Question

I’m currently trying to manually create a simple daemon process, I don’t want to use the existing externals libraries to avoid overhead.

I’m currently checking when my process runs that it doesn’t have a PID file already created (meaning it’s running), like described in this post.

I also have a daemonizing module to detach the PID from current process and redirect stdout and stderr (so my daemon will keep running even if I end my session):

import os
import sys

def daemonize(stdin="/dev/null", stdout="/dev/null", stderr="/dev/null"):
    try: 
        pid = os.fork() 
        if pid > 0:
            sys.exit(0)
    except OSError, e: 
        sys.stderr.write ("fork #1 failed: (%d) %s\n" % (e.errno, e.strerror) )
        sys.exit(1)

    os.chdir("/") 
    os.umask(0) 
    os.setsid() 

    try: 
        pid = os.fork() 
        if pid > 0:
            sys.exit(0)
    except OSError, e: 
        sys.stderr.write ("fork #2 failed: (%d) %s\n" % (e.errno, e.strerror) )
        sys.exit(1)

    stdin_par = os.path.dirname(stdin)
    stdout_par = os.path.dirname(stdout)
    stderr_par = os.path.dirname(stderr)
    if not stdin_par:
        os.path.makedirs(stdin_par)
    if not stdout_par:
        os.path.makedirs(stdout_par)
    if not stderr_par:
        os.path.makedirs(stderr_par)

    si = open(stdin, 'r')
    so = open(stdout, 'a+')
    se = open(stderr, 'a+', 0)
    os.dup2(si.fileno(), sys.stdin.fileno())
    os.dup2(so.fileno(), sys.stdout.fileno())
    os.dup2(se.fileno(), sys.stderr.fileno())

So currently I can run my process like sample line below and it will run my daemon correctly:

>$ python myapp.py

But to stop it, I have to grep the PID (or take it from the lock file) and manually erase the PID after:

>$ ps -ef | grep myapp
xxxxx  11901     1  0 19:48 ?        00:00:00 python src/myapp.py
xxxxx  12282  7600  0 19:54 pts/7    00:00:00 grep myapp
>$ kill -9 11901
>$ rm -rf /path/to/lock.pid

I’d like to have a more Unix-like daemon where I can manage the daemon lifecycle with the following commands:

>$ python myapp.py start
>$ python myapp.py stop
>$ python myapp.py restart

I can certainly do it with the argparse module, but that seems a bit tedious and ugly.

Do you know a simple and elegant solution to have a Unix-style daemon process in Python?

Answer 1

After looking a bit more I found a great example that does just this here.

It uses a generic Daemon class, which can be subclassed afterwards:

#!/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().
                """

Once you have this module, you can do the following to have all different modes of your daemon:

#!/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)

Hope that helps other people who get stuck on the same problem !