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>8.3. The Double-Parentheses Construct</H1
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><P
>Similar to the <A
HREF="internal.html#LETREF"
>let</A
> command,
	  the <B
CLASS="COMMAND"
>(( ... ))</B
> construct permits
	  arithmetic expansion and evaluation. In its simplest
	  form, <TT
CLASS="USERINPUT"
><B
>a=$(( 5 + 3 ))</B
></TT
> would set
	  <TT
CLASS="USERINPUT"
><B
>a</B
></TT
> to <TT
CLASS="USERINPUT"
><B
>5 + 3</B
></TT
>, or
	  <TT
CLASS="USERINPUT"
><B
>8</B
></TT
>. However, this double-parentheses
	  construct is also a mechanism for allowing C-style
	  manipulation of variables in Bash, for example,
	  <TT
CLASS="VARNAME"
>(( var++ ))</TT
>.</P
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>Example 8-5. C-style manipulation of variables</B
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>   1&nbsp;#!/bin/bash
   2&nbsp;# c-vars.sh
   3&nbsp;# Manipulating a variable, C-style, using the (( ... )) construct.
   4&nbsp;
   5&nbsp;
   6&nbsp;echo
   7&nbsp;
   8&nbsp;(( a = 23 ))  #  Setting a value, C-style,
   9&nbsp;              #+ with spaces on both sides of the "=".
  10&nbsp;echo "a (initial value) = $a"   # 23
  11&nbsp;
  12&nbsp;(( a++ ))     #  Post-increment 'a', C-style.
  13&nbsp;echo "a (after a++) = $a"       # 24
  14&nbsp;
  15&nbsp;(( a-- ))     #  Post-decrement 'a', C-style.
  16&nbsp;echo "a (after a--) = $a"       # 23
  17&nbsp;
  18&nbsp;
  19&nbsp;(( ++a ))     #  Pre-increment 'a', C-style.
  20&nbsp;echo "a (after ++a) = $a"       # 24
  21&nbsp;
  22&nbsp;(( --a ))     #  Pre-decrement 'a', C-style.
  23&nbsp;echo "a (after --a) = $a"       # 23
  24&nbsp;
  25&nbsp;echo
  26&nbsp;
  27&nbsp;########################################################
  28&nbsp;#  Note that, as in C, pre- and post-decrement operators
  29&nbsp;#+ have different side-effects.
  30&nbsp;
  31&nbsp;n=1; let --n &#38;&#38; echo "True" || echo "False"  # False
  32&nbsp;n=1; let n-- &#38;&#38; echo "True" || echo "False"  # True
  33&nbsp;
  34&nbsp;#  Thanks, Jeroen Domburg.
  35&nbsp;########################################################
  36&nbsp;
  37&nbsp;echo
  38&nbsp;
  39&nbsp;(( t = a&#60;45?7:11 ))   # C-style trinary operator.
  40&nbsp;#       ^  ^ ^
  41&nbsp;echo "If a &#60; 45, then t = 7, else t = 11."  # a = 23
  42&nbsp;echo "t = $t "                              # t = 7
  43&nbsp;
  44&nbsp;echo
  45&nbsp;
  46&nbsp;
  47&nbsp;# -----------------
  48&nbsp;# Easter Egg alert!
  49&nbsp;# -----------------
  50&nbsp;#  Chet Ramey seems to have snuck a bunch of undocumented C-style
  51&nbsp;#+ constructs into Bash (actually adapted from ksh, pretty much).
  52&nbsp;#  In the Bash docs, Ramey calls (( ... )) shell arithmetic,
  53&nbsp;#+ but it goes far beyond that.
  54&nbsp;#  Sorry, Chet, the secret is out.
  55&nbsp;
  56&nbsp;# See also "for" and "while" loops using the (( ... )) construct.
  57&nbsp;
  58&nbsp;# These work only with version 2.04 or later of Bash.
  59&nbsp;
  60&nbsp;exit</PRE
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>See also <A
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>Example 11-13</A
> and <A
HREF="numerical-constants.html#NUMBERS"
>Example 8-4</A
>.</P
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