In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones. [40]
Table 8.1. Operator Precedence
| Operator | Meaning | Comments |
|---|---|---|
| HIGHEST PRECEDENCE | |
var++ var-- | post-increment, post-decrement | C-style operators |
++var --var | pre-increment, pre-decrement | |
! ~ | negation | logical / bitwise, inverts sense of following operator |
** | exponentiation | arithmetic operation |
* / % | multiplication, division, modulo | arithmetic operation |
+ - | addition, subtraction | arithmetic operation |
<< >> | left, right shift | bitwise |
-z -n | unary comparison | string is/is-not null |
-e -f -t -x, etc. | unary comparison | file-test |
< -lt > -gt <= -le >= -ge | compound comparison | string and integer |
-nt -ot -ef | compound comparison | file-test |
== -eq !=
-ne | equality / inequality | test operators, string and integer |
& | AND | bitwise |
^ | XOR | exclusive OR, bitwise |
| | OR | bitwise |
&& -a | AND | logical, compound comparison |
|| -o | OR | logical, compound comparison |
?: | trinary operator | C-style |
= | assignment | (do not confuse with equality test) |
*= /= %= += -= <<= >>= &= | combination assignment | times-equal, divide-equal, mod-equal, etc. |
, | comma | links 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.