The Double Parentheses Construct

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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-type manipulation of variables in Bash.

Example 9-25. 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.

http://tldp.org/LDP/abs/html/dblparens.html (1 of 2) [7/15/2002 6:35:14 PM]

The Double Parentheses Construct

#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.

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