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 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..
Update 2009-05-31: An anonymous contributor has written a version of the Daemon class suitable for Python 3.x.. The code below is for Python 2.x
#!/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: if os.path.exists(self.pidfile): 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..
#!/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!