#!/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 ######################################################## # Note that, as in C, pre- and post-decrement operators #+ have slightly 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 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
