System Commands

[azheid]

I have a perl script that calls another set of perl scripts with the system() command.

What I want to know is the behavior of perl when a perl script is killed with ctrl-c. Specifically, if a system() command has already been called, but the system() command process is still running, what happens when the original script is killed?

Will the called process continue to execute, or will it die with the killed script?

[stevieb]

What happens when you try it? It's pretty trivial to test...

Calling script that runs another script with system() (call.pl):

use warnings;
use strict;

system 'perl', 'call.pl';

Script that gets called by the one above that we'll ^C (called.pl):

use warnings;
use strict;

my $c = 1;

while (1){
    print $c++ ."\n";
    sleep 1;
}

Run call.pl to exec called.pl:

$ perl call.pl 
1
2
3
4
5

In another terminal window, see if called.pl is running:

$ ps ax | grep called
29221 pts/9    S+     0:00 perl called.pl

Yep, it is. Back to the other terminal:

6
7
^C

Check if called.pl is still running after ^C:

$ ps ax | grep called

Nope!

The whole process chain is killed when you kill the top process, unless you explicitly set the process called by system to go into the background, whether it be a daemon, service etc.

[choroba]

Without knowing more about the details, we can't really tell. For example, pressing C-c when running the first script terminates only the inner Perl loop

perl -wE 'say 1; system $^X, "-wE", q(sleep 1, say "\t$_" for 1 .. 10)
+; say 2;'

but the second script terminates immediately, while the system command still runs in the background and can't be stopped by C-c :

perl -wE 'say 1; system q(perl -wE "sleep 1, say qq(\\t\$_) for 1 .. 1
+0" &); say 2;'

[perlfan]

It depends. If your system command involves a nohup is daemonized somehow, your forked sub-process will continue.

Do yourself a favor and convert the Perl scripts you're calling via system to Modulinos.

You will retain the functionality of your original scripts, but they will become a callable library that requires no forking.

[Anonymous Monk]

I don't know if there's a good overview of unix process control somewhere, but for starters, take a look at NOTES section in setpgid. The gory details people rarely delve into.

That said, normally your script and its descendants (or else a piped job maybe) will all run in the same process group. They are all interrupted or suspended together.