I have used the following code

my $pid = fork;
unless($pid){
    exec('script 2>output.log');
}
print "Child Process Id:$pid\n";
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In the above example got the process id of the child process but not the process which is executed the command

How can I take the process id of the process which is running the actual application?

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
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(3882 is the PGRP in question here)

Consider the following example. There I used setpgrp to make the process group and process group id as forked process

check.pl

print "process id: $$\n";
my $pid = fork;
my $FH;
unless($pid){
    setpgrp;
    exec('script');
}
print "Child process id is: $pid\n";
while(1) {sleep(5)}
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script file:

use POSIX;

while(1) {
    sleep(1);
    print "I am running\n";
    my $pid = fork;
    unless($pid)
    {
       while(1) {  sleep(2);
                   print "I am child and I'am running\n";
       }
    }
}
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In the script file once again fork the process and printing the line

From another terminal using kill -9 <Negative number of the child process id> command kill all the process belongs to this process group

The child process is running as a defunct process but the parent process is alive

Refer the ps command output
643 15141 9607 0 18:14 pts/1 00:00:00 perl check.pl
643 15142 15141 0 18:14 pts/1 00:00:00 script <defunct>


How could I avoid this problem

Also I need to redirect the script command output and error into one file . How can I achieve it?

You need to avoid the intermediate shell that gets invoked because of 2>.... See exec. Replacing the redirection functionality provided by the shell involves reopening STDOUT and STDERR to the appropriate files and then executing the target program directly.

use IPC::Open3 qw( open3 );

open(local our $TO_CHLD, '<', '/dev/null') or die;
open(local our $FR_CHLD, '>', 'output.log') or die;

my $pid = open3('<&TO_CHLD', '>&FR_CHLD', '>&FR_CHLD', 'script');

waitpid($pid, 0) or die;
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