Script::Daemonizer - Daemonize; handle signals, logs, pidfile and locks.


Version 1.01.01

Interface changed from procedural to OO across 0.92-0.93 transition.


    # Want a quickstart? Do this:

    use Script::Daemonizer;


This module turns your script into a UNIX daemon by requiring as little modification as possible, thus letting you concentrate on solving your problem, rather than on writing a daemon.

This module also provides some facilities to manage logging, restarting (on SIGHUP by default) and preventing multiple instances to run at once (by locking a pidfile).

Default and optional steps can be customized by passing arguments to new() (see "ADVANCED USAGE" for details).

daemonize() is the main routine of this module. What it does, out-of-the-box, is:

1.* it sets umask() to 0. You must then set explicitly file and directory permissions upon creating them, restore umask() after initialization, or specify umask option (see "ADVANCED USAGE" for details).
2.* it calls fork(), then the parent exits;
3.* it calls POSIX::setsid() (see POSIX::setsid()), so the process becomes session leader;
4.* it calls fork() again, then the parent exits;
5.* it changes its working directory to "/";
6.* NO LONGER IMPLEMENTED (since 0.93.00) - closing file descriptors. It's practically impossible, so this step has been removed.
7.* it ties STDOUT and STDERR to Syslog using Tie::Syslog (if available, otherwise it reopens them on /dev/null) so that all output is logged to syslog (see Tie::Syslog); open STDIN on /dev/null.

Steps marked by * are configurable; some additional steps are also available if explicitly requested; see "ADVANCED USAGE" for details.


All UNIX and UNIX-like operating systems (or so should be). Please open a BUG if you find it's not so.


I strive to make this module support "standard" daemon features out-of-the-box (for some definition of "standard"). Some of these features can be configured, and some other are enabled only if configured.


Advanced configuration syntax is the following:

    use Script::Daemonizer;

    my $daemon = new Script::Daemonizer (
        name            => "My wonderful new daemon",  # tag for logging
        umask           => $my_umask,                  # set umask to $my_umask
        working_dir     => "/var/ftp",                 # try to chdir here
        drop_privileges => {                           # call drop_privileges()
            uid  => $to_uid,
            gid  => $to_gid,
            # and others, see drop_privileges() method description
        setsid          => 'SKIP',                     # skip POSIX::setsid() call
        fork            => 2,                          # for # number of times (0, 1 or 2)
        pidfile         => '/var/run/',    # write and lock this pidfile
        output_file     => '/var/log/mydaemon.log',    # redirect stdout/stderr here
        restart_on      => [ 'HUP', 'QUIT' ],          # restart on these signals

    # To make stdout/stderr go to different places, use these:
    my $daemon = new Script::Daemonizer (
        stdout_file     => '/log/mydaemon.log',
        stderr_file     => '/log/mydaemon.err',

    # if you did not specify drop_privileges in configuration, you can still
    # do it afterwards:
        uid  => $to_uid,
        gid  => $to_gid,



    # Will skip chdir(), unless you pass 'working_dir' to new()
    use Script::Daemonizer qw(:NOCHDIR);

    # Will skip umask(), unless you pass 'umask' to new()
    use Script::Daemonizer qw(:NOUMASK);


Some options have no default and thus corresponding actions are skipped if not configured. These are:

  • Step 0.0 - privileges dropping

    It happens before anything else. This simply calls drop_privileges() internally.

  • Step 4.5 - pidfile creation (and locking)

    This implicitly provides a method to ensure that only one copy of your daemon is running at once, because pidfile is locked.

  • Handling of SIGHUP (and maybe other signals) as a restart() trigger

    See restart_on parameter.


Starting from 0.93.4, you can suppress some default actions by importing Script::Daemonizer with the corresponding tag:

:NOCHDIR - will skip the chdir to '/'
:NOUMASK - will skip setting umask to 0



Creates a new instance of a Script::Daemonizer object. If you just want to start with defaults, just call it with no args:

    # A new daemon
    my $daemon = Script::Daemonizer->new();

To customize your daemon see "ADVANCED USAGE".


It runs through all the steps required to send your program to background as a daemon. Its behavior varies depending on options specified to "new".


    # Just drop effective user/group id:
        euid => $to_euid,
        egid => $to_egid,
        # or by name
        euser  => $to_euser,
        egroup => $to_egroup,

    # Drop both effective and real ids:
        uid  => $to_uid,
        gid  => $to_gid,
        # or by name
        user  => $to_user,
        group => $to_group,

Tries to drop privileges to given EUID/EGID or UID/GID (single (e)uid/(e)gid allowed). Since 1.00 also names are allowed. See "perldoc perlvar" for details on IDs.

daemonize() will automatically call drop_privileges() if configured to do so (guess what? See "ADVANCED USAGE" for details) but this will happen before anything else (think of this as step 0). If you need to drop privileges at a later moment, do not pass the drop_privileges parameter to new() and use the drop_privileges() method, otherwise it's probably safer to do so while daemonize()-ing.


restart() is there to let you restart completely the daemon. A simple way to handle SIGHUP might be restarting, for example (see perlipc for details).

    # Restart upon sighup - use a closure to call restart()
    $SIG{HUP} = sub {

Please NOTE that with the restart_on parameter this setup is done automatically.

pidfile (see "ADVANCED USAGE" for details) is kept open upon restart (if configured), but on some platforms (see "Concerning locks" for details) the lock is not preserved, so a race condition may still happen.

You can pass command line args to restart() to modify command line on-the-fly:

    use Script::Daemonizer;
    unless( some_condition_on(@ARGV) ) {
        # Do some mangling on @ARGV

The defaul is to use a copy of @ARGV taken at compile-time (before any command-line-parsing modifies @ARGV, for example).


    $daemon->sigunmask( @signals );

Strictly related to restart(), sigunmask() is there to let you unmask signals without pain. See for details on why you should unmask signals. In short: inside a signal handler the signal that triggered the handler is blocked. If inside a signal handler you re-exec() yourself, the new process inherits the blocked signal. That is why you'll want to unmask that signal after a new start.

SGIHUP is unmasked by default, just by saying use Script::Daemonizer.

If you use restart() with other signals, remember to unmask them:

    # Restart on SIGHUP, SIGQUIT and SIGUSR1
    for my $nal in (qw/HUP QUIT USR1/) {
        $SIG{$nal} = sub {

    # Just in case we came from another instance of ourselves via an exec()
    # (no need to bother for SIGHUP since we unmask it by default, anyway
    # listing it here is harmless):

    $daemon->sigunmask(qw{QUIT USR1});

or use restart_on to handle this automatically>>.


These are all parameter you can pass to new(). Advanced options are the following:


Sets the name of the daemon. This is used for logging.

default: script name, got from $0, split on system path separator;


How many times do you want to fork?

-) 0 - perform no fork()
-) 1 - do it just once
-) 2 - fork twice
-) * any other value will result in a fork()-twice

default: fork() twice.


Redirect both STDOUT and STDERR on the file specified (appending by default). '/dev/null' will be converted to File::Spec->devnull (see File::Spec); Tie::Syslog will be skipped completely. E.g.:

    output_file => '/dev/null',


This will try to:

- open named pidfile, creating it if non-existent;
- lock it (exclusively);

If this operation fails, daemonize() will croak(). Otherwise, PID of the process will be written to the named pidfile after the last fork() is done.

        name    => 'A new daemon',
        pidfile => '/var/run/',

This lock mechanism provides the following advantages:

  • no other instances of the same daemon will overwrite the pidfile on purpose;

  • the pid file will serve as a lock file, ensuring no other instances of the same daemon will start;

  • an fuser (see fuser(1)) on the pidfile will reveal the daemon's pid. If the daemon is not running, the pidfile will not be in use by any process (hopefully). A simple:

        $ fuser -k $PIDFILE

    will end up killing the daemon (or provides an easy way to signal the daemon).


This is a ArrayRef of signal names (without the SIG- prefix) that will trigger the restart() method automatically. Signal masking is handled, too, so you'll want to use this to handle restarts. E.g.:

    restart_on => [ 'HUP', 'QUIT' ],    # trigger restart() on SIGHUP and SIGQUIT


The only accepted value is SKIP. Set this to skip setsid().


Redirect STDOUT on the file specified (appending by default). '/dev/null' will be converted to File::Spec->devnull (see File::Spec); Tie::Syslog will not be used for STDOUT.


Redirect STDERR on the file specified (appending by default). '/dev/null' will be converted to File::Spec->devnull (see File::Spec); Tie::Syslog will not be used for STDERR.


Set the specified umask.

default: 0 Setting umask can be suppressed using the :NOUMASK import tag.


Try to chdir() to the specified directory.

default: root (/) dir. chdir() can be suppressed using the :NOCHDIR import tag.


If you want to be sure no multiple instances of your daemon will be running, just use pidfile advanced option. See "pidfile" for details.


Concerning filehandles

Filehandles cannot really be closed. Following advice from Matt S. Trout I will skip filehandle-closing step completely.

Concerning locks

On some platforms (by now only Solaris is proven to be affected, but other platforms may be) the lock on the pidfile is not preserved across fork().

Starting with v1.01.00 the pidfile is created and locked after the last fork(), this should solve the various issues with different platforms.


Q: Why is there no built-in start/stop/* method?

A: Because to start/stop/* a daemon you usually don't call the daemon itself, instead you rely on system tools that allow process control (if you need an "init script" then write one) and interact with processes through signals. Lack of start/stop/* methods is regarded as a feature. Tyr Daemon::Control if you're looking for such an interface (it cal also generate init scripts).

Q: Why is Tie::Syslog not listed as prerequisites?

A: Because you may want to skip Syslog support and not install that module. Script::Daemonizer module will work nonetheless, without using Tie::Syslog them (and without the features it provides, of course).


Some ideas:

  • Let user set her own options for tied STDOUT/STDERR (i.e. logopt for syslog (see Sys::Syslog), facility, priority).

  • Provide a function to automatically parse command line (via Getopt::Long).

  • Provide a function to automatically handle configuration file (via Config::General).


Giacomo Montagner, <kromg at>, <kromg.kromg at> >


Please report any bugs or feature requests to bug-script-daemonizer at, or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


You can find documentation for this module with the perldoc command.

    perldoc Script::Daemonizer

You can also look for information at:


  • "Advanced Programming in the UNIX Environment: Second Edition", by W. Richard Stevens, Stephen A. Rago, Copyright 2005 Addison Wesley Professional

  • Thanks to Matt S. Trout for his help in "uncrappyfying" the code. It's still quite crappy but maybe better than before :)


Copyright (C) 2012-2014 Giacomo Montagner, all rights reserved.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See for more information.


Latest sources are available from

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