Daemonize Python

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This short module shows the correct way to create a UNIX daemon process in Python. Note that there are modules for this available.

  • check for existing PID lock file to determine if daemon is already running. This is a convention. This falls into the application level. This is not a UNIX daemon process requirement.
  • fork/exec
  • create independent session and group and detach from parent (setsid, between fork and exec)
  • detach from controlling tty
  • clear signal handlers.
  • create PID lock file to indicate new running daemon.
  • set uid and gid.
  • drop suid and sgid privileges.
  • close inherited file descriptors (including stdin, stdout, stderr).
  • open file descriptors for stdin, stdout, and stderr as necessary.
  • set umask (027).
  • set environment for running inside a chroot.
  • change working directory (relative to chroot if applicable).
  • register exit function (at least delete the daemon PID lock file).

Note that the following example does not handle all of the above requirements! The missing bits are left as an exercise.

#!/usr/bin/env python
import sys, os 

'''This module is used to fork the current process into a daemon.

Almost none of this is necessary (or advisable) if your daemon is being started
by inetd. In that case, stdin, stdout and stderr are all set up for you to
refer to the network connection, and the fork()s and session manipulation
should not be done (to avoid confusing inetd). Only the chdir() and umask()
steps remain useful. 

References:

    UNIX Programming FAQ
        1.7 How do I get my program to act like a daemon?
        http://www.unixguide.net/unix/programming/1.7.shtml
        http://www.faqs.org/faqs/unix-faq/programmer/faq/
   
    Advanced Programming in the Unix Environment
        W. Richard Stevens, 1992, Addison-Wesley, ISBN 0-201-56317-7.

$Id: daemonize.py 251 2008-05-05 21:51:11Z noah $
'''

def daemonize (stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):

    '''This forks the current process into a daemon. The stdin, stdout, and
    stderr arguments are file names that will be opened and be used to replace
    the standard file descriptors in sys.stdin, sys.stdout, and sys.stderr.
    These arguments are optional and default to /dev/null. Note that stderr is
    opened unbuffered, so if it shares a file with stdout then interleaved
    output may not appear in the order that you expect. '''

    # Do first fork.
    try: 
        pid = os.fork() 
        if pid > 0:
            sys.exit(0)   # Exit first parent.
    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.umask(0) 
    os.setsid() 

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

    # Now I am a daemon!
    
    # Redirect standard file descriptors.
    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())

def main ():

    '''This is an example main function run by the daemon.
    This prints a count and timestamp once per second.
    '''

    import time
    sys.stdout.write ('Daemon started with pid %d\n' % os.getpid() )
    sys.stdout.write ('Daemon stdout output\n')
    sys.stderr.write ('Daemon stderr output\n')
    c = 0
    while 1:
        sys.stdout.write ('%d: %s\n' % (c, time.ctime(time.time())) )
        sys.stdout.flush()
        c = c + 1
        time.sleep(1)

if __name__ == "__main__":
    daemonize('/dev/null','/tmp/daemon.log','/tmp/daemon.log')
    main()

Click here to download: daemonize.py <include src="/home/noahspurrier/noah.org/engineering/src/python/daemonize.py" highlight="python" />