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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 44] 44.10 Loops That Test Exit Status </TITLE><METANAME="DC.title"CONTENT="UNIX Power Tools"><METANAME="DC.creator"CONTENT="Jerry Peek, Tim O'Reilly & Mike Loukides"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1998-08-04T21:53:42Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-260-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch44_01.htm"TITLE="44. Shell Programming for the Uninitiated"><LINKREL="prev"HREF="ch44_09.htm"TITLE="44.9 Testing Your Success "><LINKREL="next"HREF="ch44_11.htm"TITLE="44.11 Set Exit Status of a Shell (Script) "></HEAD><BODYBGCOLOR="#FFFFFF"TEXT="#000000"><DIVCLASS="htmlnav"><H1><IMGSRC="gifs/smbanner.gif"ALT="UNIX Power Tools"USEMAP="#srchmap"BORDER="0"></H1><MAPNAME="srchmap"><AREASHAPE="RECT"COORDS="0,0,466,58"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="467,0,514,18"HREF="jobjects/fsearch.htm"ALT="Search this book"></MAP><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch44_09.htm"TITLE="44.9 Testing Your Success "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.9 Testing Your Success "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 44<BR>Shell Programming for the Uninitiated</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch44_11.htm"TITLE="44.11 Set Exit Status of a Shell (Script) "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.11 Set Exit Status of a Shell (Script) "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-2280">44.10 Loops That Test Exit Status </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50842"></A><ACLASS="indexterm"NAME="AUTOID-50845"></A><ACLASS="indexterm"NAME="AUTOID-50848"></A><ACLASS="indexterm"NAME="AUTOID-50851"></A><ACLASS="indexterm"NAME="AUTOID-50854"></A><ACLASS="indexterm"NAME="AUTOID-50857"></A>The Bourne shell has two kinds of loops that run a command and test its exitstatus.An <EMCLASS="emphasis">until</EM> loop will continue until the command returns a zero status.A <EMCLASS="emphasis">while</EM> loop will continue while the command returns a zero status.</P><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="UPT-ART-2280-SECT-1.1">44.10.1 Looping Until a Command Succeeds </A></H3><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50865"></A><ACLASS="indexterm"NAME="AUTOID-50868"></A><ACLASS="indexterm"NAME="AUTOID-50870"></A><ACLASS="indexterm"NAME="AUTOID-50873"></A><ACLASS="indexterm"NAME="AUTOID-50876"></A>The <EMCLASS="emphasis">until</EM> loop runs a command repeatedly until it succeeds.That is, if the command returns a non-zero status, the shell executes the bodyof the loop and then runs the loop control command again.The shell keeps running the command until it returns a zero status,as shown in the following example:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>cat sysmgr</B></CODE>#!/bin/shuntil who | grep "^barb "do sleep 60doneecho The system manager just logged on.% <CODECLASS="userinput"><B>sysmgr</B></CODE> &[1] 2345 ...<EMCLASS="emphasis">time passes</EM>...barb ttyp7 Jul 15 09:30The system manager just logged on.</PRE></BLOCKQUOTE></P><PCLASS="para">The loop runs<SPANCLASS="link"><EMCLASS="emphasis">who</EM> (<ACLASS="linkend"HREF="ch51_04.htm"TITLE="Who's On? ">51.4</A>)</SPAN>and pipes that output to<SPANCLASS="link"><EMCLASS="emphasis">grep</EM> (<ACLASS="linkend"HREF="ch27_01.htm#UPT-ART-7420"TITLE="Different Versions of grep ">27.1</A>)</SPAN>,which searches for any line starting with <EMCLASS="emphasis">barb</EM> and a space.(The space makes sure that usernames like <EMCLASS="emphasis">barbara</EM> don't match.)If <EMCLASS="emphasis">grep</EM> returns non-zero (no lines matched), the shell waits 60seconds. Then the loop repeats, and the script tries the <CODECLASS="literal">who</CODE> <CODECLASS="literal">|</CODE> <CODECLASS="literal">grep</CODE> command again. It keeps doing this until<EMCLASS="emphasis">grep</EM> returns a zero status - then the loop is broken and controlgoes past the <CODECLASS="literal">done</CODE> line. The <EMCLASS="emphasis">echo</EM> command prints amessage and the script quits. (I ran this script<SPANCLASS="link">in the background (<ACLASS="linkend"HREF="ch01_26.htm"TITLE="What a Multiuser System Can Do for You ">1.26</A>)</SPAN>so I could dosomething else while I waited for Barb.)</P><PCLASS="para">[A Bourne shell <EMCLASS="emphasis">until</EM> loop is <EMCLASS="emphasis">not</EM> identical to the<EMCLASS="emphasis">until</EM> construction in most programming languages, because thecondition is evaluated at the top of the loop. Virtually alllanguages with an <EMCLASS="emphasis">until</EM> loop evaluate the condition at thebottom. <EMCLASS="emphasis">-ML</EM> ]</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="UPT-ART-2280-SECT-1.2">44.10.2 Looping Until a Command Fails </A></H3><TABLECLASS="para.programreference"BORDER="1"><TR><THVALIGN="TOP"><ACLASS="programreference"HREF="examples/index.htm"TITLE="catsaway">catsaway</A><BR></TH><TDVALIGN="TOP"><ACLASS="indexterm"NAME="AUTOID-50909"></A><ACLASS="indexterm"NAME="AUTOID-50912"></A><ACLASS="indexterm"NAME="AUTOID-50915"></A><ACLASS="indexterm"NAME="AUTOID-50917"></A>The <EMCLASS="emphasis">while</EM> loop is the opposite of the <EMCLASS="emphasis">until</EM> loop. A<EMCLASS="emphasis">while</EM> loop runs a command and loops until the command fails(returns a non-zero status). The <EMCLASS="emphasis">catsaway</EM> program below uses a<EMCLASS="emphasis">while</EM> loop to watch the <EMCLASS="emphasis">who</EM> output for the system managerto log off. It's the opposite of the <EMCLASS="emphasis">sysmgr</EM> script. </TD></TR></TABLE><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"> <ACLASS="co"HREF="ch13_14.htm"TITLE="13.14 What Can You Do with an Empty File? ">/dev/null</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen"><ACLASS="indexterm"NAME="AUTOID-50930"></A><ACLASS="indexterm"NAME="AUTOID-50933"></A>% <CODECLASS="userinput"><B>cat catsaway</B></CODE>#!/bin/shwhile who | grep "^barb " > /dev/nulldo sleep 60doneecho "The cat's away..."% <CODECLASS="userinput"><B>catsaway</B></CODE> &[1] 4567 ...<EMCLASS="emphasis">time passes</EM>...The cat's away...</PRE></TD></TR></TABLE></P></DIV><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">JP</SPAN></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><TABLEWIDTH="515"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch44_09.htm"TITLE="44.9 Testing Your Success "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.9 Testing Your Success "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="book"HREF="index.htm"TITLE="UNIX Power Tools"><IMGSRC="gifs/txthome.gif"SRC="gifs/txthome.gif"ALT="UNIX Power Tools"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch44_11.htm"TITLE="44.11 Set Exit Status of a Shell (Script) "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.11 Set Exit Status of a Shell (Script) "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">44.9 Testing Your Success </TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><ACLASS="index"HREF="index/idx_0.htm"TITLE="Book Index"><IMGSRC="gifs/index.gif"SRC="gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172">44.11 Set Exit Status of a Shell (Script) </TD></TR></TABLE><HRALIGN="LEFT"WIDTH="515"TITLE="footer"><IMGSRC="gifs/smnavbar.gif"SRC="gifs/smnavbar.gif"USEMAP="#map"BORDER="0"ALT="The UNIX CD Bookshelf Navigation"><MAPNAME="map"><AREASHAPE="RECT"COORDS="0,0,73,21"HREF="../index.htm"ALT="The UNIX CD Bookshelf"><AREASHAPE="RECT"COORDS="74,0,163,21"HREF="index.htm"ALT="UNIX Power Tools"><AREASHAPE="RECT"COORDS="164,0,257,21"HREF="../unixnut/index.htm"ALT="UNIX in a Nutshell"><AREASHAPE="RECT"COORDS="258,0,321,21"HREF="../vi/index.htm"ALT="Learning the vi Editor"><AREASHAPE="RECT"COORDS="322,0,378,21"HREF="../sedawk/index.htm"ALT="sed & awk"><AREASHAPE="RECT"COORDS="379,0,438,21"HREF="../ksh/index.htm"ALT="Learning the Korn Shell"><AREASHAPE="RECT"COORDS="439,0,514,21"HREF="../lrnunix/index.htm"ALT="Learning the UNIX Operating System"></MAP></DIV></BODY></HTML>⌨️ 快捷键说明
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