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<HTML><HEAD><TITLE>Recipe 16.3. Replacing the Current Program with a Different One (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="1999-07-02T01:43:40Z"><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_03.htm"TITLE="16.2. Running Another Program"><LINKREL="next"HREF="ch16_05.htm"TITLE="16.4. Reading or Writing to Another Program"></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_03.htm"TITLE="16.2. Running Another Program"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 16.2. Running 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_05.htm"TITLE="16.4. Reading or Writing to Another Program"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 16.4. Reading or Writing to Another Program"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch16-95997">16.3. Replacing the Current Program with a <ACLASS="indexterm"NAME="ch16-idx-1000006257-0"></A><ACLASS="indexterm"NAME="ch16-idx-1000006257-1"></A>Different One</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-942">Problem</A></H3><PCLASS="para">You want to replace the running program with another, as when checking parameters and setting up the initial environment before running another program.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-948">Solution</A></H3><PCLASS="para">Use the built-in <CODECLASS="literal">exec</CODE><ACLASS="indexterm"NAME="ch16-idx-1000006258-0"></A> function. If <CODECLASS="literal">exec</CODE> is called with a single argument containing metacharacters, the shell will be used to run the program:</P><PRECLASS="programlisting">exec("archive *.data") or die "Couldn't replace myself with archive: $!\n";</PRE><PCLASS="para">If you pass <CODECLASS="literal">exec</CODE> more than one argument, the shell will not be used:</P><PRECLASS="programlisting">exec("archive", "accounting.data") or die "Couldn't replace myself with archive: $!\n";</PRE><PCLASS="para">If called with a single argument containing no shell metacharacters, the argument will be split on whitespace and then interpreted as though the resulting list had been passed to <CODECLASS="literal">exec</CODE>:</P><PRECLASS="programlisting">exec("archive accounting.data") or die "Couldn't replace myself with archive: $!\n";</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-970">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">exec</CODE> function in Perl is a direct interface to the <ICLASS="filename">execlp </I>(2) system call, which replaces the current program with another, leaving the process intact. The program that calls <CODECLASS="literal">exec</CODE> gets wiped clean, and its place in the operating system's process table is taken by the program specified in the arguments to <CODECLASS="literal">exec</CODE>. As a result, the new program has the same process ID (<CODECLASS="literal">$$</CODE>) as the original program. If the specified program couldn't be run, <CODECLASS="literal">exec</CODE> returns a false value and the original program continues. Be sure to check for this.</P><PCLASS="para">If you <CODECLASS="literal">exec</CODE> yourself into a different program, neither your END blocks nor any object destructors will be automatically run as they would if your process actually exited.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch16-pgfId-978">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch03_033.htm"><CODECLASS="literal">exec</CODE></A> 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); your system's <EMCLASS="emphasis">execlp </EM>(2) manpage (if you have it); <ACLASS="xref"HREF="ch16_03.htm"TITLE="Running Another Program">Recipe 16.2</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_03.htm"TITLE="16.2. 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