min_daemon.py

#! /usr/bin/env python3
""" Linux / *NIX daemon base code.
This script daemonizes its self with the start option and kills an already 
running daemon with the stop option. It's kept minimal and simple, but it works 
and can be extended/re-factored as base-code for a useful daemon.

Note: point of daemons is that they are quite and run in the background, add 
      some logging if you are developing a daemon, saves a lot of headaches.
"""

import os, sys, signal

PIDFILE = '/tmp/mydaemon.pid'

def daemonize(pidfile, workpath='/'):
        pid = os.fork() 
        if pid > 0:
                sys.exit(0) 

        os.chdir(workpath)
        os.setsid() 
        os.umask(0) 

        pid = os.fork() 
        if pid > 0:
                sys.exit(0) 

        sys.stdout.flush()
        sys.stderr.flush()
        si = open(os.devnull, 'r')
        so = open(os.devnull, 'a+')
        se = open(os.devnull, 'a+')
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())

        pid = str(os.getpid())
        with open(pidfile,'w+') as f:
                f.write(pid + '\n')

def cleanup_handler(signum, frame):
        os.remove(PIDFILE)

if __name__ == '__main__':
        if sys.argv[1] == 'start':
                if os.path.isfile(PIDFILE):
                        sys.stderr.write(
                            'pidfile found, daemon already running?\n')
                        sys.exit(2)
        
                signal.signal(signal.SIGTERM, cleanup_handler)
                daemonize(PIDFILE)
                # daemon program logic
                signal.pause()

        elif sys.argv[1] == 'stop':
                try:
                        with open(PIDFILE,'r') as pf:
                                pid = int(pf.read().strip())
                except:
                        raise Exception('no pid found, daemon not running?')
                os.kill(pid, signal.SIGTERM)