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
#!/bin/bash # 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" (( a++ )) # Post-increment 'a', C-style. echo "a (after a++) = $a" (( a-- )) # Post-decrement 'a', C-style. echo "a (after a--) = $a" (( ++a )) # Pre-increment 'a', C-style. echo "a (after ++a) = $a" (( --a )) # Pre-decrement 'a', C-style. echo "a (after --a) = $a" echo (( t = a<45?7:11 )) # C-style trinary operator. echo "If a < 45, then t = 7, else t = 11." echo "t = $t " # Yes! echo # ----------------- # Easter Egg alert! # ----------------- # Chet Ramey apparently 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 now out. # See also "for" and "while" loops using the ((...)) construct. # These work only with Bash, version 2.04 or later. exit 0 |
See also Example 10-12.