The 2004 Perl Advent Calendar
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On the 9th day of Advent my True Language brought to me..

Sometimes we want to run a script forever. We want it to monitor what's going on, or perform some action periodically, and we don't want it to go away when we log out.

Proc::Daemon is a handy little utility class that can handle all the complicated operating system related tasks that are involved in making the script completely fork into the background. With one simple command your script will can be detached from the terminal login session you're using. Along with Proc::PID::File you can manage background tasks with the smallest amount of effort.

One of the more common ways of tackling spam is to is maintaining a whitelist of addresses of people you trust that you're willing to always accept mail from (or at the very least, trust more than other addresses) and blacklists of people we don't ever want to hear from again.

The hard thing about maintaining the whitelist and blacklist is providing a mechanism for updating the lists. The solution I came up with was a special mailbox on my mailserver that if I put a message in it would be added to the correct list and then moved back into my inbox or spam folder. This means I'm able to update my whitelist/blacklists using any of the mail clients I normally use (Apple Mail, pine and Squirrel Mail) over IMAP without any problem.

The script that runs on the server that updates the blacklist is fairly simple. I keep my mail in maildir format meaning that any mail I've put for blacklisting will be in the .blacklist folder. Let's start by writing the part of the script that finds that file:

  # turn on my message
  use strict;
  use warnings;
  # load my collection of modules
  use File::Copy;
  use File::Find::Rule;
  use Email::Simple;
  use Email::Address;
  # find all messages in the blacklist folder
  my @files = (
  # stop here unless we found any mails
  exit unless @files;

We then need to open the blacklist file for appending - we're hopefully going to add some addresses to the end.

  # open the file we're updating
  open my $list_fh,">>", "/home/mark/.my_blacklist"
   or die "Can't open the blacklist: $!";

And then start a loop that works though each of the files we found and loads them one by one into memory, works out what the from address was, and prints it out to $list_fh.

  # read in each of the messages
  foreach my $file (@files)
    print "Looking at '$file'\n";
    # read in the file
    open my $in_fh,"<", $file
      or next;  # can't read it?  Skip it
    my $message_text = join '', <$in_fh>;
    close $in_fh;
    # make it an email simple object
    my $email = Email::Simple->new($message_text);
    # get the email addresses and store them in the blacklist
      my $header = $email->header("From")
        or die "No From";
      my @address = Email::Address->parse($header)
        or die "No addresses parsed";
      foreach my $address (@address)
        my $from = $address->address
          or next;
        print "...Found address '$from'\n";
        print $list_fh "$from\n";
      # move the mail to my spambox
      move($file, "/home/mark/Maildir/.spam.blacklisted/cur");
    };  # ignore all errors

Running The Script Forever

Making this script run forever is pretty simple...we just need to wrap the whole code in a while loop with a sleep command so that it waits five seconds between runs and doesn't overload the server:

  # run forever
  while (1)
    { of code from the script...

Note the use of eval so that if we get die from any errors then the whole run isn't aborted - we just wait five seconds and try again. Along these lines we also need to change the script to not exit if there are new mails and rather go back to waiting again. Changing:

  # stop here unless we found any mails
  exit unless @files;


  # stop here unless we found any mails
  die "not this time" unless @files;

causes it to skip to the end of the loop and start the next sleep.

Running The Script In The Background

If we run the script from the shell then this runs forever

  bash$ perl & least until we log out and close the shell.

  bash$ exit
  [1]+  Done                    perl
  Connection to closed.

At which point the script is killed because it's parent is killed. What we need to do is reparent the process to a higher process (normally init) and free up all other resources it's using (like closing STDIN, STDOUT and STDERR and moving to another directory) This is actually quite complicated to get right. Luckily, Proc::Daemon does all this for us. We simply just have to insert at the top of the script the code:

  # make this run in the background.
  use Proc::Daemon;

And magically it'll all fork into the background for us. And that's all there is to it! Told you this was easy.

Controlling The Forked Process

It's be nice if we could control the forked process, to tell it to stop running when we want for example. This is actually quite complicated. Let's walk though what needs to be done.

Firstly, we need to keep track of if the process is running or not, and what process it's running as. To do this we use a PID file, a file that is stored somewhere on disk that has simply the process id of the process that's running. To do this we use Proc::PID::File. This module has one routine that we want to use:

  use Proc::PID::File;
  my $pid = Proc::PID::File->running(name => "foo");

The running routine returns the pid contained in the file if one existed (i.e. another process is already running) or returns undef if one wasn't and writes to the file with the current pid of the process we're currently using. Crucially, when the current process exits it deletes the pid file.

We can start to adapt the script like so:

  # turn on the safety features
  use Proc::Daemon;
  use Proc::PID::File;
  # did we get a stop command?
  if (@ARGV && $ARGV[0] eq "stop")
    # we need to send a signal to the running process to tell it
    # to quit
    # get the pid file (in /var/run by default)
    my $pid = Proc::PID::File->running(name => "blacklist");
    unless ($pid)
     { die "Not already running!" }
    # and send a signal to that process
    kill(2,$pid);  # you may need a different signal for your system
    print "Stop signal sent!\n";
  # fork into the background
  # do this first because our process id will change
  # write the pid file, exiting if there's one there already.
  # this pid file will automatically be deleted when this script
  # exits.
  if (Proc::PID::File->running(name => "blacklist"))
   { die "Already running!" }

The second problem that remains is that we're killing the process straight out without giving it a chance to do anything else. This isn't a good idea - it might be in the middle of something important like processing a mail and removing it might cause corruption. What we need to do is add a signal handler which catches the kill signal and logs that it's been received and then when it's safe to exit it'll check the flag and exit then . At the top of the file we add:

  # when we get a INT signal, set the exit flag
  $SIG{INT} = sub { $::exit = 1 }

Then in our while loop we write:

  while (1)
    exit if $::exit;
    exit if $::exit;

  • Proc::PID::File
  • Email::Simple
  • Email::Address
  • File::Copy