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<HTML><HEAD><TITLE>Recipe 7.9. Modifying a File in Place with -i Switch (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:36:32Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"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="ch07_01.htm"TITLE="7. File Access"><LINKREL="prev"HREF="ch07_09.htm"TITLE="7.8. Modifying a File in Place with Temporary File"><LINKREL="next"HREF="ch07_11.htm"TITLE="7.10. Modifying a File in Place Without a Temporary File"></HEAD><BODYBGCOLOR="#FFFFFF"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch07_09.htm"TITLE="7.8. Modifying a File in Place with Temporary File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 7.8. Modifying a File in Place with Temporary File"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch07_01.htm"TITLE="7. File Access"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch07_11.htm"TITLE="7.10. Modifying a File in Place Without a Temporary File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 7.10. Modifying a File in Place Without a Temporary File"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch07-31788">7.9. Modifying a File in Place with -i Switch</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch07-pgfId-1000011060">Problem<ACLASS="indexterm"NAME="ch07-idx-1000011059-0"></A><ACLASS="indexterm"NAME="ch07-idx-1000011059-1"></A><ACLASS="indexterm"NAME="ch07-idx-1000011059-2"></A></A></H3><PCLASS="para">You need to modify a file in place from the command line, and you're too lazy[<ACLASS="footnote"HREF="#ch07-pgfId-1000001159">5</A>] for the file manipulation of <ACLASS="xref"HREF="ch07_09.htm"TITLE="Modifying a File in Place with Temporary File">Recipe 7.8</A>.</P><BLOCKQUOTECLASS="footnote"><DIVCLASS="footnote"><PCLASS="para"><ACLASS="footnote"NAME="ch07-pgfId-1000001159">[5]</A> Lazy-as-virtue, not lazy-as-sin.</P></DIV></BLOCKQUOTE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch07-pgfId-874">Solution</A></H3><PCLASS="para">Use the <BCLASS="emphasis.bold">-i</B> and <BCLASS="emphasis.bold">-p</B> switches to Perl. Write your program on the command line:</P><PRECLASS="programlisting">% perl -i.orig -p -e 'FILTER COMMAND' file1 file2 file3 ...</PRE><PCLASS="para">Or use the switches in programs:</P><PRECLASS="programlisting">#!/usr/bin/perl -i.orig -p# filter commands go here</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch07-pgfId-888">Discussion</A></H3><PCLASS="para">The <BCLASS="emphasis.bold">-i</B> command-line switch modifies each file in place. It creates a temporary file as in the previous recipe, but Perl takes care of the tedious file manipulation for you. Use it with <BCLASS="emphasis.bold">-p</B> (explained in <ACLASS="xref"HREF="ch07_08.htm"TITLE="Writing a Filter">Recipe 7.7</A>) to turn:</P><PRECLASS="programlisting">% perl -pi.orig -e 's/DATE/localtime/e'</PRE><PCLASS="para">into:</P><PRECLASS="programlisting">while (<>) { if ($ARGV ne $oldargv) { # are we at the next file? rename($ARGV, $ARGV . '.orig'); open(ARGVOUT, ">$ARGV"); # plus error check select(ARGVOUT); $oldargv = $ARGV; } s/DATE/localtime/e;}continue{ print;}select (STDOUT); # restore default output</PRE><PCLASS="para">The <BCLASS="emphasis.bold">-i</B> switch takes care of making a backup (say <CODECLASS="literal">-i</CODE> instead of <CODECLASS="literal">-i.orig</CODE> to discard the original file contents instead of backing them up), and <BCLASS="emphasis.bold">-p</B> makes Perl loop over filenames given on the command line (or STDIN if no files were given).</P><PCLASS="para">The preceding one-liner would turn a file containing the following:</P><PRECLASS="programlisting">Dear Sir/Madam/Ravenous Beast, As of DATE, our records show your accountis overdue. Please settle by the end of the month.Yours in cheerful usury, --A. Moneylender</PRE><PCLASS="para">into:</P><PRECLASS="programlisting">Dear Sir/Madam/Ravenous Beast, As of Sat Apr 25 12:28:33 1998, our records show your accountis overdue. Please settle by the end of the month.Yours in cheerful usury, --A. Moneylender</PRE><PCLASS="para">This switch makes in-place translators a lot easier to write and to read. For instance, this changes isolated instances of <CODECLASS="literal">"hisvar"</CODE> to <CODECLASS="literal">"hervar"</CODE> in all C, C++, and <EMCLASS="emphasis">yacc</EM> files:</P><PRECLASS="programlisting">% perl -i.old -pe 's{\bhisvar\b}{hervar}g' *.[Cchy]</PRE><PCLASS="para">Turn on and off the <BCLASS="emphasis.bold">-i</B> behavior with the special variable <CODECLASS="literal">$^I</CODE>. Set <CODECLASS="literal">@ARGV</CODE>, and then use <> as you would with <BCLASS="emphasis.bold">-i</B> on the command line:</P><PRECLASS="programlisting"># set up to iterate over the *.c files in the current directory,# editing in place and saving the old file with a .orig extensionlocal $^I = '.orig'; # emulate -i.origlocal @ARGV = glob("*.c"); # initialize list of fileswhile (<>) { if ($. == 1) { print "This line should appear at the top of each file\n"; } s/\b(p)earl\b/${1}erl/ig; # Correct typos, preserving case print;} continue {close ARGV if eof} </PRE><PCLASS="para">Beware that creating a backup file under a particular name when that name already exists clobbers the previously backed up version.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch07-pgfId-1000009274">See Also</A></H3><PCLASS="para"><ICLASS="filename">perlrun</I> (1), and the <ACLASS="olink"HREF="../prog/ch06_01.htm#PERL2-CH-6-SECT-1.2">"Switches"</A> section of <ACLASS="olink"HREF="../prog/ch06_01.htm">Chapter 6</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the <CODECLASS="literal">$^I</CODE> and <CODECLASS="literal">$.</CODE> variables in <ICLASS="filename">perlvar </I>(1), and in the <ACLASS="olink"HREF="../prog/ch02_09.htm#">"Special Variables"</A> section of <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the <CODECLASS="literal">..</CODE> operator in the "Range Operator" sections of <ICLASS="filename">perlop</I> (1) and <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch07_09.htm"TITLE="7.8. 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