Perl fork pid Not Correcto for Xterm Spawn

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

I am trying to spawn an xterm window that does a "tail -f" to a summary log file. However the pid is not the correct one! When my script tries to kill the window, the $pid does not has the right process Id. Can someone please help? I have attached my code! Thanks in advance PerlMonks!

#!/usr/bin/perl

my $fname = "/tmp/help.txt";
system("echo help me > $fname");

my $pid = fork();
if ($pid eq 0)
{
   sleep (1);
   system ("xterm -T hello -e tail -f $fname &");
   exit(-1);
}
print "This is not the correct pid: $pid \n";
sleep(5);
print "After some point in time, I want to kill $pid! \n");
system ("kill -9 $pid");
print "$pid is incorrect!\n";
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Re: Perl fork pid Not Correcto for Xterm Spawn by ikegami (Patriarch) on Jul 08, 2010 at 00:09 UTC
Parent creates a forked copy and sleeps for 5 seconds The forked copy sleeps for a second. The forked copy launches a shell. The shell executes `xterm` in the background and exits. The forked copy exits. The sleep ends You try to kill a process that exited almost 4 seconds earlier. You want the shell to stick around until the `xterm` exits. Do so by removing the "`&`". Since we don't need to keep the forked copy around since at all it does is exit (with a wrong code), use `exec` instead of `system`. `#!/usr/bin/perl my $fname = "/tmp/help.txt"; system("echo help me > $fname"); my $pid = fork(); if ($pid == 0) { exec("xterm -T hello -e tail -f $fname"); exit($! \|\| 255); } ...` [download] Even better: Avoid the shell and use a safer interface. `#!/usr/bin/perl ... my $pid = fork(); if ($pid == 0) { exec('xterm', '-T', 'hello', '-e', 'tail', '-f', $fname); exit($! \|\| 255); } ...` [download]	[reply] [d/l] [select]

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Re: Perl fork pid Not Correcto for Xterm Spawn
by ikegami (Patriarch) on Jul 08, 2010 at 00:09 UTC

Parent creates a forked copy and sleeps for 5 seconds
The forked copy sleeps for a second.
The forked copy launches a shell.
The shell executes xterm in the background and exits.
The forked copy exits.
The sleep ends
You try to kill a process that exited almost 4 seconds earlier.

You want the shell to stick around until the xterm exits. Do so by removing the "&".

Since we don't need to keep the forked copy around since at all it does is exit (with a wrong code), use exec instead of system.

#!/usr/bin/perl

my $fname = "/tmp/help.txt";
system("echo help me > $fname");

my $pid = fork();
if ($pid == 0)
{
   exec("xterm -T hello -e tail -f $fname");
   exit($! || 255);
}

...
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Even better: Avoid the shell and use a safer interface.

#!/usr/bin/perl

...

my $pid = fork();
if ($pid == 0)
{
   exec('xterm', '-T', 'hello', '-e', 'tail', '-f', $fname);
   exit($! || 255);
}

...
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