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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-type manipulation of variables in Bash.
Example 9-29. C-type manipulation of variables
1 #!/bin/bash 2 # Manipulating a variable, C-style, using the ((...)) construct. 3 4 5 echo 6 7 (( a = 23 )) # Setting a value, C-style, with spaces on both sides of the "=". 8 echo "a (initial value) = $a" 9 10 (( a++ )) # Post-increment 'a', C-style. 11 echo "a (after a++) = $a" 12 13 (( a-- )) # Post-decrement 'a', C-style. 14 echo "a (after a--) = $a" 15 16 17 (( ++a )) # Pre-increment 'a', C-style. 18 echo "a (after ++a) = $a" 19 20 (( --a )) # Pre-decrement 'a', C-style. 21 echo "a (after --a) = $a" 22 23 echo 24 25 ######################################################## 26 # Note that, as in C, pre- and post-decrement operators 27 #+ have slightly different side-effects. 28 29 n=1; let --n && echo "True" || echo "False" # False 30 n=1; let n-- && echo "True" || echo "False" # True 31 32 # Thanks, Jeroen Domburg. 33 ######################################################## 34 35 echo 36 37 (( t = a<45?7:11 )) # C-style trinary operator. 38 echo "If a < 45, then t = 7, else t = 11." 39 echo "t = $t " # Yes! 40 41 echo 42 43 44 # ----------------- 45 # Easter Egg alert! 46 # ----------------- 47 # Chet Ramey apparently snuck a bunch of undocumented C-style constructs 48 #+ into Bash (actually adapted from ksh, pretty much). 49 # In the Bash docs, Ramey calls ((...)) shell arithmetic, 50 #+ but it goes far beyond that. 51 # Sorry, Chet, the secret is now out. 52 53 # See also "for" and "while" loops using the ((...)) construct. 54 55 # These work only with Bash, version 2.04 or later. 56 57 exit 0 |
See also Example 10-12.