Trenin:

I just reviewed the docs for Proc::Daemon and don't see where the difficulty lies in running multiple instances. What's giving you the problem?

As you mention, the code in the link provided does want to use a single daemon, though it doesn't look like it would be all that difficult to modify. To do so, I'd suggest adding a PID command line argument to tell the daemons apart, and make sure the run function displays the pid of the new daemon. Finally, if the PID argument passed in was 0 or LIST or some such, it would just list all matching daemons.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Yeah I guess I just wasn't firing on all cylinders yesterday; I'm not sure why I couldn't get it to work. I've run into a slightly different issue: the daemon doesn't return input control to the user. So for example the behavior of the script I borrowed from is:

[user@linux ~] perl original.pl --start

Starting...

[user@linux ~]

But my version hangs, like

[user@linux ~] perl original.pl --start

Starting...

and I have to ctrl-z before I can enter anything else

Here's my modified example daemon:

#!/usr/bin/perl
use warnings;
use strict;
use Getopt::Long;
use Proc::Daemon;
use Cwd;
use File::Spec::Functions;

my $daemon = Proc::Daemon->new(
    work_dir => getcwd()    
);

my $pid;
my $daemonize = 1;

GetOptions(
    'daemon!'    => \$daemonize,
    "start"     => \&run,
    "status"     => \&status,
    "stop"         => \&stop
);

sub stop{
    if($pid){
        print "Stopping pid $pid... \n";
        if($daemon->Kill_Daemon($pid)){
            print "Successfully stopped. \n";
        }else{
            print "Could not find $pid. Was it running? \n";
        }
    }else{
        print "not Running, nothing to stop. \n";
    }
}

sub status{
    if($pid){
        print "Running with pid $pid. \n";
    }else{
        print "Not running. \n";
    }
}

sub run{
    if (!$pid){
        print "Starting...\n";
        if($daemonize){
            $pid = $daemon->Init;
    
        }
        while(1){
            open(my $FH, '>>', catfile(getcwd(), "log.txt"));
            print $FH "Logging from PID $pid at " . time() . "\n";
            close $FH;
            sleep 5;
        }
    }else{
        print "No active daemons. \n";
    }
}
[download]

I've narrowed the problem down to the assignment $pid = $daemon->Init;

But removing the assignment obviously means the pid isn't stored. I guess I could just go with command line arguments.

EDIT: OR I could write a temp pid file, write to and then read from the file, and then destroy it.

Trenin:

Are you certain that it's $daemon->Init? The reason I ask is that immediately after doing that you enter an infinite loop, so I wouldn't expect you to get control again until you either put the job in the background or terminate it.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

One idea might be to require the location of the PID file on the command line.

daemon.pl -p /var/run/task1.pid ...
daemon.pl -p /var/run/task2.pid ...
[download]

Thanks for the suggestion; I may go with this.

When it comes to writing daemons, I prefer not to have to think about daemons at all. I write a program that runs "in foreground" (whatever that means), it does not fork to "run in background", it logs by simply writing to STDERR, and it does not maintain a PID file. That reduces the amount of code needed.

How do I get my programs to run as daemons "in background"? Easy, I use djb's daemontools, or one of the various toolsets using the same concept, e.g. runsv from busybox.

Daemontools handle all of the daemonizing, i.e. running "in background", switching user IDs, configuration, reliable logging even across program restarts and crashes, starting and stopping, sending signals to the daemon, and so on. See also Re^3: Proc::PID::File problem generating pid files, or: does it matter where a pid file lives?, Re^3: How to capture system return code when command launched in the background?, Re: Executing script in normal/daemon mode and adding start/stop capability, Re^5: A few questions regarding Proc::Daemon, daemontools at "the djb way".

And by the way: you simply do not need any PID file with daemontools. Of course, you can run multiple instances of the same program with different options via daemontools.

Note that you also do not need any daemonizing code in your program when you use daemontools, not even Proc::Daemon. In fact, such code makes it harder to use daemontools.

So, how hard is it to write a daemon using daemontools? Less than 10 lines of simple shell code, including configuration and logging.

Alexander

Awesome, thanks for all of the links and information.