perlquestion
pjf
<p>Ahoy there monks,
</p><p>
I'm writing some material which is primarily aimed at system administrators who'd like to use Perl to make their life easier. Part of that discussion involves demonstrating how to use <tt>system()</tt> to call external programs, but unfortunately I've hit a snag.
</p><p>
Checking <tt>system()</tt> to see if anything has gone wrong is a pain. Our command may not actually get run, in which case <tt>$? == -1</tt>, or it may have been killed by a signal, in which case <tt>WIFSIGNALED($?)</tt> will be true. It may have run to completion, but the exit code may not have been something that we expect.
</p><p>
Handling the error status from system is always a pain, even in the most simpliest case:
</p>
<blockquote><tt>
system("some_command") and die "...";
</tt></blockquote>
<p>
Some less experienced programmers may see the <i>and die</i> and think it is a mistake, since for all other Perl built-ins one would use <i>or die</i>. Even the experienced programmers would be hard-pressed to say what should actually go in the die message. Surely <tt>$!</tt> should be reported if <tt>$? == -1</tt>, but otherwise one ends up with an awkward series of statements to unpack signals and exit status. And what if our requirements change later on, and it's okay if <tt>some_command</tt> also returns an exit status of <tt>1</tt>? Surely there must be a better way?
</p><p>
Unfortunately, I can't seem to find a Perl module that makes this <i>easy</i>. I'd like to be able to write code that reads:
</p>
<blockquote><tt>
use Some::Module qw(run);<br />
<br />
run("some_command");
</tt></blockquote>
<p>
and have <tt>run</tt> throw an appropriately verbose exception if the command doesn't start, gets zapped by a signal, or returns non-zero. If I later need to allow my command to return a couple of different status values (say 0, 1 and 5), then I can change my code to be:
</p>
<blockquote><tt>
my $exit_value = run( [0,1,5], "some_command");
</tt></blockquote>
<p>
and be <i>certain</i> that <tt>$exit_value</tt> will be either 0, 1 or 5, or an exception will be thrown.
</p><p>
Have I been missing this obviously simple module all this time, and if so, what is it called? (If not, then watch this space for a new module announcement.)
</p><p>
Many thanks,
</p><p>
<b>Update:</b> You can find my solution to the problem using <a href="http://search.cpan.org/perldoc?IPC::System::Simple">IPC::System::Simple</a>.
</p>
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<div class="pmsig"><div class="pmsig-116014">
<i>Paul Fenwick</i><br>
<a href="http://perltraining.com.au/">Perl Training Australia</a>
</div></div>