perl run background job and force job to finish?

Question

I've got a Perl script that starts a background job to print some logging information into a file, executes something, and then finishes.

I would like to end the background process when the script is done, but as far as I know, if I use a double fork as below, then waitpid($pid,0) will wait for the background process to finish, instead of killing it, and kill(9,$pid) won't get rid of the background process.

Example script:

#!/usr/bin/perl
use strict;

# Do a first general top
my $ret = `top -b -n 1 > logfile`;

# Do a background $USER top every minute
my $cmd = "top -b -d 60 -n 300 -u $ENV{USER} >> logfile";
my $pid;
unless ($pid = fork) {
  unless (fork) {       #second_fork
    exec "$cmd";
    die "exec failed!";
  }                     #second_fork
  exit 0;
}

# Script does something else for 1-2 hours
# [...]
# We are finished now, let's finish the logging

# waitpid($pid,0) would wait until top command is finished
force_background_job_to_finish($pid) if (background_job_still_running($pid));

# Add final top to the end
$ret = `top -b -n 1 >> logfile`;

1;

Any ideas how to force_background_job_to_finish?

Answer 1

This works for me:

$SIG{CHLD}="IGNORE";
my $cmd = "top -b -d 60 -n 300 -u $ENV{USER} >> logfile";
if (my $logger_pid=fork) {
    # parent
    do_something_for_1to2_hours();
    kill 15, $logger_pid;
} else {
    # child (logger)
    exec($cmd);
}

Answer 2

Usually double fork is used so that you don't have to worry about reaping child processes. Since you want to control the child process, you can just do a single fork, which will give you the process id which you can then use to kill your child process using kill.