SIG and system

=sjs= has asked for the wisdom of the Perl Monks concerning the following question:

Hi All

I'm having problems understanding how SIG works on UNIX I guess.

I have a script that looks something like:

$SIG{INT} = sub {$Abort++};

while (1) {
    if($Abort) exit(0);
    system(do something);
}
[download]

As far as I can tell, my signal handeler is never called, so I assume the forked process under the system call is getting the signal -- it does exit. How do I get this passed up to the parent?

Thanks
Steve S.

Edited by Chady -- added code tags, minor formatting.

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Re: SIG and system by Roy Johnson (Monsignor) on Dec 06, 2004 at 18:49 UTC
According to `perldoc -f system`, `Because "system" and backticks block "SIGINT" and "SIGQUIT", killing the program they're running doesn't actually interrupt your program.` [download] However, I find that I get different results using backticks compared to system: use strict; use warnings; my $Abort=0; $SIG{INT} = sub { ++$Abort }; for (1..10) { print "Iteration $_: Abort = $Abort\n"; `sleep 5`; } [download] $Abort will increment if I use backticks, not if I use system. Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re: SIG and system by bluto (Curate) on Dec 06, 2004 at 18:53 UTC
The easiest way to do this in this particular case is not to catch a signal, but to check the lower eight bits of the exit code from system. It will usually indicate the signal number the child received. I say "usually" since if you are actually launching a shell, esp. with pipelineing, they don't always behave nicely (i.e. make sure you test).	[reply]
Re: SIG and system by =sjs= (Initiate) on Dec 06, 2004 at 21:36 UTC
Thanks all for the suggestions and thanks for formatting previous message. This was exactly what I needed. Steve S.	[reply]