628 lines
10 KiB
HTML
628 lines
10 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><TITLE
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>Loop Control</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Loops and Branches"
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HREF="loops.html"><LINK
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REL="PREVIOUS"
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TITLE="Nested Loops"
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HREF="nestedloops.html"><LINK
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TITLE="Testing and Branching"
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HREF="testbranch.html"></HEAD
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Advanced Bash-Scripting Guide: </TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="nestedloops.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 11. Loops and Branches</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="testbranch.html"
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ACCESSKEY="N"
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>Next</A
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="LOOPCONTROL"
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></A
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>11.3. Loop Control</H1
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><TABLE
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BORDER="0"
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WIDTH="100%"
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CELLSPACING="0"
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CELLPADDING="0"
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CLASS="EPIGRAPH"
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><TR
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><TD
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WIDTH="45%"
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> </TD
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><TD
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WIDTH="45%"
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ALIGN="LEFT"
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VALIGN="TOP"
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><I
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><P
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><I
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>Tournez cent tours, tournez mille tours,</I
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></P
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><P
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><I
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>Tournez souvent et tournez toujours . . .</I
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></P
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><P
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><I
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>--Verlaine, <SPAN
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CLASS="QUOTE"
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>"Chevaux de bois"</SPAN
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></I
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></P
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></I
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></TD
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></TR
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></TABLE
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><P
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></P
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><DIV
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CLASS="VARIABLELIST"
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><P
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><B
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><A
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NAME="BRKCONT1"
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></A
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>Commands affecting loop behavior</B
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></P
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><DL
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><DT
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><B
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CLASS="COMMAND"
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>break</B
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>, <B
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CLASS="COMMAND"
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>continue</B
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></DT
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><DD
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><P
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>The <B
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CLASS="COMMAND"
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>break</B
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> and <B
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CLASS="COMMAND"
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>continue</B
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>
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loop control commands
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<A
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NAME="AEN6981"
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HREF="#FTN.AEN6981"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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>
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correspond exactly to their counterparts in other
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programming languages. The <B
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CLASS="COMMAND"
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>break</B
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>
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command terminates the loop (<EM
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>breaks</EM
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>
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out of it), while <B
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CLASS="COMMAND"
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>continue</B
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> causes a jump
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to the next <A
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HREF="loops1.html#ITERATIONREF"
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>iteration</A
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>
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of the loop, skipping all the remaining commands in that
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particular loop cycle.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="EX28"
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></A
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><P
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><B
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>Example 11-21. Effects of <I
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CLASS="FIRSTTERM"
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>break</I
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> and
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<B
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CLASS="COMMAND"
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>continue</B
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> in a loop</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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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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LIMIT=19 # Upper limit
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echo
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echo "Printing Numbers 1 through 20 (but not 3 and 11)."
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a=0
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while [ $a -le "$LIMIT" ]
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do
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a=$(($a+1))
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if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11.
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then
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continue # Skip rest of this particular loop iteration.
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fi
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echo -n "$a " # This will not execute for 3 and 11.
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done
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# Exercise:
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# Why does the loop print up to 20?
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echo; echo
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echo Printing Numbers 1 through 20, but something happens after 2.
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##################################################################
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# Same loop, but substituting 'break' for 'continue'.
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a=0
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while [ "$a" -le "$LIMIT" ]
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do
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a=$(($a+1))
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if [ "$a" -gt 2 ]
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then
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break # Skip entire rest of loop.
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fi
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echo -n "$a "
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done
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echo; echo; echo
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exit 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><P
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><A
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NAME="BREAKPARAM"
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></A
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></P
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><P
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>The <B
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CLASS="COMMAND"
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>break</B
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> command may optionally take a
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parameter. A plain <B
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CLASS="COMMAND"
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>break</B
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> terminates
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only the innermost loop in which it is embedded,
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but a <B
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CLASS="COMMAND"
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>break N</B
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> breaks out of
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<TT
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CLASS="PARAMETER"
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><I
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>N</I
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></TT
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> levels of loop.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="BREAKLEVELS"
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></A
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><P
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><B
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>Example 11-22. Breaking out of multiple loop levels</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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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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# break-levels.sh: Breaking out of loops.
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# "break N" breaks out of N level loops.
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for outerloop in 1 2 3 4 5
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do
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echo -n "Group $outerloop: "
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# --------------------------------------------------------
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for innerloop in 1 2 3 4 5
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do
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echo -n "$innerloop "
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if [ "$innerloop" -eq 3 ]
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then
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break # Try break 2 to see what happens.
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# ("Breaks" out of both inner and outer loops.)
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fi
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done
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# --------------------------------------------------------
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echo
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done
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echo
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exit 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><P
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>The <B
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CLASS="COMMAND"
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>continue</B
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> command, similar to
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<B
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CLASS="COMMAND"
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>break</B
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>, optionally takes a parameter. A
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plain <B
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CLASS="COMMAND"
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>continue</B
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> cuts short the
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current iteration within its loop and begins the next.
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A <B
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CLASS="COMMAND"
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>continue N</B
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> terminates all remaining
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iterations at its loop level and continues with the
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next iteration at the loop, <TT
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CLASS="OPTION"
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>N</TT
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> levels
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above.</P
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="CONTINUELEVELS"
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></A
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><P
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><B
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>Example 11-23. Continuing at a higher loop level</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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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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# The "continue N" command, continuing at the Nth level loop.
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for outer in I II III IV V # outer loop
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do
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echo; echo -n "Group $outer: "
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# --------------------------------------------------------------------
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for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
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do
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if [[ "$inner" -eq 7 && "$outer" = "III" ]]
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then
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continue 2 # Continue at loop on 2nd level, that is "outer loop".
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# Replace above line with a simple "continue"
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# to see normal loop behavior.
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fi
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echo -n "$inner " # 7 8 9 10 will not echo on "Group III."
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done
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# --------------------------------------------------------------------
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done
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echo; echo
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# Exercise:
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# Come up with a meaningful use for "continue N" in a script.
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exit 0</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="EXAMPLE"
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><A
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NAME="CONTINUENEX"
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></A
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><P
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><B
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>Example 11-24. Using <I
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CLASS="FIRSTTERM"
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>continue N</I
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> in an actual task</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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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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># Albert Reiner gives an example of how to use "continue N":
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# ---------------------------------------------------------
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# Suppose I have a large number of jobs that need to be run, with
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#+ any data that is to be treated in files of a given name pattern
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#+ in a directory. There are several machines that access
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#+ this directory, and I want to distribute the work over these
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#+ different boxen.
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# Then I usually nohup something like the following on every box:
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while true
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do
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for n in .iso.*
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do
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[ "$n" = ".iso.opts" ] && continue
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beta=${n#.iso.}
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[ -r .Iso.$beta ] && continue
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[ -r .lock.$beta ] && sleep 10 && continue
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lockfile -r0 .lock.$beta || continue
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echo -n "$beta: " `date`
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run-isotherm $beta
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date
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ls -alF .Iso.$beta
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[ -r .Iso.$beta ] && rm -f .lock.$beta
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continue 2
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done
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break
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done
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exit 0
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# The details, in particular the sleep N, are particular to my
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#+ application, but the general pattern is:
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while true
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do
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for job in {pattern}
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do
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{job already done or running} && continue
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{mark job as running, do job, mark job as done}
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continue 2
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done
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break # Or something like `sleep 600' to avoid termination.
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done
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# This way the script will stop only when there are no more jobs to do
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#+ (including jobs that were added during runtime). Through the use
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#+ of appropriate lockfiles it can be run on several machines
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#+ concurrently without duplication of calculations [which run a couple
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#+ of hours in my case, so I really want to avoid this]. Also, as search
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#+ always starts again from the beginning, one can encode priorities in
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#+ the file names. Of course, one could also do this without `continue 2',
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#+ but then one would have to actually check whether or not some job
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#+ was done (so that we should immediately look for the next job) or not
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#+ (in which case we terminate or sleep for a long time before checking
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#+ for a new job).</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><DIV
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CLASS="CAUTION"
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><P
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></P
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><TABLE
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CLASS="CAUTION"
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WIDTH="90%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/caution.gif"
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HSPACE="5"
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ALT="Caution"></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>The <B
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CLASS="COMMAND"
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>continue N</B
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> construct is
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difficult to understand and tricky to use in any meaningful
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context. It is probably best avoided.</P
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></TD
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></TR
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></TABLE
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></DIV
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></DD
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></DL
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></DIV
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
|
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
|
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><A
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NAME="FTN.AEN6981"
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|
HREF="loopcontrol.html#AEN6981"
|
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></TD
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><TD
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|
ALIGN="LEFT"
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|
VALIGN="TOP"
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WIDTH="95%"
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><P
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>These are shell <A
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HREF="internal.html#BUILTINREF"
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>builtins</A
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>,
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whereas other loop commands, such as <A
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HREF="loops1.html#WHILELOOPREF"
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>while</A
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> and <A
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HREF="testbranch.html#CASEESAC1"
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>case</A
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>, are <A
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HREF="internal.html#KEYWORDREF"
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>keywords</A
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>.</P
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></TD
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></TR
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></TABLE
|
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><DIV
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CLASS="NAVFOOTER"
|
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
|
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><A
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HREF="nestedloops.html"
|
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
|
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
|
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><A
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HREF="testbranch.html"
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ACCESSKEY="N"
|
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>Next</A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Nested Loops</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="loops.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Testing and Branching</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |