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Chez Marcel

By Marty McGowan

Marcel Gagné's article (Linux Journal #65, September 1999) on French cooking inspired me to share some recipes of my own. The cooking metaphor is not new to computing, as Donald Knuth, in his forward to "Fundamental Algorithms" confesses he almost used "Recipes for the Computer" as its title. Without stirring the metaphor too vigorously, Gagné's article gives me the opportunity to share two items of interest and give them the needed cooking flavor.

For some time, I've been concerned about what I regard are overuse or misuse of two programming constructs:

To continue the cooking analogy, these two may be thought of respectively as inconsistent or lumpy sauce, and uneven temperature. Realizing that we chefs like to work on one another's recipes, lets see what happens when we apply them to Marcel Gagné's recipe, "Check User Mail".

Before I'd read Marcel's article, my style of programming used the tool metaphor. While not much of a chef, I now prefer the cooking metaphor, as it connotes more of a learning, and sharing model, which is what we do in programming.

Marcel's recipe is an excellent starting point for my school of cooking, as his recipe is complete, all by itself, and offers the opportunity to visit each of the points once. First, here is a copy of his recipe, without the comment header.

for user_name in 'cat /usr/local/etc/mail_notify'
	no_messages='frm $user_name |
		grep -v "Mail System Internal Data" |
		wc -l'
	if [ "$no_messages" -gt "0" ]
		echo "You have $no_messages e-mail message(s) waiting." > /tmp/$user_name.msg
		echo " " >> /tmp/$user_name.msg
		echo "Please start your e-mail client to collect mail." >> /tmp/$user_name.msg
		/usr/bin/smbclient -M $user_name < /tmp/$user_name.msg
This script isn't hard to maintain or understand, but I think the chefs in the audience will profit from the seasonings I offer here.

A by-product of my cooking school is lots of short functions. There are those who are skeptical about adopting this approach. Let's suspend belief for just a moment as we go through the method. I'll introduce my seasonings one at a time, and then put Marcel Gagné's recipe back together at the end. Then you may judge the sauce.

One of the languages in my schooling was Pascal, which if you recall puts the main procedure last. So, I've learned to read scripts backwards, as that's usually where the action is anyway. In Marcel Gagné's script, we come to the point in the last line, where he sends the message to each client. (I don't know Samba, but I assume this will make a suitable function):

	function to_samba { /usr/bin/smbclient -M $1; }
This presumes samba reads from its standard input without another flag or argument. It's used: "to_samba {user_name}", reading the standard input, writing to the samba client.

And, what are we going to send the user, but a message indicating they have new mail. That function looks like this:

	function you_have_mail {
		echo "You have $1 e-mail message(s) waiting."
		echo " " 
		echo "Please start your e-mail client to collect mail."
and it is used: you_have_mail {num_messages}. writing the message on the standard output.

At this point, you've noticed a couple of things. The file names and the redirection of output and input are missing. We'll use them if we need them. But let me give you a little hint: we won't. Unix(Linux) was designed with the principle that recipes are best made from simple ingredients. Temporary files are OK, but Linux has other means to reduce your reliance on them. Introducing temporary files does a few things:

Therefore, we seek to save ourselves these tasks. We'll see how this happens in a bit.

A key piece of the recipe is deciding whether or not our user needs to be alerted to incoming mail. Let's take care of that now:

	function num_msg { frm $1 | but_not "Mail System Internal Data" | linecount; }
This is almost identical with Marcel's code fragment. We'll deal with the differences later. The curious among you have already guessed. This function is used: num_msg {user_name}, returning a count of the number of lines.

What does the final recipe look like. All of Marcel Gagné's recipe is wrapped up in this one line of shell program:

	foreach user_notify  'cat /usr/etc/local/mail_notify'
And that's exactly how it's used. This single line is the entire program, supported of course, by the functions, or recipe fragments we have been building. We peeked ahead, breaking with Pascal tradition, because, after looking at some low-level ingredients, I thought it important to see where we are going at this point. You can see the value of a single-line program. It now can be moved around in your operations plan at will. You may serve your users with the frequency and taste they demand. Note, at this point, you won't have much code to change if you wanted to serve your A-M diners at 10 minute intervals beginning at 5 after the hour and your N-Z diners on the 10-minute marks.

So what does "user_notify" look like? I toiled with this one. Let me share the trials. First I did this:

	function user_notify { do_notify $(num_msg $1) $1; }
thinking that if I first calculated the number of messages for the user, and supplied that number and the user name to the function, then that function (do_notify) could perform the decision and send the message. Before going further, we have to digress. In the Korn shell, which I use exclusively, the result of the operation in the expression: $( ... ) is returned to the command line. So, in our case, the result of "num_mag {user_name}" is a number 0 through some positive number, indicating the number of mail messages the user has waiting.

This version of user_notify would expect a "do_notify" to look like this:

	function do_notify { if [ "$1" -gt "0" ]; then notify_user $2 $1; fi; }
This is OK, but it means yet another "notify" function, and even for this one-line fanatic, that's a bit much. So, what to do? Observe, the only useful piece of information in this function is another function name "notify_user". This is where culinary art, inspiration, and experience come in. Let's try a function which looks like this:
	function foo { { if [ "$X" -gt "0" ]; then eval $*; fi }
This is different than the "do_notify" we currently have. First of all, $X, is not an actual shell variable, but here the X stands for "lets see what is the best argument number to use for the numeric test". And the "eval $*" performs an evaluation of all its arguments. And here's the spice that gives this whole recipe it's flavor! The first argument may be another command or function name! A remarkable, and little used property of the shell is to pass command names as arguments.

So, let's give "foo" a name. What does it do? If one of its arguments is non-zero, then it performs a function (it's first argument) on all the other arguments. Let's solve for X. It could be any of the positional parameters, but to be completely general, it probably should be the next one, as it's the only other one this function ever has to know about. So, let's call this thing:

	if_non_zero {function} {number} ....
Using another convenient shorthand, it all becomes:
	function if_non_zero { [ $2 -gt 0 ] && eval $*; }
and we'll see how it's used later. With this function, user_notify now looks like:
	function user_notify { if_non_zero do_notify $(num_msg $1) $1; }
and is used: user_notify {user_name}. Note the dual use of the first argument, which is the user's name. In one case, it is a further argument to the num_msg function which return the number for that user, in the other case, it merely stands for itself, but now as the 2nd argument to "do_notify". So, what does "do_notify" look like. We've already written the sub pieces, so, it's simply:
	function do_notify { you_have_mail $1 | to_samba $2; }
At this point, we have (almost) all the recipe ingredients. The careful reader has noted the omission of "but_not", "linecount", and "foreach". Permit me another gastronomic aside. Ruth Reichel, recently food editor of the New York Times, is now the editor for Gourmet magazine. One of the things she promises to do is correct the complicated recipes so frequently seen in their pages. For example, "use 1/4 cup lemon juice" will replace the paragraph of instructions on how to extract that juice from a lemon.

In that spirit, I'll let you readers write your own "but_not" and "linecount". Let me show you the "foreach" you can use:

      function foreach { cmd=$1; shift; for arg in $*; do eval $cmd $arg; done; }
A slightly more elegant version avoids the temporary file name:
      function foreach { for a $(shifted $*); do eval $1 $a; done; }
which requires "shifted":
      function shifted { shift; echo $*; }
The former "foreach", to be completely secure, needs a "typeset" qualifier in front of the "cmd" variable. It's another reason to avoid the use of variable names. This comes under the general rule that not every programming feature needs to be used.

We need one final "Chapters of the Cookbook" instruction before putting this recipe back together. Let's imagine by now, that we are practicing student chefs and we have a little repertoire of our own. So what's an easy way to re-use those cooking tricks of the past. In the programming sense, we put them in a function library and invoke the library in our scripts. In this case, let's assume we have "foreach", "but_not", and "linecount" in the cookbook. Put that file "cookbook" either in the current directory, but more usefully, somewhere along your PATH variable. Using Marcel Gagné's example, we might expect to put it in, say, /usr/local/recipe/cookbook, so you might do this in your environment:

and then, in your shell files, or recipes, you would have a line like this:
    . cookbook		#  "dot - cookbook"
where the "dot" reads, or "sources" the contents of the cookbook file into the current shell. So, let's put it together:
# -- Mail Notification, Marty McGowan, mcfly@workmail.com, 9/9/99
  . cookbook
# -------------------------------------------- General Purpose --
function if_non_zero	{ [ $2 -gt 0 ] && eval $*; }
function to_samba	{ /usr/bin/smbclient -M $1; }
# --------------------------------------- Application Specific --
function num_msg	{ frm $1 | but_not "Mail System Internal Data" | linecount; }
function you_have_mail	{
	echo "You have $1 e-mail message(s) waiting."
	echo " " 
	echo "Please start your e-mail client to collect mail."
function do_notify	{ you_have_mail $1 | to_samba $2; }
function user_notify	{ if_non_zero do_notify $(num_msg $1) $1; }
# ------------------------------------------ Mail Notification --
  foreach user_notify  'cat /usr/etc/local/mail_notify'
On closing, there are a few things that suggest themselves here. "if_non_zero" probably belongs in the cookbook. It may already be in mine. And also "to_samba". Where does that go? I keep one master cookbook, for little recipes that may be used in any type of cooking. Also, I keep specialty cookbooks for each style that needs its own repertoire. So, I may have a Samba cookbook as well. After I've done some cooking, and in a new recipe, I might find the need for some fragment I've used before. Hopefully, it's in the cookbook. If it's not there, I ask myself, "is this little bit ready for wider use?". If so, I put it in the cookbook, or, after a while other little fragments might find their way into the specialty books. So, in the not too distant future, I might have a file, called "samba_recipe", which starts out like:
# --------------- Samba Recipes, uses the Cookbook, Adds SAMBA --
. cookbook
# -------------------------------------------- General Purpose --
function to_samba	{ /usr/bin/smbclient -M $1; }
This leads to a recipe with three fewer lines and the cookbook has been replace with 'samba_recipes" at the start.

Let me say just two things about style: my functions either fit on one line or not. If they do, each phrase needs to be separated by a semi-colon (;), if not, a newline is sufficient. My multi-line function closes with a curly brace on it's own line. Also, my comments are "right-justified", with two trailing dashes. Find your style, and stick to it.

In conclusion, note how we've eliminated temporary files and variables. Nor are there nested decisions or program flow. How was this achieved? Each of these are now "atomic" actions. The one decision in this recipe, "does Marcel have any mail now?" has been encapsulated in the "if_non_zero" function, which is supplied the result of the "num_msg" query. Also, the looping construct has been folded into the "foreach" function. This one function has simplified my recipes greatly. (I've also found it necessary to write a "foreach" function which passes a single argument to each function executed.)

The temporary files disappeared into the pipe, which was Unix's (Linux's) single greatest invention. The idea that one program might read its input from the output from another was not widely understood when Unix was invented. And the temporary names disappeared into the shell variable arguments. The shell function, which is very well defined in the Korn shell, adds greatly to this simplification.

To debug in this style, I've found it practical to add two things to a function to tell me what's going on in the oven. For example:

   function do_notify	{ comment do_notify $*
	    you_have_mail $1 | tee do_notify.$$ |  to_samba $2
where "comment" looks like:
      function comment { echo $* 1>&2; } 
Hopefully, the chefs in the audience will find use for these approaches to their recipes. I'll admit this style is not the easiest to adapt, but soon it will yield recipes of more even consistency, both in taste and temperature. And a programming style that will expand each chef's culinary art.

Copyright © 1999, Marty McGowan
Published in Issue 47 of Linux Gazette, November 1999

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