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App::Daemon - Start an Application as a Daemon


     # Program:
   use App::Daemon qw( daemonize );
   do_something_useful(); # your application

     # Then, in the shell: start application,
     # which returns immediately, but continues 
     # to run do_something_useful() in the background
   $ app start

     # stop application
   $ app stop

     # start app in foreground (for testing)
   $ app -X

     # show if app is currently running
   $ app status


App::Daemon helps running an application as a daemon. The idea is that you prepend your script with the

    use App::Daemon qw( daemonize ); 

and 'daemonize' it that way. That means, that if you write

    use App::Daemon qw( daemonize ); 


you'll get a script that, when called from the command line, returns immediatly, but continues to run as a daemon for 10 seconds.

Along with the common features offered by similar modules on CPAN, it

  • supports logging with Log4perl: In background mode, it logs to a logfile. In foreground mode, log messages go directly to the screen.

  • detects if another instance is already running and ends itself automatically in this case.

  • shows with the 'status' command if an instance is already running and which PID it has:

        ./my-app status
        Pid file:    /tmp/
        Pid in file: 14914
        Running:     no
        Name match:  0


App::Daemon recognizes three different actions:

my-app start

will start up the daemon. "start" itself is optional, as this is the default action,

        $ ./my-app

will also run the 'start' action. If the -X option is given, the program is run in foreground mode for testing purposes.


will find the daemon's PID in the pidfile and send it a kill signal. It won't verify if this actually shut down the daemon or if it's immune to the kill signal.


will print out diagnostics on what the status of the daemon is. Typically, the output look like this:

    Pid file:    /tmp/
    Pid in file: 15562
    Running:     yes
    Name match:  1
        /usr/local/bin/perl -w

This indicates that the pidfile says that the daemon has PID 15562 and that a process with this PID is actually running at this moment. Also, a name grep on the process name in the process table results in 1 match, according to the output above.

Note that the name match is unreliable, as it just looks for a command line that looks approximately like the script itself. So if the script is, it will match lines like "perl -w" or "perl start", but unfortunately also lines like "vi".

If the process is no longer running, the status output might look like this instead:

    Pid file:    /tmp/
    Pid in file: 14914
    Running:     no
    Name match:  0

Command Line Options


Foreground mode. Log messages go to the screen.

-l logfile

Logfile to send Log4perl messages to in background mode. Defaults to /tmp/[appname].log.

-u as_user

User to run as if started as root. Defaults to 'nobody'.

-l4p l4p.conf

Path to Log4perl configuration file. Note that in this case the -v option will be ignored.

-p pidfile

Where to save the pid of the started process. Defaults to /tmp/[appname].pid.


Increase default Log4perl verbosity from $INFO to $DEBUG. Note that this option will be ignored if Log4perl is initialized independently or if a user-provided Log4perl configuration file is used.

Setting Parameters

Instead of setting paramteters like the logfile, the pidfile etc. from the command line, you can directly manipulate App::Daemon's global variables:

    use App::Daemon qw(daemonize);

    $App::Daemon::logfile    = "mylog.log";
    $App::Daemon::pidfile    = "mypid.log";
    $App::Daemon::l4p_conf   = "myconf.l4p";
    $App::Daemon::background = 1;
    $App::Daemon::as_user    = "nobody";

    use Log::Log4perl qw(:levels);
    $App::Daemon::loglevel   = $DEBUG;


Application-specific command line options

If an application needs additional command line options, it can use whatever is not yet taken by App::Daemon, as described previously in the "Command Line Options" section.

However, it needs to make sure to remove these additional options before calling daemonize(), or App::Daemon will complain. To do this, create an options hash %opts and store application-specific options in there while removing them from @ARGV:

    my %opts = ();

    for my $opt (qw(-k -P -U)) {
        my $v = App::Daemon::find_option( $opt, 1 );
        $opts{ $opt } = $v if defined $v;

After this, options -k, -P, and -U will have disappeared from @ARGV and can be checked in $opts{k}, $opts{P}, and $opts{U}.


If the process is started as root but later drops permissions to a non-priviledged user for security purposes, it's important that logfiles are created with correct permissions.

If they're created as root when the program starts, the non-priviledged user won't be able to write to them later (unless they're world-writable which is also undesirable because of security concerns).

The best strategy to handle this case is to specify the non-priviledged user as the owner of the logfile in the Log4perl configuration:

    log4perl.logger = DEBUG, FileApp
    log4perl.appender.FileApp = Log::Log4perl::Appender::File
    log4perl.appender.FileApp.filename = /var/log/foo-app.log
    log4perl.appender.FileApp.owner    = nobody
    log4perl.appender.FileApp.layout   = PatternLayout
    log4perl.appender.FileApp.layout.ConversionPattern = %d %m%n

This way, the process starts up as root, creates the logfile if it doesn't exist yet, and changes its owner to 'nobody'. Later, when the process assumes the identity of the user 'nobody', it will continue to write to the logfile without permission problems.

Detach only

If you want to create a daemon without the fancy command line parsing and PID file checking functions, use

    use App::Daemon qw(detach);
    # ... some code here

This will fork a child, terminate the parent and detach the child from the terminal. Issued from the command line, the program above will continue to run the code following the detach() call but return to the shell prompt immediately.


Mike Schilli,


Copyright (C) 2008 by Mike Schilli

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.5 or, at your option, any later version of Perl 5 you may have available.