Your usage of "&" in the command is not only unnecessary (the fork already runs "in the background"), it also is the primary reason your code doesn't work.  Otherwise, your approach to create a new process group is perfectly fine, in case you need or want to kill an entire tree of child processes.

Compare the following (I replaced tcpdump with a simple sleep, which is irrelevant to the discussion):

#!/usr/bin/perl -w
use strict;
sub ps { system "ps Tf -o pid,ppid,pgrp,sid,cmd"; }
$SIG{CHLD} = 'IGNORE';
my $snoop = fork();
if ( $snoop == 0 ){
    setpgrp;
    system("sleep 20 &");
    exit;
}
ps();
sleep 1;

print  "PID:  $snoop\n";
printf "PGRP: %d\n", getpgrp($snoop);
kill -1, getpgrp($snoop);
ps();

__END__
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19529  2360 19529  2360  \_ /usr/bin/perl -w ./964597.pl
19531 19529 19529  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
19533     1 19530  2360 sleep 20
PID:  19530
PGRP: -1
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19529  2360 19529  2360  \_ /usr/bin/perl -w ./964597.pl
19534 19529 19529  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
19533     1 19530  2360 sleep 20
[download]

#!/usr/bin/perl -w
use strict;
sub ps { system "ps Tf -o pid,ppid,pgrp,sid,cmd"; }
$SIG{CHLD} = 'IGNORE';
my $snoop = fork();
if ( $snoop == 0 ){
    setpgrp;
    system("sleep 20 ;");
    exit;
}
ps();
sleep 1;

print  "PID:  $snoop\n";
printf "PGRP: %d\n", getpgrp($snoop);
kill -1, getpgrp($snoop);
ps();

__END__
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19537  2360 19537  2360  \_ /usr/bin/perl -w ./964597.pl
19538 19537 19538  2360      \_ /usr/bin/perl -w ./964597.pl
19540 19538 19538  2360      |   \_ sh -c sleep 20 ;
19541 19540 19538  2360      |       \_ sleep 20
19539 19537 19537  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
PID:  19538
PGRP: 19538
  PID  PPID  PGRP   SID CMD
 2360  2358  2360  2360 bash -rcfile .bashrc
19537  2360 19537  2360  \_ /usr/bin/perl -w ./964597.pl
19542 19537 19537  2360      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
[download]

As you can see in the first example, the child process (sleep) dissociates, i.e. the original process (your $snoop PID) is no longer alive, so getpgrp($snoop) fails, and you effectively kill nothing...

In the second example, however, without using "&"¹, things work as intended. I.e., you have created a new process group (19538 here), which you are then killing successfully.

That said, unless you actually are running multiple child processes, there is no need to use the process group technique. As long as you can make sure that tcpdump is the only and immediate child process (for example using exec, as noted by halfcountplus (but without your "&" !)), killing that process directly would work without a problem.

BTW, to check whether a process is running, you can send it the "0" pseudo signal, e.g.

     printf "child%s alive\n", kill(0, $snoop) ? "":" not";
[download]

___

¹note that I'm using a ";" here in place of the "&", because the example is also meant to show that using an extra process group to kill multiple processes in one go, is working just fine.  Without a shell meta character in the command (the semicolon in this case), Perl would optimize away the extra shell, so there would only be a single child process, which kind of defeats the purpose of using a process group...

You're sample of forks without & is very close to how I solved my problem, based on your original reply. I used Proc::ProcessTable to get the pid associated with snoop.

The following is a mock-up of pstree.

sshd,30982
  --bash,30983
      --su,31002
          --bash,31003
              --my.pl,6893 ./my.pl debug
                  --sh,6934 -c ...
                       --tcpdump,6935
                  --my.pl,6933 ./my.pl debug
[download]

I achieved these results by using open, as in "open, $snoop, $snoop_cmd". Then, while $snoop is running, I start a counter, and test whether the actual snoop pid is running.

while ( $counter < 31 ) {
        my $snoop_running = 1;
        for my $p ( @{ $ps->table } ) {
            if (   ( $p->pid == $snoop_pid )
                && ( $p->ppid == getpgrp($fork_pid) )
                && ( ( $p->cmndline ) =~ '$snoop_cmd' ) )
            {
                $snoop_running = 0;
                print "packet capture still running (", $p->pid, ") ",
                  $p->cmndline, "\n"
                  if (( $debug eq 0 ) && (( $p->cmndline ) =~ '$snoop_
+cmd' ));
            }
        }
        if ( $snoop_running == 1 ) {

            #snoop must have finished already
            for my $p ( @{ $ps->table } ) {
                kill 9, ( $p->pid ) if ( $p->ppid == getpgrp($fork_pid
+) );
            }
            last;
        }
$counter++;
sleep 1;
[download]

I used this timer method, because at certain check points; 5, 10, and 20 seconds; I wanted to execute additional system commands, but I did not want to execute any if my snoop had already completed. Next, I kill the fork once packet capture is done. That level of fine pid control is admittedly overly fine, and not for everyone.

I don't think I would have figured this out, without your initial reply. Thank you very much!

$snoop in the parent is the child pid. Unfortunately, system() performs a further fork, (and I think tcpdump would be still another child of that) -- and killing the parent doesn't kill the child.

HOWEVER, if you use exec() instead of system(), there are no additional children spawned, so you can stop the process by killing the fork. Just beware, there should not be any calls after exec(), because it doesn't return.

my $pid = fork();

if (!$pid) {
    exec("tcpdump ...");
    # no more code here!!!
}

sleep(11);
kill 9, $pid;
[download]

Should work. qv. exec

I did something similar a while ago at Timing and timing out Unix commands -- this node includes a complete example Unix program that does something very similar to what you describe.