The system command and waiting for the child process to finish

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

man perlfunc tells me (about the system command):

"Does exactly the same thing as "exec LIST", except that a fork is done first, and the parent process waits for the child process to complete."

I recently wrote a perl program:

#!/usr/bin/perl -w
use strict;

[set up some stuff]

foreach my $val (@vals)
    {

    [set up some more stuff]

    system "/usr/bin/mplayer -arg1 -arg2 $file";
    }
[download]

I expected these to get executed sequentially. Instead, the program started, let's just say "a lot," of these simultaneously, leading to a significant disaster.

I eventually got around this by slamming a sleep statement in the loop, but I want to know what I missed, or basically why it didn't behave as I expected (allowing mplayer to finish executing before continuing).

Comment on The system command and waiting for the child process to finish Download Code

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Re: The system command and waiting for the child process to finish by betterworld (Curate) on Sep 29, 2008 at 11:38 UTC
If `$file` contains "&" or other funny characters, you should use the several-arguments way of calling system. Otherwise "&" will instruct the shell to start mplayer in the background, which would explain your problem. `system('mplayer', '-arg1', '-arg2', $file) and die "could not run mpla +yer";` [download] I prefer this form unless I really need the shell's functionality.	[reply] [d/l] [select]
Re: The system command and waiting for the child process to finish by Corion (Patriarch) on Sep 29, 2008 at 10:41 UTC
Most likely it's `mplayer` itself that doesn't wait. If you take a small test program like `perl -e 'sleep 5; echo $$'` [download] then likely the whole setup works.	[reply] [d/l] [select]
Re: The system command and waiting for the child process to finish by JavaFan (Canon) on Sep 29, 2008 at 11:04 UTC
It could happen if mplayer detaches itself from the parent process. But the version of mplayer I have doesn't do so, and a brief skim through its manual page doesn't show an option. Furthermore, I can't reproduce your problem; when I do call mplayer from a Perl program using system, the program waits for mplayer to finish before continuing. You don't have a & in your real code, do you?	[reply]
Re: The system command and waiting for the child process to finish by Fletch (Bishop) on Sep 29, 2008 at 15:22 UTC
Ampersands and accidental backgrounding aside, it's also possible that system encountered a problem but since you have no error checking on the call it's blithely continuing on to the next iteration immediately with no indication a failure occured. `$ perl -le 'for (0..3) { system( "Idonotexist $_" ) }' $ perl -le 'for (0..3) { system( "Idonotexist $_" ) == 0 or warn "syst +em problem (exit $?): $!\n" }' system problem (exit -1): No such file or directory system problem (exit -1): No such file or directory system problem (exit -1): No such file or directory system problem (exit -1): No such file or directory $` [download] Update: Not to say that's what happened here since as you say you did see a lot of processes running; just that in general it pays to take the extra 15 seconds and put the error handling in place. The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l]