9.7. The Double-Parentheses Construct

Similar to the let 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 9-33. 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 Example 10-12 and Example 8-4.