From https://zhuanlan.zhihu.com/p/21250530?refer=auxten

daemon_demo.py

#!/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='nbMon.pid', stdin='/dev/null', stdout='nbMon.log', stderr='nbMon.log'):
    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().
    """