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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 9-30. 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

########################################################
#  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<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.


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