In general, you can't, unless you somehow have the actual application communicate its PID.  Problem is that the child process may fork any number of subprocesses, so the immediate child is not necessarily the application you're interested in.

But if the idea is to kill a hanging application, you could maybe create a new process group, and then kill that entire process group.

Update: 

#!/usr/bin/perl

if (my $pid = fork()) {

    # kill process group after timeout
    sleep 3;
    show_procs();
    kill -9, $pid;  # negative signal number kills group
    wait;
    print "killed children\n";
    show_procs();   # show remaining processes

    # ...
    
} elsif (defined $pid) {
    
    setpgrp;  # create new process group
    
    # run something (consisting of several processes) that takes a whi
+le
    exec 'find / | grep foo';

} else {
    die "couldn't fork";
}

sub show_procs {
    system "ps Tf -o pid,ppid,pgrp,sid,cmd";
}

__END__
$ ./841709.pl
  PID  PPID  PGRP   SID CMD
12046 12044 12046 12046 bash -rcfile .bashrc
 3881 12046  3881 12046  \_ /usr/bin/perl ./841709.pl
 3882  3881  3882 12046      \_ sh -c find / | grep foo
 3883  3882  3882 12046      |   \_ find /
 3884  3882  3882 12046      |   \_ grep foo
 3885  3881  3881 12046      \_ ps Tf -o pid,ppid,pgrp,sid,cmd
killed children
  PID  PPID  PGRP   SID CMD
12046 12044 12046 12046 bash -rcfile .bashrc
 3881 12046  3881 12046  \_ /usr/bin/perl ./841709.pl
 3886  3881  3881 12046      \_ ps Tf -o pid,ppid,pgrp,sid,cmd

[download]

(3882 is the PGRP in question here)

In reply to Re^5: Obtain the child process id in perl by almut
in thread Obtain the child process id in perl by Anonymous Monk

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