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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 (( t = a<45?7:11 )) # C-style trinary operator. 26 echo "If a < 45, then t = 7, else t = 11." 27 echo "t = $t " # Yes! 28 29 echo 30 31 32 # ----------------- 33 # Easter Egg alert! 34 # ----------------- 35 # Chet Ramey apparently snuck a bunch of undocumented C-style constructs 36 #+ into Bash (actually adapted from ksh, pretty much). 37 # In the Bash docs, Ramey calls ((...)) shell arithmetic, 38 #+ but it goes far beyond that. 39 # Sorry, Chet, the secret is now out. 40 41 # See also "for" and "while" loops using the ((...)) construct. 42 43 # These work only with Bash, version 2.04 or later. 44 45 exit 0 |
See also Example 10-12.