example for double fork

daemon_fork_twice.py

# c.f.  https://stackoverflow.com/a/5386753

import os
import sys

print('pre-fork: pid=%d, pgid=%d, sid=%d'  % (os.getpid(), os.getpgid(0), os.getsid(0)))

pid = os.fork()
if pid > 0:
        sys.exit(0)

print('1st_fork: pid=%d, pgid=%d, sid=%d'  % (os.getpid(), os.getpgid(0), os.getsid(0)))

os.setsid()
print('setsid  : pid=%d, pgid=%d, sid=%d'  % (os.getpid(), os.getpgid(0), os.getsid(0)))

pid = os.fork()
if pid > 0:
        sys.exit(0)

print('2nd_fork: pid=%d, pgid=%d, sid=%d'  % (os.getpid(), os.getpgid(0), os.getsid(0)))