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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 44] 44.8 Test Exit Status with the if Statement </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="1998-08-04T21:53:39Z"><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_07.htm"TITLE="44.7 Exit Status of UNIX Processes "><LINKREL="next"HREF="ch44_09.htm"TITLE="44.9 Testing Your Success "></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_07.htm"TITLE="44.7 Exit Status of UNIX Processes "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.7 Exit Status of UNIX Processes "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_09.htm"TITLE="44.9 Testing Your Success "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.9 Testing Your Success "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-7080">44.8 Test Exit Status with the if Statement </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50650"></A><ACLASS="indexterm"NAME="AUTOID-50653"></A><ACLASS="indexterm"NAME="AUTOID-50656"></A><ACLASS="indexterm"NAME="AUTOID-50659"></A><ACLASS="indexterm"NAME="AUTOID-50662"></A><ACLASS="indexterm"NAME="AUTOID-50665"></A><ACLASS="indexterm"NAME="AUTOID-50668"></A>If you are going to write a shell script of any complexity at all, you need some way to write "conditional expressions." Conditionalexpressions are nothing more than statements that have a value of"true" or "false": like "Have I gotten dressed today?" or "Is itbefore 5 p.m.?" or "Does the file <EMCLASS="emphasis">indata</EM> exist?" or "Is the valueof <CODECLASS="literal">$aardvark</CODE> greater than 60?"</P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50674"></A><ACLASS="indexterm"NAME="AUTOID-50676"></A><ACLASS="indexterm"NAME="AUTOID-50679"></A>The UNIX shell is a complete programming language. Therefore, itallows you to write "if" statements with conditional expressions - justlike C, Basic, Pascal, or any other language. Conditional expressionscan also be used in several other situations; but most obviously,they're the basis for any sort of <EMCLASS="emphasis">if</EM> statement. Here's thesyntax of an <EMCLASS="emphasis">if</EM> statement for the Bourne shell:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">if <CODECLASS="replaceable"><I>conditional</I></CODE>then # do this if <CODECLASS="replaceable"><I>conditional</I></CODE> returns a zero ("true") status <CODECLASS="replaceable"><I>one-or-more-commands</I></CODE>else # do this if <CODECLASS="replaceable"><I>conditional</I></CODE> returns non-zero ("false") status <CODECLASS="replaceable"><I>one-or-more-commands</I></CODE>fi</PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50691"></A><ACLASS="indexterm"NAME="AUTOID-50694"></A><ACLASS="indexterm"NAME="AUTOID-50697"></A><ACLASS="indexterm"NAME="AUTOID-50700"></A><ACLASS="indexterm"NAME="AUTOID-50703"></A><ACLASS="indexterm"NAME="AUTOID-50706"></A>You can omit the <EMCLASS="emphasis">else</EM> and the block of code following it.However, you can't omit the <EMCLASS="emphasis">then</EM> or the <EMCLASS="emphasis">fi</EM>. If you want to omit the<EMCLASS="emphasis">then</EM> (i.e., if you want to do something special when<EMCLASS="emphasis">condition</EM> is false, but nothing when it is true), write thestatement like this:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">if <CODECLASS="replaceable"><I>conditional</I></CODE>then : # do nothingelse # do this if <CODECLASS="replaceable"><I>conditional</I></CODE> returns non-zero ("false") status <CODECLASS="replaceable"><I>one-or-more-commands</I></CODE>fi</PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50720"></A><ACLASS="indexterm"NAME="AUTOID-50722"></A><ACLASS="indexterm"NAME="AUTOID-50725"></A><ACLASS="indexterm"NAME="AUTOID-50728"></A>Note that this uses a special null command, a<SPANCLASS="link">colon (<CODECLASS="literal">:</CODE>) (<ACLASS="linkend"HREF="ch45_09.htm"TITLE='The Unappreciated Bourne Shell ":" Operator'>45.9</A>)</SPAN>.There's another, more useful way of expressing the inverse of acondition (do something if <EMCLASS="emphasis">conditional</EM> is not "true"), the<SPANCLASS="link"><CODECLASS="literal">||</CODE> operator (<ACLASS="linkend"HREF="ch44_09.htm"TITLE="Testing Your Success ">44.9</A>)</SPAN>(two vertical bars).</P><PCLASS="para">Don't forget the <EMCLASS="emphasis">fi</EM> terminating the statement. This is asurprisingly common source of bugs. (At least for me.)</P><PCLASS="para">Another common debugging problem:the manual pages that discuss this material imply that you can smashthe <EMCLASS="emphasis">if</EM>, the <EMCLASS="emphasis">then</EM>, and the <EMCLASS="emphasis">else</EM> onto one line.Well, it's true, but it's not always easy.Do yourself a favor: writeyour <EMCLASS="emphasis">if</EM> statements <EMCLASS="emphasis">exactly</EM> like the one above. You'llrarely be disappointed, and you may even start writing programs thatwork correctly the first time.</P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50746"></A>Here's a real-life example: a shell script named <EMCLASS="emphasis">bkedit</EM> that makes abackup copy of a file before editing it.If <EMCLASS="emphasis">cp</EM> returns a zero status, the script edits the file; otherwise, itprints a message.(The <CODECLASS="literal">$1</CODE> is replaced with the first filename from the commandline - see article<ACLASS="xref"HREF="ch44_15.htm"TITLE="Handling Command-Line Arguments in Shell Scripts ">44.15</A>.)</P><PCLASS="para"><TABLECLASS="screen.co"BORDER="1"><TR><THVALIGN="TOP"><PRECLASS="calloutlist"> <ACLASS="co"HREF="ch08_06.htm"TITLE="8.6 Output Command-Line Arguments ">1>&2</A> </PRE></TH><TDVALIGN="TOP"><PRECLASS="screen">#!/bin/shif cp "$1" "$1.bak"then vi "$1"else echo "bkedit quitting: can't make backup?" 1>&2fi</PRE></TD></TR></TABLE></P><PCLASS="para">You can try typing in that shell script and running it.Or, just type inthe lines (starting with the <CODECLASS="literal">if</CODE>) on a terminal running theBourne shell; use a real filename instead of <CODECLASS="literal">$1</CODE>.</P><PCLASS="para">The <EMCLASS="emphasis">if</EM> statement is often used with a command named<SPANCLASS="link"><EMCLASS="emphasis">test</EM> (<ACLASS="linkend"HREF="ch44_20.htm"TITLE="test: Testing Files and Strings ">44.20</A>)</SPAN>.The <EMCLASS="emphasis">test</EM> command does a test and returns an exit status of 0 or 1.</P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">ML</SPAN>, <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_07.htm"TITLE="44.7 Exit Status of UNIX Processes "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.7 Exit Status of UNIX Processes "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_09.htm"TITLE="44.9 Testing Your Success "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.9 Testing Your Success "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">44.7 Exit Status of UNIX Processes </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">44.9 Testing Your Success </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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