8.4. Operator Precedence (Revision #8)

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

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