Run a Perl script in the background and send it commands

Question

I want to make a script that will prevent requests to certain domains for a certain amount of time, and kill specific processes during the same amount of time.

I would like to have something like a daemon, to which I could send commands. For example, to see how much time is left, with some_script timeleft, to start the daemon which would be created by something like some_script start, or to add a new domain/process, etc.

What I'm stuck on is :

• How to create a daemon? I have seen this

• I don't know how to send a command to a daemon from a command line

I hope I have been clear enough in my explanation.

Answer 1

I would probably use the bones of the answer you refer to, but add:

• a handler for SIGHUP which re-reads the config file of IPs to suppress, and,

• a handler for SIGUSR1 which reports how much time there is remaining.

So, it would look like this roughly:

#!/usr/bin/perl

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

Proc::Daemon::Init;

my $continue = 1;
################################################################################
# Exit on SIGTERM
################################################################################
$SIG{TERM} = sub { $continue = 0 };

################################################################################
# Re-read config file on SIGHUP
################################################################################
$SIG{HUP} = sub {
   # Re-read some config file - probably using same sub that we used at startup
   open(my $fh, '>', '/tmp/status.txt');
   print $fh "Re-read config file\n";
   close $fh;
};

################################################################################
# Report remaining time on SIGUSR1
################################################################################
$SIG{USR1} = sub {
   # Subtract something from something and report difference
   open(my $fh, '>', '/tmp/status.txt');
   print $fh "Time remaining = 42\n";
   close $fh;
};

################################################################################
# Main loop
################################################################################
while ($continue) {
     sleep 1;
}

You would then send the HUP signal or USR1 signal with:

pkill -HUP daemon.pl

or

pkill -USR1 daemon.pl 

and look in /tmp/status.txt for the output from the daemon. The above commands assume you stored the Perl script as daemon.pl - adjust if you used a different name.

Or you could have the daemon write its own pid in a file on startup and use the -F option to pkill.

Answer 2

There are a few ways to communicate with a daemon, but I would think UNIX domain sockets would make the most sense. In perl, IO::Socket::UNIX would be a thing to look at.