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<h1 class="chapter">Chapter 14. Process Management</h1>
<div class="htmltoc"><h4 class="tochead">Contents:</h4>
<p> <a href="#lperl3-CHP-14-SECT-1">The system Function</a><br />
<a href="ch14_02.htm">The exec Function</a><br />
<a href="ch14_03.htm">The Environment Variables</a><br />
<a href="ch14_04.htm">Using Backquotes to Capture Output</a><br />
<a href="ch14_05.htm">Processes as Filehandles</a><br />
<a href="ch14_06.htm">Getting Down and Dirty with Fork</a><br />
<a href="ch14_07.htm">Sending and Receiving Signals</a><br />
<a href="ch14_08.htm">Exercises</a><br /></p></div>
<p><a name="INDEX-952" /></a> <a name="INDEX-953" /></a> <a name="INDEX-954" /></a>One
of the best parts of being a programmer is launching someone
else's code so that you don't have to write it yourself.
It's time to learn how to manage your children<a href="#FOOTNOTE-304">[304]</a> by launching other
programs directly from Perl.
</p><blockquote class="footnote">
<a name="FOOTNOTE-304" /></a><p>[304]Child processes, that is.</p> </blockquote>
<p>And like everything else in Perl, There's More Than One Way To
Do It, with lots of overlap, variations, and special features. So if
you don't like the first way, just read on for another page or
two for a solution more to your liking.
</p>
<p>Perl is very portable; most of the rest of this book doesn't
need many notes saying that it works this way on Unix systems and
that way on Windows and the other way on VMS. But when you're
starting other programs on your machine, different programs are
available on a Macintosh than you'll likely find on a Cray. The
examples in this chapter are primarily Unix-based; if you have a
non-Unix system, you can expect to see some differences.
</p>
<div class="sect1"><a name="lperl3-CHP-14-SECT-1" /></a>
<h2 class="sect1">14.1. The system Function</h2>
<p><a name="INDEX-955" /></a>The simplest way to launch a child
process in Perl to run a program is the <tt class="literal">system</tt>
function. For example, to invoke the Unix
<i class="command">date</i><a name="INDEX-956" /></a> command from within Perl, it looks like:
</p>
<blockquote><pre class="code">system "date";</pre></blockquote>
<p>The child process runs the <i class="command">date</i> command, which
inherits Perl's standard input, standard output, and standard
error. This mean that the normal short date-and-time string generated
by <i class="command">date</i> ends up wherever Perl's
<tt class="literal">STDOUT</tt> was already going.
</p>
<p>The parameter to the system function is generally whatever
you'd normally type at the shell. So if it were a more
complicated command, like "<em class="emphasis">ls -l $HOME</em>
", we'd just have put all that into the
parameter:
</p>
<blockquote><pre class="code">system 'ls -l $HOME';</pre></blockquote>
<p>Note that we had to switch here from double quotes to single quotes,
since <tt class="literal">$HOME</tt> is the shell's variable.
Otherwise, the shell would never have seen the dollar sign, since
that's also an indicator for Perl to interpolate.
Alternatively, we could write:
</p>
<blockquote><pre class="code">system "ls -l \$HOME";</pre></blockquote>
<p>But that can get quickly unwieldly.</p>
<p>Now, the <i class="command">date</i> command is output-only, but
let's say it had been a chatty command, asking first "for
which time zone do you want the time?"<a href="#FOOTNOTE-305">[305]</a> That'll end up on standard
output, and then the program will listen on standard input (inherited
from Perl's <tt class="literal">STDIN</tt>) for the response.
You'll see the question, and type in the answer (like
"Zimbabwe time"), and then <i class="command">date</i> will
finish its duty.
</p><blockquote class="footnote"> <a name="FOOTNOTE-305" /></a><p>[305]As far
as we know, no one has made a <i class="command">date</i> command that
works like this.</p> </blockquote>
<p>While the child process is running, Perl is patiently waiting for it
to finish. So if the <i class="command">date</i> command took 37 seconds,
then Perl is paused for those 37 seconds. You can use the
shell's facility to launch a background process,<a href="#FOOTNOTE-306">[306]</a> however:
</p><blockquote class="footnote">
<a name="FOOTNOTE-306" /></a><p>[306]See what we mean about this depending upon your system? The
Unix shell (<i class="command">/bin/sh</i>) lets you use the ampersand on
this kind of command to make a background process. If your non-Unix
system doesn't support this way to launch a background process,
then you can't do it this way, that's all.</p>
</blockquote>
<blockquote><pre class="code">system "long_running_command with parameters &";</pre></blockquote>
<p>Here, the shell gets launched, which then notices the ampersand at
the end of the command line, causing the
<i class="command">long_running_command</i> to be made into a background
process. And then the shell exits rather quickly, which Perl notices
and moves on. In this case, the
<i class="command">long_running_command</i> is really a grandchild of the
Perl process, to which Perl really has no direct access or knowledge.
</p>
<p>When the command is "simple enough," no shell gets
involved, so for the <i class="command">date</i> and
<i class="command">ls</i> commands earlier, the requested command is
launched directly by Perl, which searches the inherited
<tt class="literal">PATH<a href="#FOOTNOTE-307">[307]</a></tt> to find the command, if necessary. But if
there's anything weird in the string (such as shell
metacharacters like the dollar sign, semicolon, or vertical bar),
then the standard Bourne Shell (<em class="emphasis">/bin/sh<a href="#FOOTNOTE-308">[308]</a></em>) gets invoked to work through the complicated
stuff. In that case, the shell is the child process, and the
requested commands are grandchildren (or further offspring). For
example, you can write an entire little shell script in the argument:
</p><blockquote class="footnote"> <a name="FOOTNOTE-307" /></a><p>[307]The <tt class="literal">PATH</tt> can be
changed by adjusting <tt class="literal">$ENV{'PATH'}</tt> at any time.
Initially, this is the environment variable inherited from the parent
process (usually the shell). Changing this value affects new child
processes, but cannot affect any preceding parent processes. The
<tt class="literal">PATH</tt> is the list of directories where executable
programs (commands) are found, even on some non-Unix systems.</p>
</blockquote><blockquote class="footnote"> <a name="FOOTNOTE-308" /></a><p>[308]Or
whatever was determined when Perl was built. Practically always, this
is just <em class="emphasis">/bin/sh</em> on Unix-like systems.</p>
</blockquote>
<blockquote><pre class="code">system 'for in *; do echo == $i ==; cat $i; done';</pre></blockquote>
<p>Here again, we're using single quotes, because the dollar signs
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