Re^2: wait syntax for background process to complete

Hi, I am running my process in background using system command in perl. I only have one child process and I have tried using wait and waitpid but still my script exits but I can see the process running in background. Also, I found that when running system command with only one parameter, Perl will spawn a command shell to do the work. Can this be the reason that my script exits? Here's my code:

system(" some command >&file &");
wait;
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I would appreciate any help. Thanks.

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Re^3: wait syntax for background process to complete by jettero (Monsignor) on Jun 24, 2008 at 19:13 UTC
You are causing your external process to background and detach from your process group. You can't really wait for it to end like that in a reliable way... Instead, do this: `if( my $pid = fork ) { print "I am the parent pid ($$), and I'm going to wait for $pid to + exit.\n"; my $kid; do { $kid = waitpid $pid, 0; } while $kid > 0; exit 0; } else { exec(qw(ls -al /tmp)); die "hrm, exec() failed: $!"; }` [download] Using a '&' in the system() call causes perl to fork a shell and asks the shell to fork a kid. Then the shell returns and perl completely looses track of the kids. UPDATE: Also, I just noticed that you probably need some kind of redirection: `# exec(qw(ls -al /tmp)); die ... blah open my $out, ">", "filename" or die $!; open my $in, "-\|", qw(ls -al /tmp) or die $!; print $out $_ while <$in>; close $out; close $in;` [download] -Paul	[reply] [d/l] [select]

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Re^3: wait syntax for background process to complete
by jettero (Monsignor) on Jun 24, 2008 at 19:13 UTC

and detach

You can't really wait for it to end like that in a reliable way...

Instead, do this:

if( my $pid = fork ) {
    print "I am the parent pid ($$), and I'm going to wait for $pid to
+ exit.\n";
    my $kid;
    do {

     $kid = waitpid $pid, 0;

    } while $kid > 0;
    exit 0;

} else {
    exec(qw(ls -al /tmp)); die "hrm, exec() failed: $!";
}
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Using a '&' in the system() call causes perl to fork a shell and asks the shell to fork a kid. Then the shell returns and perl completely looses track of the kids.

UPDATE: Also, I just noticed that you probably need some kind of redirection:

    # exec(qw(ls -al /tmp)); die ... blah
    open my $out, ">", "filename" or die $!;
    open my $in, "-|", qw(ls -al /tmp) or die $!;
    print $out $_ while <$in>;
    close $out;
    close $in;
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-Paul

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