8.4. Operator Precedence

In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones. [1]

Table 8-1. Operator Precedence

HIGHEST PRECEDENCE
var++ var--post-increment, post-decrementC-style operators
++var --varpre-increment, pre-decrement

! ~negationlogical / bitwise, inverts sense of following operator

**exponentiationarithmetic operation
* / %multiplication, division, moduloarithmetic operation
+ -addition, subtractionarithmetic operation

<< >>left, right shiftbitwise

-z -nunary comparisonstring is/is-not null
-e -f -t -x, etc.unary comparisonfile-test
< -lt > -gt <= -le >= -gecompound comparisonstring and integer
-nt -ot -efcompound comparisonfile-test
== -eq != -neequality / inequalitytest operators, string and integer

&ANDbitwise
^XORexclusive OR, bitwise
|ORbitwise

&& -aANDlogical, compound comparison
|| -oORlogical, compound comparison

?:trinary operatorC-style
=assignment(do not confuse with equality test)
*= /= %= += -= <<= >>= &=combination assignmenttimes-equal, divide-equal, mod-equal, etc.

,commalinks a sequence of operations
LOWEST PRECEDENCE

In practice, all you really need to remember is the following:

• The "My Dear Aunt Sally" mantra (multiply, divide, add, subtract) for the familiar arithmetic operations.

• The compound logical operators, &&, ||, -a, and -o have low precedence.

• The order of evaluation of equal-precedence operators is usually left-to-right.

Now, let's utilize our knowledge of operator precedence to analyze a couple of lines from the /etc/init.d/functions file, as found in the Fedora Core Linux distro.

 ```while [ -n "\$remaining" -a "\$retry" -gt 0 ]; do # This looks rather daunting at first glance. # Separate the conditions: while [ -n "\$remaining" -a "\$retry" -gt 0 ]; do # --condition 1-- ^^ --condition 2- # If variable "\$remaining" is not zero length #+ AND (-a) #+ variable "\$retry" is greater-than zero #+ then #+ the [ expresion-within-condition-brackets ] returns success (0) #+ and the while-loop executes an iteration. # ============================================================== # Evaluate "condition 1" and "condition 2" ***before*** #+ ANDing them. Why? Because the AND (-a) has a lower precedence #+ than the -n and -gt operators, #+ and therefore gets evaluated *last*. ################################################################# if [ -f /etc/sysconfig/i18n -a -z "\${NOLOCALE:-}" ] ; then # Again, separate the conditions: if [ -f /etc/sysconfig/i18n -a -z "\${NOLOCALE:-}" ] ; then # --condition 1--------- ^^ --condition 2----- # If file "/etc/sysconfig/i18n" exists #+ AND (-a) #+ variable \$NOLOCALE is zero length #+ then #+ the [ test-expresion-within-condition-brackets ] returns success (0) #+ and the commands following execute. # # As before, the AND (-a) gets evaluated *last* #+ because it has the lowest precedence of the operators within #+ the test brackets. # ============================================================== # Note: # \${NOLOCALE:-} is a parameter expansion that seems redundant. # But, if \$NOLOCALE has not been declared, it gets set to *null*, #+ in effect declaring it. # This makes a difference in some contexts.```

To avoid confusion or error in a complex sequence of test operators, break up the sequence into bracketed sections.
 ```if [ "\$v1" -gt "\$v2" -o "\$v1" -lt "\$v2" -a -e "\$filename" ] # Unclear what's going on here... if [[ "\$v1" -gt "\$v2" ]] || [[ "\$v1" -lt "\$v2" ]] && [[ -e "\$filename" ]] # Much better -- the condition tests are grouped in logical sections.```

Notes

 [1] Precedence, in this context, has approximately the same meaning as priority