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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 44] 44.9 Testing Your Success </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="1998-08-04T21:53:41Z"><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_08.htm"TITLE="44.8 Test Exit Status with the if Statement "><LINKREL="next"HREF="ch44_10.htm"TITLE="44.10 Loops That Test Exit Status "></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_08.htm"TITLE="44.8 Test Exit Status with the if Statement "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.8 Test Exit Status with the if Statement "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_10.htm"TITLE="44.10 Loops That Test Exit Status "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.10 Loops That Test Exit Status "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-0230">44.9 Testing Your Success </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50769"></A><ACLASS="indexterm"NAME="AUTOID-50772"></A><ACLASS="indexterm"NAME="AUTOID-50776"></A><ACLASS="indexterm"NAME="AUTOID-50779"></A>The shells let you test for success right on the command line. This gives you a very efficient way to write quick andcomprehensible shell scripts.</P><PCLASS="para">I'm referring to the <CODECLASS="literal">||</CODE> and <CODECLASS="literal">&&</CODE> operators; in particular,the <CODECLASS="literal">||</CODE> operator.<CODECLASS="replaceable"><I>comm1</I></CODE><CODECLASS="literal"> || </CODE><CODECLASS="replaceable"><I>comm2</I></CODE> is typically explained as "execute thecommand on the right if the command on the left failed."I prefer to explain it as an "either-or" construct: "execute either<EMCLASS="emphasis">comm1</EM> or <EMCLASS="emphasis">comm2</EM>."While this isn't really precise, let's see what it means incontext:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen"><ACLASS="indexterm"NAME="AUTOID-50794"></A><ACLASS="indexterm"NAME="AUTOID-50797"></A>cat filea fileb > filec || exit</PRE></BLOCKQUOTE></P><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-50801"></A><ACLASS="indexterm"NAME="AUTOID-50804"></A><ACLASS="indexterm"NAME="AUTOID-50807"></A>This means "either <EMCLASS="emphasis">cat</EM> the files or <EMCLASS="emphasis">exit</EM>."If you can't<EMCLASS="emphasis">cat</EM> the files (if <EMCLASS="emphasis">cat</EM> returns an exit status of 1), you<SPANCLASS="link">exit (<ACLASS="linkend"HREF="ch38_04.htm"TITLE="Subshells ">38.4</A>)</SPAN>.If you can<EMCLASS="emphasis">cat</EM> the files, you don't exit; you execute the left side<EMCLASS="emphasis">or</EM> the right side.</P><PCLASS="para">I'm stretching normal terminology a bit here, but I think it'snecessary to clarify the purpose of <CODECLASS="literal">||</CODE>.By the way, we could give the poor user an error message before flaming out:</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"><ACLASS="indexterm"NAME="AUTOID-50821"></A>cat filea fileb > filec || { echo sorry, no dice 1>&2 exit 1}</PRE></TD></TR></TABLE></P><PCLASS="para"> Similarly, <CODECLASS="replaceable"><I>comm1</I></CODE><CODECLASS="literal"> && </CODE><CODECLASS="replaceable"><I>comm2</I></CODE>means "execute <EMCLASS="emphasis">comm1</EM> AND <EMCLASS="emphasis">comm2</EM>," orexecute <EMCLASS="emphasis">comm2</EM> if <EMCLASS="emphasis">comm1</EM> succeeds. (But if you can'texecute the first, don't do any.)This might be helpful if you want toprint a temporary file and delete it immediately.</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">lpr file && rm file</PRE></BLOCKQUOTE></P><PCLASS="para">If <EMCLASS="emphasis">lpr</EM> fails for some reason, you want to leave the file around.Again, I want to stress how to read this: print the file and deleteit.(Implicitly: if you don't print it, don't delete it.)</P><DIVCLASS="sect1info"><PCLASS="SECT1INFO">- <SPANCLASS="authorinitials">ML</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_08.htm"TITLE="44.8 Test Exit Status with the if Statement "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 44.8 Test Exit Status with the if Statement "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_10.htm"TITLE="44.10 Loops That Test Exit Status "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 44.10 Loops That Test Exit Status "BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="172">44.8 Test Exit Status with the if Statement </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">44.10 Loops That Test Exit Status </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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