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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

OperatorMeaningComments
 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.

Tip
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


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