Operator Precedence

Precedence

var++ var--

post-increment, post-decrement (C-style)

++var --var

pre-increment, pre-decrement

! ~

negation

**

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)

Mental shortcut

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.

Examples

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.