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<HTML><!--Distributed by F --><HEAD><TITLE>[Chapter 13] 13.6 Safe I/O Redirection with noclobber </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="1998-08-04T21:36:44Z"><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="ch13_01.htm"TITLE="13. Redirecting Input and Output"><LINKREL="prev"HREF="ch13_05.htm"TITLE="13.5 Redirection in C Shell: Capture Errors, Too? "><LINKREL="next"HREF="ch13_07.htm"TITLE="13.7 The () Subshell Operators "></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="ch13_05.htm"TITLE="13.5 Redirection in C Shell: Capture Errors, Too? "><IMGSRC="gifs/txtpreva.gif"SRC="gifs/txtpreva.gif"ALT="Previous: 13.5 Redirection in C Shell: Capture Errors, Too? "BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="171"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1">Chapter 13<BR>Redirecting Input and Output</FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="172"><ACLASS="SECT1"HREF="ch13_07.htm"TITLE="13.7 The () Subshell Operators "><IMGSRC="gifs/txtnexta.gif"SRC="gifs/txtnexta.gif"ALT="Next: 13.7 The () Subshell Operators "BORDER="0"></A></TD></TR></TABLE> <HRALIGN="LEFT"WIDTH="515"TITLE="footer"></DIV><DIVCLASS="SECT1"><H2CLASS="sect1"><ACLASS="title"NAME="UPT-ART-1190">13.6 Safe I/O Redirection with noclobber </A></H2><PCLASS="para"><ACLASS="indexterm"NAME="AUTOID-14353"></A><ACLASS="indexterm"NAME="AUTOID-14355"></A>Have you ever destroyed a file accidentally?If you set the <EMCLASS="emphasis">noclobber</EM> C shell variable,or the <EMCLASS="emphasis">noclobber</EM> option in <EMCLASS="emphasis">bash</EM> and <EMCLASS="emphasis">ksh</EM>,it can help you avoid these mistakes.Setting <EMCLASS="emphasis">noclobber</EM> preventsyou from destroying a file when you are<SPANCLASS="link">redirecting standard output (<ACLASS="linkend"HREF="ch13_01.htm#UPT-ART-1023"TITLE="Using Standard Input and Output">13.1</A>)</SPAN>.</P><PCLASS="para">Consider the following situation:</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="replaceable"><I>anycommand </I></CODE><CODECLASS="userinput"><B>> outputfile</B></CODE></PRE></BLOCKQUOTE></P><PCLASS="para">The command above destroys the old <EMCLASS="emphasis">outputfile</EM> and creates a newone. If you have misspelled the name of your output file, or if youhave forgotten that the file already exists and contains importantdata, or (most common) if you really meant to type <CODECLASS="literal">>></CODE> instead of <CODECLASS="literal">></CODE>(i.e., if you really meant to append to the end of <EMCLASS="emphasis">outputfile</EM>,rather than start a new one), tough luck; your old data is gone.</P><PCLASS="para">Setting the variable <EMCLASS="emphasis">noclobber</EM> prevents this problem. If<EMCLASS="emphasis">noclobber</EM> exists, the C shell will not allow I/O redirection todestroy an existing file, unless you explicitly tell it to by adding anexclamation point (<CODECLASS="literal">!</CODE>) after the C shell redirect symbol - or a<ACLASS="indexterm"NAME="AUTOID-14378"></A>vertical bar (<CODECLASS="literal">|</CODE>) in <EMCLASS="emphasis">ksh</EM> and <EMCLASS="emphasis">bash</EM>.Here are examples.The left column shows <EMCLASS="emphasis">csh</EM> and <EMCLASS="emphasis">tcsh</EM>;the right column is for <EMCLASS="emphasis">bash</EM> (<EMCLASS="emphasis">ksh</EM> is similar):</P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>set noclobber</B></CODE> $ <CODECLASS="userinput"><B>set -o noclobber</B></CODE>% <CODECLASS="userinput"><B>ls</B></CODE> $ <CODECLASS="userinput"><B>ls</B></CODE>filea fileb filea fileb% <CODECLASS="userinput"><B>anyprogram > fileb</B></CODE> $ <CODECLASS="userinput"><B>anyprogram > fileb</B></CODE>fileb: File exists. bash: fileb: Cannot clobber existing file% <CODECLASS="userinput"><B>anyprogram >! fileb</B></CODE> $ <CODECLASS="userinput"><B>anyprogram >| fileb</B></CODE>% $</PRE></BLOCKQUOTE></P><PCLASS="para">Be sure to put space after the <CODECLASS="literal">!</CODE>.If you don't, the C shell thinks you're making a history reference and it(usually) prints an error like <CODECLASS="literal">fileb: Event not found.</CODE></P><PCLASS="para">Remember that <EMCLASS="emphasis">noclobber</EM> is not an environment variable,so any new shells you create<SPANCLASS="link">won't inherit it (<ACLASS="linkend"HREF="ch06_08.htm"TITLE="Shell Variables ">6.8</A>)</SPAN>.Therefore, if you want this feature, put the <EMCLASS="emphasis">set</EM> command (above) inyour shell's<SPANCLASS="link">setup file (<ACLASS="linkend"HREF="ch02_02.htm"TITLE="Shell Setup Files-Which, Where, and Why ">2.2</A>)</SPAN>.</P><BLOCKQUOTECLASS="note"><PCLASS="para"><STRONG>NOTE:</STRONG> In some C shells, <EMCLASS="emphasis">noclobber</EM> will prevent you from redirecting standardoutput to<SPANCLASS="link"><EMCLASS="emphasis">/dev/null</EM> (<ACLASS="linkend"HREF="ch13_14.htm"TITLE="What Can You Do with an Empty File? ">13.14</A>)</SPAN>or to a terminal unless you add the <CODECLASS="literal">!</CODE>.</P></BLOCKQUOTE><PCLASS="para">The C shell <EMCLASS="emphasis">noclobber</EM> variable has one other feature that's worth noting.Normally, the C shell lets you append to a file that doesn't exist.If <EMCLASS="emphasis">noclobber</EM> is set under <EMCLASS="emphasis">csh</EM> and <EMCLASS="emphasis">tcsh</EM>,it won't; you can only append to files thatalready exist unless you use an exclamation point: </P><PCLASS="para"><BLOCKQUOTECLASS="screen"><PRECLASS="screen">% <CODECLASS="userinput"><B>ls</B></CODE>filea fileb% <CODECLASS="replaceable"><I>anyprogram </I></CODE><CODECLASS="userinput"><B>>> filec</B></CODE>filec: No such file or directory% <CODECLASS="replaceable"><I>anyprogram </I></CODE><CODECLASS="userinput"><B>>>! filec</B></CODE>%</PRE></BLOCKQUOTE></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="ch13_05.htm"TITLE="13.5 Redirection in C Shell: Capture Errors, Too? 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