----------------+-----------------+-----------------------------------| | -f | noglob | Filename expansion (globbing) | | | | disabled | |----------------+-----------------+-----------------------------------| | globstar | globbing | Enables the ** globbing operator | | | star-match | (version 4+ of Bash). Usage: | | | | shopt -s globstar | |----------------+-----------------+-----------------------------------| | -i | interactive | Script runs in interactive mode | |----------------+-----------------+-----------------------------------| | -n | noexec | Read commands in script, but do | | | | not execute them (syntax check) | |----------------+-----------------+-----------------------------------| | -o Option-Name | (none) | Invoke the Option-Name option | |----------------+-----------------+-----------------------------------| | -o posix | POSIX | Change the behavior of Bash, or | | | | invoked script, to conform to | | | | POSIX standard. | |----------------+-----------------+-----------------------------------| | -o pipefail | pipe failure | Causes a pipeline to return the | | | | exit status of the last command | | | | in the pipe that returned a | | | | non-zero return value. | |----------------+-----------------+-----------------------------------| | -p | privileged | Script runs as "suid" (caution!) | |----------------+-----------------+-----------------------------------| | -r | restricted | Script runs in restricted mode | | | | (see Chapter 21). | |----------------+-----------------+-----------------------------------| | -s | stdin | Read commands from stdin | |----------------+-----------------+-----------------------------------| | -t | (none) | Exit after first command | |----------------+-----------------+-----------------------------------| | -u | nounset | Attempt to use undefined variable | | | | outputs error message, and forces | | | | an exit | |----------------+-----------------+-----------------------------------| | -v | verbose | Print each command to stdout | | | | before executing it | |----------------+-----------------+-----------------------------------| | -x | xtrace | Similar to -v, but expands | | | | commands | |----------------+-----------------+-----------------------------------| | - | (none) | End of options flag. All other | | | | arguments are positional | | | | parameters. | |----------------+-----------------+-----------------------------------| | -- | (none) | Unset positional parameters. If | | | | arguments given (-- arg1 arg2), | | | | positional parameters set to | | | | arguments. | +----------------------------------------------------------------------+ ------------------------------------------------------------------------ Chapter 31. Gotchas Turandot: Gli enigmi sono tre, la morte una! Caleph: No, no! Gli enigmi sono tre, una la vita! --Puccini Here are some (non-recommended!) scripting practices that will bring excitement into an otherwise dull life. * Assigning reserved words or characters to variable names. case=value0 # Causes problems. 23skidoo=value1 # Also problems. # Variable names starting with a digit are reserved by the shell. # Try _23skidoo=value1. Starting variables with an underscore is okay. # However . . . using just an underscore will not work. _=25 echo $_ # $_ is a special variable set to last arg of last command. # But . . . _ is a valid function name! xyz((!*=value2 # Causes severe problems. # As of version 3 of Bash, periods are not allowed within variable names. * Using a hyphen or other reserved characters in a variable name (or function name). var-1=23 # Use 'var_1' instead. function-whatever () # Error # Use 'function_whatever ()' instead. # As of version 3 of Bash, periods are not allowed within function names. function.whatever () # Error # Use 'functionWhatever ()' instead. * Using the same name for a variable and a function. This can make a script difficult to understand. do_something () { echo "This function does something with \"$1\"." } do_something=do_something do_something do_something # All this is legal, but highly confusing. * Using whitespace inappropriately. In contrast to other programming languages, Bash can be quite finicky about whitespace. var1 = 23 # 'var1=23' is correct. # On line above, Bash attempts to execute command "var1" # with the arguments "=" and "23". let c = $a - $b # Instead: let c=$a-$b or let "c = $a - $b" if [ $a -le 5] # if [ $a -le 5 ] is correct. # ^^ if [ "$a" -le 5 ] is even better. # [[ $a -le 5 ]] also works. * Not terminating with a semicolon the final command in a code block within curly brackets. { ls -l; df; echo "Done." } # bash: syntax error: unexpected end of file { ls -l; df; echo "Done."; } # ^ ### Final command needs semicolon. * Assuming uninitialized variables (variables before a value is assigned to them) are "zeroed out". An uninitialized variable has a value of null, not zero. #!/bin/bash echo "uninitialized_var = $uninitialized_var" # uninitialized_var = * Mixing up = and -eq in a test. Remember, = is for comparing literal variables and -eq for integers. if [ "$a" = 273 ] # Is $a an integer or string? if [ "$a" -eq 273 ] # If $a is an integer. # Sometimes you can interchange -eq and = without adverse consequences. # However . . . a=273.0 # Not an integer. if [ "$a" = 273 ] then echo "Comparison works." else echo "Comparison does not work." fi # Comparison does not work. # Same with a=" 273" and a="0273". # Likewise, problems trying to use "-eq" with non-integer values. if [ "$a" -eq 273.0 ] then echo "a = $a" fi # Aborts with an error message. # test.sh: [: 273.0: integer expression expected * Misusing string comparison operators. Example 31-1. Numerical and string comparison are not equivalent #!/bin/bash # bad-op.sh: Trying to use a string comparison on integers. echo number=1 # The following while-loop has two errors: #+ one blatant, and the other subtle. while [ "$number" < 5 ] # Wrong! Should be: while [ "$number" -lt 5 ] do echo -n "$number " let "number += 1" done # Attempt to run this bombs with the error message: #+ bad-op.sh: line 10: 5: No such file or directory # Within single brackets, "<" must be escaped, #+ and even then, it's still wrong for comparing integers. echo "---------------------" while [ "$number" \< 5 ] # 1 2 3 4 do # echo -n "$number " # It *seems* to work, but . . . let "number += 1" #+ it actually does an ASCII comparison, done #+ rather than a numerical one. echo; echo "---------------------" # This can cause problems. For example: lesser=5 greater=105 if [ "$greater" \< "$lesser" ] then echo "$greater is less than $lesser" fi # 105 is less than 5 # In fact, "105" actually is less than "5" #+ in a string comparison (ASCII sort order). echo exit 0 * Attempting to use let to set string variables. let "a = hello, you" echo "$a" # 0 * Sometimes variables within "test" brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause unexpected behavior. See Example 7-6, Example 19-5, and Example 9-6. * Quoting a variable containing whitespace prevents splitting. Sometimes this produces unintended consequences. * Commands issued from a script may fail to execute because the script owner lacks execute permission for them. If a user cannot invoke a command from the command-line, then putting it into a script will likewise fail. Try changing the attributes of the command in question, perhaps even setting the suid bit (as root, of course). * Attempting to use - as a redirection operator (which it is not) will usually result in an unpleasant surprise. command1 2> - | command2 # Trying to redirect error output of command1 into a pipe . . . # . . . will not work. command1 2>& - | command2 # Also futile. Thanks, S.C. * Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines may have version 1.XX of Bash as the default installation. #!/bin/bash minimum_version=2 # Since Chet Ramey is constantly adding features to Bash, # you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate. E_BAD_VERSION=80 if [ "$BASH_VERSION" \< "$minimum_version" ] then echo "This script works only with Bash, version $minimum or greater." echo "Upgrade strongly recommended." exit $E_BAD_VERSION fi ... * Using Bash-specific functionality in a Bourne shell script (#!/bin/sh) on a non-Linux machine may cause unexpected behavior. A Linux system usually aliases sh to bash, but this does not necessarily hold true for a generic UNIX machine. * Using undocumented features in Bash turns out to be a dangerous practice. In previous releases of this book there were several scripts that depended on the "feature" that, although the maximum value of an exit or return value was 255, that limit did not apply to negative integers. Unfortunately, in version 2.05b and later, that loophole disappeared. See Example 23-9. * A script with DOS-type newlines (\r\n) will fail to execute, since #!/bin/bash\r\n is not recognized, not the same as the expected #!/bin/bash\n. The fix is to convert the script to UNIX-style newlines. #!/bin/bash echo "Here" unix2dos $0 # Script changes itself to DOS format. chmod 755 $0 # Change back to execute permission. # The 'unix2dos' command removes execute permission. ./$0 # Script tries to run itself again. # But it won't work as a DOS file. echo "There" exit 0 * A shell script headed by #!/bin/sh will not run in full Bash-compatibility mode. Some Bash-specific functions might be disabled. Scripts that need complete access to all the Bash-specific extensions should start with #!/bin/bash. * Putting whitespace in front of the terminating limit string of a here document will cause unexpected behavior in a script. * Putting more than one echo statement in a function whose output is captured. add2 () { echo "Whatever ... " # Delete this line! let "retval = $1 + $2" echo $retval } num1=12 num2=43 echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)" # Sum of 12 and 43 = Whatever ... # 55 # The "echoes" concatenate. This will not work. * A script may not export variables back to its parent process, the shell, or to the environment. Just as we learned in biology, a child process can inherit from a parent, but not vice versa. WHATEVER=/home/bozo export WHATEVER exit 0 +-----------------------------------------------------------+ |bash$ echo $WHATEVER | | | |bash$ | +-----------------------------------------------------------+ Sure enough, back at the command prompt, $WHATEVER remains unset. * Setting and manipulating variables in a subshell, then attempting to use those same variables outside the scope of the subshell will result an unpleasant surprise. Example 31-2. Subshell Pitfalls #!/bin/bash # Pitfalls of variables in a subshell. outer_variable=outer echo echo "outer_variable = $outer_variable" echo ( # Begin subshell echo "outer_variable inside subshell = $outer_variable" inner_variable=inner # Set echo "inner_variable inside subshell = $inner_variable" outer_variable=inner # Will value change globally? echo "outer_variable inside subshell = $outer_variable" # Will 'exporting' make a difference? # export inner_variable # export outer_variable # Try it and see. # End subshell ) echo echo "inner_variable outside subshell = $inner_variable" # Unset. echo "outer_variable outside subshell = $outer_variable" # Unchanged. echo exit 0 # What happens if you uncomment lines 19 and 20? # Does it make a difference? * Piping echo output to a read may produce unexpected results. In this scenario, the read acts as if it were running in a subshell. Instead, use the set command (as in Example 14-18). Example 31-3. Piping the output of echo to a read #!/bin/bash # badread.sh: # Attempting to use 'echo and 'read' #+ to assign variables non-interactively. a=aaa b=bbb c=ccc echo "one two three" | read a b c # Try to reassign a, b, and c. echo echo "a = $a" # a = aaa echo "b = $b" # b = bbb echo "c = $c" # c = ccc # Reassignment failed. # ------------------------------ # Try the following alternative. var=`echo "one two three"` set -- $var a=$1; b=$2; c=$3 echo "-------" echo "a = $a" # a = one echo "b = $b" # b = two echo "c = $c" # c = three # Reassignment succeeded. # ------------------------------ # Note also that an echo to a 'read' works within a subshell. # However, the value of the variable changes *only* within the subshell. a=aaa # Starting all over again. b=bbb c=ccc echo; echo echo "one two three" | ( read a b c; echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" ) # a = one # b = two # c = three echo "-----------------" echo "Outside subshell: " echo "a = $a" # a = aaa echo "b = $b" # b = bbb echo "c = $c" # c = ccc echo exit 0 In fact, as Anthony Richardson points out, piping to any loop can cause a similar problem. # Loop piping troubles. # This example by Anthony Richardson, #+ with addendum by Wilbert Berendsen. foundone=false find $HOME -type f -atime +30 -size 100k | while true do read f echo "$f is over 100KB and has not been accessed in over 30 days" echo "Consider moving the file to archives." foundone=true # ------------------------------------ echo "Subshell level = $BASH_SUBSHELL" # Subshell level = 1 # Yes, we're inside a subshell. # ------------------------------------ done # foundone will always be false here since it is #+ set to true inside a subshell if [ $foundone = false ] then echo "No files need archiving." fi # =====================Now, here is the correct way:================= foundone=false for f in $(find $HOME -type f -atime +30 -size 100k) # No pipe here. do echo "$f is over 100KB and has not been accessed in over 30 days" echo "Consider moving the file to archives." foundone=true done if [ $foundone = false ] then echo "No files need archiving." fi # ==================And here is another alternative================== # Places the part of the script that reads the variables #+ within a code block, so they share the same subshell. # Thank you, W.B. find $HOME -type f -atime +30 -size 100k | { foundone=false while read f do echo "$f is over 100KB and has not been accessed in over 30 days" echo "Consider moving the file to archives." foundone=true done if ! $foundone then echo "No files need archiving." fi } A lookalike problem occurs when trying to write the stdout of a tail -f piped to grep. tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log # The "error.log" file will not have anything written to it. # As Samuli Kaipiainen points out, this results from grep #+ buffering its output. # The fix is to add the "--line-buffered" parameter to grep. * Using "suid" commands within scripts is risky, as it may compromise system security. [111] * Using shell scripts for CGI programming may be problematic. Shell script variables are not "typesafe," and this can cause undesirable behavior as far as CGI is concerned. Moreover, it is difficult to "cracker-proof" shell scripts. * Bash does not handle the double slash (//) string correctly. * Bash scripts written for Linux or BSD systems may need fixups to run on a commercial UNIX (or Apple OSX) machine. Such scripts often employ the GNU set of commands and filters, which have greater functionality than their generic UNIX counterparts. This is particularly true of such text processing utilites as tr. Danger is near thee -- Beware, beware, beware, beware. Many brave hearts are asleep in the deep. So beware -- Beware. --A.J. Lamb and H.W. Petrie ------------------------------------------------------------------------ Chapter 32. Scripting With Style Get into the habit of writing shell scripts in a structured and systematic manner. Even on-the-fly and "written on the back of an envelope" scripts will benefit if you take a few minutes to plan and organize your thoughts before sitting down and coding. Herewith are a few stylistic guidelines. This is not (necessarily) intended as an Official Shell Scripting Stylesheet. ------------------------------------------------------------------------ 32.1. Unofficial Shell Scripting Stylesheet * Comment your code. This makes it easier for others to understand (and appreciate), and easier for you to maintain. PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" # It made perfect sense when you wrote it last year, #+ but now it's a complete mystery. # (From Antek Sawicki's "pw.sh" script.) Add descriptive headers to your scripts and functions. #!/bin/bash #************************************************# # xyz.sh # # written by Bozo Bozeman # # July 05, 2001 # # # # Clean up project files. # #************************************************# E_BADDIR=85 # No such directory. projectdir=/home/bozo/projects # Directory to clean up. # --------------------------------------------------------- # # cleanup_pfiles () # # Removes all files in designated directory. # # Parameter: $target_directory # # Returns: 0 on success, $E_BADDIR if something went wrong. # # --------------------------------------------------------- # cleanup_pfiles () { if [ ! -d "$1" ] # Test if target directory exists. then echo "$1 is not a directory." return $E_BADDIR fi rm -f "$1"/* return 0 # Success. } cleanup_pfiles $projectdir exit $? * Avoid using "magic numbers," [112] that is, "hard-wired" literal constants. Use meaningful variable names instead. This makes the script easier to understand and permits making changes and updates without breaking the application. if [ -f /var/log/messages ] then ... fi # A year later, you decide to change the script to check /var/log/syslog. # It is now necessary to manually change the script, instance by instance, #+ and hope nothing breaks. # A better way: LOGFILE=/var/log/messages # Only line that needs to be changed. if [ -f "$LOGFILE" ] then ... fi * Choose descriptive names for variables and functions. fl=`ls -al $dirname` # Cryptic. file_listing=`ls -al $dirname` # Better. MAXVAL=10 # All caps used for a script constant. while [ "$index" -le "$MAXVAL" ] ... E_NOTFOUND=95 # Uppercase for an errorcode, #+ and name prefixed with E_. if [ ! -e "$filename" ] then echo "File $filename not found." exit $E_NOTFOUND fi MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental export MAIL_DIRECTORY #+ variable. GetAnswer () # Mixed case works well for a { #+ function name, especially prompt=$1 #+ when it improves legibility. echo -n $prompt read answer return $answer } GetAnswer "What is your favorite number? " favorite_number=$? echo $favorite_number _uservariable=23 # Permissible, but not recommended. # It's better for user-defined variables not to start with an underscore. # Leave that for system variables. * Use exit codes in a systematic and meaningful way. E_WRONG_ARGS=95 ... ... exit $E_WRONG_ARGS See also Appendix D. Ender suggests using the exit codes in /usr/include/sysexits.h in shell scripts, though these are primarily intended for C and C++ programming. * Use standardized parameter flags for script invocation. Ender proposes the following set of flags. -a All: Return all information (including hidden file info). -b Brief: Short version, usually for other scripts. -c Copy, concatenate, etc. -d Daily: Use information from the whole day, and not merely information for a specific instance/user. -e Extended/Elaborate: (often does not include hidden file info). -h Help: Verbose usage w/descs, aux info, discussion, help. See also -V. -l Log output of script. -m Manual: Launch man-page for base command. -n Numbers: Numerical data only. -r Recursive: All files in a directory (and/or all sub-dirs). -s Setup & File Maintenance: Config files for this script. -u Usage: List of invocation flags for the script. -v Verbose: Human readable output, more or less formatted. -V Version / License / Copy(right|left) / Contribs (email too). See also Section F.1. * Break complex scripts into simpler modules. Use functions where appropriate. See Example 34-4. * Don't use a complex construct where a simpler one will do. COMMAND if [ $? -eq 0 ] ... # Redundant and non-intuitive. if COMMAND ... # More concise (if perhaps not quite as legible). ... reading the UNIX source code to the Bourne shell (/bin/sh). I was shocked at how much simple algorithms could be made cryptic, and therefore useless, by a poor choice of code style. I asked myself, "Could someone be proud of this code?" --Landon Noll ------------------------------------------------------------------------ Chapter 33. Miscellany Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help. --Tom Duff ------------------------------------------------------------------------ 33.1. Interactive and non-interactive shells and scripts An interactive shell reads commands from user input on a tty. Among other things, such a shell reads startup files on activation, displays a prompt, and enables job control by default. The user can interact with the shell. A shell running a script is always a non-interactive shell. All the same, the script can still access its tty. It is even possible to emulate an interactive shell in a script. #!/bin/bash MY_PROMPT='$ ' while : do echo -n "$MY_PROMPT" read line eval "$line" done exit 0 # This example script, and much of the above explanation supplied by # Stéphane Chazelas (thanks again). Let us consider an interactive script to be one that requires input from the user, usually with read statements (see Example 14-3). "Real life" is actually a bit messier than that. For now, assume an interactive script is bound to a tty, a script that a user has invoked from the console or an xterm. Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out for automation by non-interactive scripts. Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never comes. Handle that difficulty by having an expect script or embedded here document feed input to an interactive script running as a background job. In the simplest case, redirect a file to supply input to a read statement (read variable > | left, right shift | bitwise | |----------------+------------------------+----------------------------| | | | | |----------------+------------------------+----------------------------|