Advanced Bash-Scripting Guide
# -----------------
# Chet Ramey seems to have snuck a bunch of undocumented C-style
#+ constructs into Bash (actually adapted from ksh, pretty much).
# In the Bash docs, Ramey calls (( ... )) shell arithmetic,
#+ but it goes far beyond that.
# Sorry, Chet, the secret is out.
# See also "for" and "while" loops using the (( ... )) construct.
# These work only with version 2.04 or later of Bash.
exit
See also Example 11-13 and Example 8-4.
8.4. Operator Precedence
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
var++ var--
++var --var
! ~
**
* / %
+ -
Meaning Comments
HIGHEST PRECEDENCE
post-increment, post-decrement
C-style operators
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
-z -n
-e -f -t -x, etc.
unary comparison
unary comparison
< -lt > -gt <= -le >= -ge
compound comparison
-nt -ot -ef
compound comparison equality / inequality
AND
XOR
OR
bitwise
string is/is-not null
file-test
string and integer file-test test operators, string and integer bitwise
exclusive OR, bitwise bitwise
Chapter 8. Operations and Related Topics 81
&& -a
|| -o
?:
=
*= /= %= += -= <<= >>= &=
Advanced Bash-Scripting Guide
AND
OR
logical, compound comparison
logical, compound comparison
trinary operator
assignment
combination assignment
C-style
(do not confuse with equality
test) 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
Chapter 8. Operations and Related Topics 82
Advanced Bash-Scripting Guide
#+ 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.
Chapter 8. Operations and Related Topics 83
