How to get PID of perl daemon in init script? - linux

Question

I have the following perl script:
#!/usr/bin/perl
use strict;
use warnings;
use Proc::Daemon;
Proc::Daemon::Init;
my $continue = 1;
$SIG{TERM} = sub { $continue = 0 };
while ($continue) {
# stuff
}
I have the following in my init script:
DAEMON='/path/to/perl/script.pl'
start() {
PID=`$DAEMON > /dev/null 2>&1 & echo $!`
echo $PID > /var/run/mem-monitor.pid
}
The problem is, this returns the wrong PID! This returns the PID of the parent process which is started when the daemon is run, but that process is immediately killed off. I need to get the PID of the child process!

Answer 1

The Proc::Daemon says
Proc::Daemon does the following:
...
9. The first child transfers the PID of the second child (daemon) to the parent. Additionally the PID of the daemon process can be written into a file if 'pid_file' is defined. Then the first child exits.
and then later, under new ( %ARGS )
pid_file
Defines the path to a file (owned by the parent user) where the PID of the daemon process will be stored. Defaults to undef (= write no file).
Also look at Init() method description. This all implies that you may want to use new first.
The point is that it is the grand-child process that is the daemon. However, the childr passes the pid along and it is available to the parent. If pid_file => $file_name is set in the constructor (the daemon's) pid is written to that file.
A comment asks to not have shell script rely on a file written by another script.
I can see two ways to do that.
Print the pid, returned by the $daemon->Init(), from the parent and pick it up in the shell. This is defeated by redirects in the question, but I don't know why they are needed. The parent and child exit right as all is set up, while the daemon is detached from everything.
Shell script can start the Perl script with the desired log-file name as an argument, letting it write the daemon pid to that file by the above process. The file is still output by Perl, but what matters about it is decided by the shell script.
I'd like to include a statement from my comment below. I consider these superior to two other things that come to mind: picking the filename from a config-style file kept by the shell is more complicated, while parsing the process table may be unreliable.

Answer 2

I've seen this before and had to resort to using STDERR to send back the childs PID to the calling shell script. I've always assumed it was due to the mentioned unreliability of exit codes - but details were not clear in the documentation. Please try something like this:
#!/usr/bin/perl
use strict;
use warnings;
use Proc::Daemon;
if( my $pid = Proc::Daemon::Init() ) {
print STDERR $pid;
exit;
}
my $continue = 1;
$SIG{TERM} = sub { $continue = 0 };
while ($continue) {
sleep(20);
exit;
}
With a calling script like this:
#!/bin/bash
DAEMON='./script.pl'
start() {
PID=$($DAEMON 2>&1 >/dev/null)
echo $PID > ./mem-monitor.pid
}
start;
When the bash script is ran, it will capture the STDERR output (containing the correct PID), and store it in the file. Any STDOUT the Perl script produces would be sent to /dev/null - though this is unlikely as the 1st level Perl script does (in this case) exit fairly early on.

Answer 3

Thank you to the suggestions from zdim and Hakon. They are certainly workable, and got me on the right track, but ultimately I went a different route. Rather than relying on $!, I used ps and awk to get the PID, as follows:
DAEMON='/path/to/perl/script.pl'
start() {
$DAEMON > /dev/null 2>&1
PID=`ps aux | grep -v 'grep' | grep "$DAEMON" | awk '{print $2}'`
echo $PID > /var/run/mem-monitor.pid
}
This works and satisfies my OCD! Note the double quotes around "$DAEMON" in grep "$DAEMON".