The Double-Parentheses Construct

Bash Unix Shell Scripting:
Chapter 8. Operations and Related Topics

The Double-Parentheses Construct

Similar to the command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ )).

Example 8-5. C-style manipulation of variables 

#!/bin/bash
# c-vars.sh
# Manipulating a variable, C-style, using the (( ... )) construct.
echo
(( a = 23 ))  #  Setting a value, C-style,
              #+ with spaces on both sides of the "=".
echo "a (initial value) = $a"   # 23
(( a++ ))     #  Post-increment 'a', C-style.
echo "a (after a++) = $a"       # 24
(( a-- ))     #  Post-decrement 'a', C-style.
echo "a (after a--) = $a"       # 23
(( ++a ))     #  Pre-increment 'a', C-style.
echo "a (after ++a) = $a"       # 24
(( --a ))     #  Pre-decrement 'a', C-style.
echo "a (after --a) = $a"       # 23
echo
########################################################
#  Note that, as in C, pre- and post-decrement operators
#+ have different side-effects.
n=1; let --n && echo "True" || echo "False"  # False
n=1; let n-- && echo "True" || echo "False"  # True
#  Thanks, Jeroen Domburg.
########################################################
echo
(( t = a<45?7:11 ))   # C-style trinary operator.
#       ^  ^ ^
echo "If a < 45, then t = 7, else t = 11."  # a = 23
echo "t = $t "                              # t = 7
echo
# -----------------
# Easter Egg alert!
# -----------------
#  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 and .