285 lines
4.6 KiB
HTML
285 lines
4.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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>The Double-Parentheses Construct</TITLE
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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TITLE="Advanced Bash-Scripting Guide"
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TITLE="Operations and Related Topics"
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TITLE="Numerical Constants"
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TITLE="Operator Precedence"
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>Advanced Bash-Scripting Guide: </TH
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>Prev</A
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><TD
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>Chapter 8. Operations and Related Topics</TD
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><H1
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><A
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NAME="DBLPARENS"
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></A
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>8.3. The Double-Parentheses Construct</H1
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><P
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><A
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NAME="DBLPARENSREF"
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></A
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></P
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><P
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>Similar to the <A
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HREF="internal.html#LETREF"
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>let</A
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> command,
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the <B
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CLASS="COMMAND"
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>(( ... ))</B
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> construct permits
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arithmetic expansion and evaluation. In its simplest
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form, <TT
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CLASS="USERINPUT"
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><B
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>a=$(( 5 + 3 ))</B
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></TT
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> would set
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<TT
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CLASS="USERINPUT"
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><B
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>a</B
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></TT
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> to <TT
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CLASS="USERINPUT"
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><B
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>5 + 3</B
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></TT
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>, or
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<TT
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CLASS="USERINPUT"
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><B
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>8</B
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></TT
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>. However, this double-parentheses
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construct is also a mechanism for allowing C-style
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manipulation of variables in Bash, for example,
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<TT
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CLASS="VARNAME"
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>(( var++ ))</TT
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>.</P
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><P
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><A
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NAME="PLUSPLUSREF"
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></A
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></P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="CVARS"
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></A
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><P
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><B
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>Example 8-5. C-style manipulation of variables</B
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></P
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><TABLE
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>#!/bin/bash
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# c-vars.sh
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# Manipulating a variable, C-style, using the (( ... )) construct.
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echo
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(( a = 23 )) # Setting a value, C-style,
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#+ with spaces on both sides of the "=".
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echo "a (initial value) = $a" # 23
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(( a++ )) # Post-increment 'a', C-style.
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echo "a (after a++) = $a" # 24
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(( a-- )) # Post-decrement 'a', C-style.
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echo "a (after a--) = $a" # 23
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(( ++a )) # Pre-increment 'a', C-style.
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echo "a (after ++a) = $a" # 24
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(( --a )) # Pre-decrement 'a', C-style.
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echo "a (after --a) = $a" # 23
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echo
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########################################################
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# Note that, as in C, pre- and post-decrement operators
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#+ have different side-effects.
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n=1; let --n && echo "True" || echo "False" # False
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n=1; let n-- && echo "True" || echo "False" # True
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# Thanks, Jeroen Domburg.
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########################################################
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echo
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(( t = a<45?7:11 )) # C-style trinary operator.
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# ^ ^ ^
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echo "If a < 45, then t = 7, else t = 11." # a = 23
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echo "t = $t " # t = 7
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echo
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# -----------------
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# Easter Egg alert!
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# -----------------
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# Chet Ramey seems to have snuck a bunch of undocumented C-style
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#+ constructs into Bash (actually adapted from ksh, pretty much).
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# In the Bash docs, Ramey calls (( ... )) shell arithmetic,
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#+ but it goes far beyond that.
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# Sorry, Chet, the secret is out.
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# See also "for" and "while" loops using the (( ... )) construct.
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# These work only with version 2.04 or later of Bash.
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exit</PRE
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></FONT
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></TD
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></TABLE
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></DIV
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><P
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>See also <A
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HREF="loops1.html#FORLOOPC"
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>Example 11-13</A
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> and <A
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HREF="numerical-constants.html#NUMBERS"
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>Example 8-4</A
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>.</P
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