Lone Wolves

I've written a simple Python class for creating daemons on unix/linux systems. It was pieced together for various other examples, mostly corrections to various Python Cookbook articles and a couple of examples posted to the Python mailing lists. It has support for a pidfile to keep track of the process. I hope it's useful to someone.

Below is the Daemon class. To use it, simply subclass it and implement the run() method. Download this file.

#!/usr/bin/env python

import sys, os, time, atexit
from signal import SIGTERM 

class Daemon:
	"""
	A generic daemon class.
	
	Usage: subclass the Daemon class and override the run() method
	"""
	def __init__(self, pidfile, stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
		self.stdin = stdin
		self.stdout = stdout
		self.stderr = stderr
		self.pidfile = pidfile
	
	def daemonize(self):
		"""
		do the UNIX double-fork magic, see Stevens' "Advanced 
		Programming in the UNIX Environment" for details (ISBN 0201563177)
		http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
		"""
		try: 
			pid = os.fork() 
			if pid > 0:
				# exit first parent
				sys.exit(0) 
		except OSError, e: 
			sys.stderr.write("fork #1 failed: %d (%s)\n" % (e.errno, e.strerror))
			sys.exit(1)
	
		# decouple from parent environment
		os.chdir("/") 
		os.setsid() 
		os.umask(0) 
	
		# do second fork
		try: 
			pid = os.fork() 
			if pid > 0:
				# exit from second parent
				sys.exit(0) 
		except OSError, e: 
			sys.stderr.write("fork #2 failed: %d (%s)\n" % (e.errno, e.strerror))
			sys.exit(1) 
	
		# redirect standard file descriptors
		sys.stdout.flush()
		sys.stderr.flush()
		si = file(self.stdin, 'r')
		so = file(self.stdout, 'a+')
		se = file(self.stderr, 'a+', 0)
		os.dup2(si.fileno(), sys.stdin.fileno())
		os.dup2(so.fileno(), sys.stdout.fileno())
		os.dup2(se.fileno(), sys.stderr.fileno())
	
		# write pidfile
		atexit.register(self.delpid)
		pid = str(os.getpid())
		file(self.pidfile,'w+').write("%s\n" % pid)
	
	def delpid(self):
		os.remove(self.pidfile)

	def start(self):
		"""
		Start the daemon
		"""
		# Check for a pidfile to see if the daemon already runs
		try:
			pf = file(self.pidfile,'r')
			pid = int(pf.read().strip())
			pf.close()
		except IOError:
			pid = None
	
		if pid:
			message = "pidfile %s already exist. Daemon already running?\n"
			sys.stderr.write(message % self.pidfile)
			sys.exit(1)
		
		# Start the daemon
		self.daemonize()
		self.run()

	def stop(self):
		"""
		Stop the daemon
		"""
		# Get the pid from the pidfile
		try:
			pf = file(self.pidfile,'r')
			pid = int(pf.read().strip())
			pf.close()
		except IOError:
			pid = None
	
		if not pid:
			message = "pidfile %s does not exist. Daemon not running?\n"
			sys.stderr.write(message % self.pidfile)
			return # not an error in a restart

		# Try killing the daemon process	
		try:
			while 1:
				os.kill(pid, SIGTERM)
				time.sleep(0.1)
		except OSError, err:
			err = str(err)
			if err.find("No such process") > 0:
				os.remove(self.pidfile)
			else:
				print str(err)
				sys.exit(1)

	def restart(self):
		"""
		Restart the daemon
		"""
		self.stop()
		self.start()

	def run(self):
		"""
		You should override this method when you subclass Daemon. It will be called after the process has been
		daemonized by start() or restart().
		"""

And here is an example implementation. It implements the daemon as well as it's controlling client. Simply invoke this script with start, stop or restart as it's first argument. Download this file.

#!/usr/bin/env python

import sys, time
from daemon import Daemon

class MyDaemon(Daemon):
	def run(self):
		while True:
			time.sleep(1)

if __name__ == "__main__":
	daemon = MyDaemon('/tmp/daemon-example.pid')
	if len(sys.argv) == 2:
		if 'start' == sys.argv[1]:
			daemon.start()
		elif 'stop' == sys.argv[1]:
			daemon.stop()
		elif 'restart' == sys.argv[1]:
			daemon.restart()
		else:
			print "Unknown command"
			sys.exit(2)
		sys.exit(0)
	else:
		print "usage: %s start|stop|restart" % sys.argv[0]
		sys.exit(2)

That's it! I hope this is of some use to someone. Happy coding!