108 lines
3.0 KiB
HTML
108 lines
3.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>BASH Programming - Introduction HOW-TO: Loops for, while and until</TITLE>
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<LINK HREF="Bash-Prog-Intro-HOWTO-8.html" REL=next>
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<LINK HREF="Bash-Prog-Intro-HOWTO-6.html" REL=previous>
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<LINK HREF="Bash-Prog-Intro-HOWTO.html#toc7" REL=contents>
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<A HREF="Bash-Prog-Intro-HOWTO-8.html">Next</A>
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<A HREF="Bash-Prog-Intro-HOWTO.html#toc7">Contents</A>
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<HR>
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<H2><A NAME="s7">7. Loops for, while and until</A> </H2>
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<P> In this section you'll find for, while and until loops.
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<P> The <B>for</B> loop is a little bit different from other programming
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languages. Basically, it let's you iterate over a series of
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'words' within a string.
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<P> The <B>while</B> executes a piece of code if the control expression
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is true, and only stops when it is false (or a explicit break is found
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within the executed code.
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<P> The <B>until</B> loop is almost equal to the while loop, except that
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the code is executed while the control expression evaluates to false.
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<P> If you suspect that while and until are very similar you are right.
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<H2><A NAME="ss7.1">7.1 For sample</A>
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</H2>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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#!/bin/bash
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for i in $( ls ); do
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echo item: $i
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done
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</PRE>
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</CODE></BLOCKQUOTE>
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<P>
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<P> On the second line, we declare i to be the variable that
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will take the
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different values contained in $( ls ).
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<P> The third line could be longer if needed, or there could
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be more lines
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before the done (4).
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<P> 'done' (4) indicates that the code that used the value of $i has
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finished and $i can take a new value.
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<P> This script has very little sense, but a more useful way to use the
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for loop would be to use it to match only certain files on the previous
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example
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<P>
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<H2><A NAME="ss7.2">7.2 C-like for</A>
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</H2>
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<P> fiesh suggested adding this form of looping. It's a for loop
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more similar to C/perl... for.
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<BLOCKQUOTE><CODE>
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<PRE>
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#!/bin/bash
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for i in `seq 1 10`;
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do
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echo $i
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done
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</PRE>
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</CODE></BLOCKQUOTE>
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<H2><A NAME="ss7.3">7.3 While sample</A>
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</H2>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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#!/bin/bash
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COUNTER=0
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while [ $COUNTER -lt 10 ]; do
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echo The counter is $COUNTER
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let COUNTER=COUNTER+1
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done
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</PRE>
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</CODE></BLOCKQUOTE>
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<P>
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<P> This script 'emulates' the well known
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(C, Pascal, perl, etc) 'for' structure
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<H2><A NAME="ss7.4">7.4 Until sample</A>
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</H2>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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#!/bin/bash
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COUNTER=20
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until [ $COUNTER -lt 10 ]; do
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echo COUNTER $COUNTER
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let COUNTER-=1
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done
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</PRE>
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</CODE></BLOCKQUOTE>
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<HR>
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<A HREF="Bash-Prog-Intro-HOWTO.html#toc7">Contents</A>
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