How can I run a Perl script as a system daemon in linux?

What's a simple way to get a Perl script to run as a daemon in linux?

Currently, this is on CentOS. I'd want it to start up with the system and shutdown with the system, so some /etc/rc.d/init.d integration would also be nice, but I could always add a custom line to /etc/rc.d/rc.local.

The easiest way is to use Proc::Daemon.

#!/usr/bin/perl

use strict;
use warnings;
use Proc::Daemon;

Proc::Daemon::Init;

my $continue = 1;
$SIG{TERM} = sub { $continue = 0 };

while ($continue) {
     #do stuff
}

Alternately you could do all of the things Proc::Daemon does:

1. Fork a child and exits the parent process.
2. Become a session leader (which detaches the program from the controlling terminal).
3. Fork another child process and exit first child. This prevents the potential of acquiring a controlling terminal.
4. Change the current working directory to "/".
5. Clear the file creation mask.
6. Close all open file descriptors.

Integrating with the runlevel system is easy. You need a script like the following (replace XXXXXXXXXXXX with the Perl script's name, YYYYYYYYYYYYYYYYYYY with a description of what it does, and /path/to with path to the Perl script) in /etc/init.d. Since you are using CentOS, once you have the script in /etc/init.d, you can just use chkconfig to turn it off or on in the various runlevels.

#!/bin/bash
#
# XXXXXXXXXXXX This starts and stops XXXXXXXXXXXX
#
# chkconfig: 2345 12 88
# description: XXXXXXXXXXXX is YYYYYYYYYYYYYYYYYYY
# processname: XXXXXXXXXXXX
# pidfile: /var/run/XXXXXXXXXXXX.pid
### BEGIN INIT INFO
# Provides: $XXXXXXXXXXXX
### END INIT INFO

# Source function library.
. /etc/init.d/functions

binary="/path/to/XXXXXXXXXXXX"

[ -x $binary ] || exit 0

RETVAL=0

start() {
	echo -n "Starting XXXXXXXXXXXX: "
	daemon $binary
	RETVAL=$?
	PID=$!
	echo
	[ $RETVAL -eq 0 ] && touch /var/lock/subsys/XXXXXXXXXXXX

	echo $PID > /var/run/XXXXXXXXXXXX.pid
}

stop() {
	echo -n "Shutting down XXXXXXXXXXXX: "
	killproc XXXXXXXXXXXX
	RETVAL=$?
	echo
	if [ $RETVAL -eq 0 ]; then
		rm -f /var/lock/subsys/XXXXXXXXXXXX
		rm -f /var/run/XXXXXXXXXXXX.pid
	fi
}

restart() {
	echo -n "Restarting XXXXXXXXXXXX: "
	stop
	sleep 2
	start
}

case "$1" in
	start)
		start
	;;
	stop)
		stop
	;;
	status)
		status XXXXXXXXXXXX
	;;
	restart)
		restart
	;;
	*)
		echo "Usage: $0 {start|stop|status|restart}"
	;;
esac

exit 0

If you don't have [Proc::Daemon][1] as suggested by Chas. Owens, here's how you'd do it by hand:

sub daemonize {
   use POSIX;
   POSIX::setsid or die "setsid: $!";
   my $pid = fork() // die $!; #//
   exit(0) if $pid;

   chdir "/";
   umask 0;
   for (0 .. (POSIX::sysconf (&POSIX::_SC_OPEN_MAX) || 1024))
      { POSIX::close $_ }
   open (STDIN, "</dev/null");
   open (STDOUT, ">/dev/null");
   open (STDERR, ">&STDOUT");
 }

[1]: http://search.cpan.org/dist/Proc-Daemon/Daemon.pm "Proc::Daemon"

I think the easiest way is to use daemon. It allows you to run any process as a daemon. This means you don't have to worry about libraries if you, for example, decided to change to python. To use it, just use:

daemon myscript args

This should be available on most distros, but it might not be installed by default.

I used supervisor for running a perl script.

As a system administrator, I like to minimise changes and variations among server and like to stick to core services or bare minimum.

Supervisor was already installed and available for a python-flask application running on the same box. So, I just added a conf file for the perl script I wanted to run as a service. Now, I can do

supervisorctl start/stop/restart my_perl_script_supervisor_service_name