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<HTML><HEAD><TITLE>Recipe 16.5. Filtering Your Own Output (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="1999-07-02T01:43:43Z"><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="ch16_01.htm"TITLE="16. Process Management and Communication"><LINKREL="prev"HREF="ch16_05.htm"TITLE="16.4. Reading or Writing to Another Program"><LINKREL="next"HREF="ch16_07.htm"TITLE="16.6. Preprocessing Input"></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="ch16_05.htm"TITLE="16.4. Reading or Writing to Another Program"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 16.4. Reading or Writing to Another Program"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch16_01.htm"TITLE="16. Process Management and Communication"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch16_07.htm"TITLE="16.6. Preprocessing Input"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 16.6. Preprocessing Input"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch16-38102">16.5. Filtering Your Own Output</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1074">Problem<ACLASS="indexterm"NAME="ch16-idx-1000006271-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006271-1"></A><ACLASS="indexterm"NAME="ch16-idx-1000006271-2"></A><ACLASS="indexterm"NAME="ch16-idx-1000006271-3"></A><ACLASS="indexterm"NAME="ch16-idx-1000006271-4"></A><ACLASS="indexterm"NAME="ch16-idx-1000006271-5"></A></A></H3><PCLASS="para">You want to postprocess your program's output without writing a separate program to do so.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1080">Solution</A></H3><PCLASS="para">Use the forking form of <CODECLASS="literal">open</CODE> to attach a filter to yourself. For example, this will restrict your program to a hundred lines of output:</P><PRECLASS="programlisting">head(100);while (<>) { print;} sub head { my $lines = shift || 20; return if $pid = open(STDOUT, "|-"); die "cannot fork: $!" unless defined $pid; while (<STDIN>) { print; last unless --$lines ; } exit;} </PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1116">Discussion</A></H3><PCLASS="para">It's easy to add an output filter. Just use the forking <EMCLASS="emphasis">open</EM> on your own STDOUT, and let the child filter STDIN to STDOUT, performing whatever alterations you care about. Notice that we install the output filter <EMCLASS="emphasis">before</EM> we generate the output. This makes sense - you can't filter your output if it has already left your program. Any such filters should be applied in LIFO order - the last one inserted is the first one run.</P><PCLASS="para">Here's an example that uses two output filters. One numbers lines; the other quotes the lines like a mail reply. When run on <EMCLASS="emphasis">/etc/motd</EM>, you get something like:</P><PRECLASS="programlisting"><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>1: > Welcome to Linux, version 2.0.33 on a i686</I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>2: > </I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>3: > "The software required `Windows 95 or better', </I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>4: > so I installed Linux." </I></CODE></B></CODE></PRE><PCLASS="para">If you reversed the order of the two filters, you'd get:</P><PRECLASS="programlisting"><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>> 1: Welcome to Linux, Kernel version 2.0.33 on a i686</I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>> 2: </I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>> 3: "The software required `Windows 95 or better', </I></CODE></B></CODE><CODECLASS="userinput"><B><CODECLASS="replaceable"><I>> 4: so I installed Linux." </I></CODE></B></CODE></PRE><PCLASS="para">The program is in <ACLASS="xref"HREF="ch16_06.htm#ch16-29490"TITLE="qnumcat">Example 16.1</A>.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch16-29490">Example 16.1: qnumcat</A></H4><PRECLASS="programlisting">#!/usr/bin/perl# qnumcat - demo additive output filtersnumber(); # push number filter on STDOUTquote(); # push quote filter on STDOUTwhile (<>) { # act like /bin/cat print;} close STDOUT; # tell kids we're done--politelyexit;sub number { my $pid; return if $pid = open(STDOUT, "|-"); die "cannot fork: $!" unless defined $pid; while (<STDIN>) { printf "%d: %s", $., $_ } exit;} sub quote { my $pid; return if $pid = open(STDOUT, "|-"); die "cannot fork: $!" unless defined $pid; while (<STDIN>) { print "> $_" } exit;} </PRE></DIV><PCLASS="para">As with all process forks, doing this a zillion times has some cost, but it's fine for a couple of processes, or even a couple dozen. If the system was actually designed to be multitasking right from the start, as Unix was, this is far cheaper than you imagine. Virtual memory and copy-on-write makes this efficient. Forking is an elegant and inexpensive solution to many, if not most, multitasking needs.<ACLASS="indexterm"NAME="ch16-idx-1000006276-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006276-1"></A><ACLASS="indexterm"NAME="ch16-idx-1000006276-2"></A><ACLASS="indexterm"NAME="ch16-idx-1000006276-3"></A><ACLASS="indexterm"NAME="ch16-idx-1000006276-4"></A><ACLASS="indexterm"NAME="ch16-idx-1000006276-5"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-1200">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch03_102.htm"><CODECLASS="literal">open</CODE></A> function in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A> and in <ICLASS="filename">perlfunc </I>(1); <ACLASS="xref"HREF="ch16_05.htm"TITLE="Reading or Writing to Another Program">Recipe 16.4</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="ch16_05.htm"TITLE="16.4. Reading or Writing to Another Program"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 16.4. Reading or Writing to Another Program"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch16_07.htm"TITLE="16.6. Preprocessing Input"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 16.6. Preprocessing Input"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">16.4. Reading or Writing to Another Program</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">16.6. 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