Bring the current Python program to background

Question

A python script prints information to screen and then should go to background.

How to do that from within the script itself?

Answer 1

Copying related code from Creating a daemon the Python way; please read the comments as to why that code is quite thought-out.

def createDaemon():
   """Detach a process from the controlling terminal and run it in the
   background as a daemon.
   """

   try:
      # Fork a child process so the parent can exit.  This returns control to
      # the command-line or shell.  It also guarantees that the child will not
      # be a process group leader, since the child receives a new process ID
      # and inherits the parent's process group ID.  This step is required
      # to insure that the next call to os.setsid is successful.
      pid = os.fork()
   except OSError, e:
      raise Exception, "%s [%d]" % (e.strerror, e.errno)

   if (pid == 0):   # The first child.
      # To become the session leader of this new session and the process group
      # leader of the new process group, we call os.setsid().  The process is
      # also guaranteed not to have a controlling terminal.
      os.setsid()

      # Is ignoring SIGHUP necessary?
      #
      # It's often suggested that the SIGHUP signal should be ignored before
      # the second fork to avoid premature termination of the process.  The
      # reason is that when the first child terminates, all processes, e.g.
      # the second child, in the orphaned group will be sent a SIGHUP.
      #
      # "However, as part of the session management system, there are exactly
      # two cases where SIGHUP is sent on the death of a process:
      #
      #   1) When the process that dies is the session leader of a session that
      #      is attached to a terminal device, SIGHUP is sent to all processes
      #      in the foreground process group of that terminal device.
      #   2) When the death of a process causes a process group to become
      #      orphaned, and one or more processes in the orphaned group are
      #      stopped, then SIGHUP and SIGCONT are sent to all members of the
      #      orphaned group." [2]
      #
      # The first case can be ignored since the child is guaranteed not to have
      # a controlling terminal.  The second case isn't so easy to dismiss.
      # The process group is orphaned when the first child terminates and
      # POSIX.1 requires that every STOPPED process in an orphaned process
      # group be sent a SIGHUP signal followed by a SIGCONT signal.  Since the
      # second child is not STOPPED though, we can safely forego ignoring the
      # SIGHUP signal.  In any case, there are no ill-effects if it is ignored.
      #
      # import signal           # Set handlers for asynchronous events.
      # signal.signal(signal.SIGHUP, signal.SIG_IGN)

      try:
         # Fork a second child and exit immediately to prevent zombies.  This
         # causes the second child process to be orphaned, making the init
         # process responsible for its cleanup.  And, since the first child is
         # a session leader without a controlling terminal, it's possible for
         # it to acquire one by opening a terminal in the future (System V-
         # based systems).  This second fork guarantees that the child is no
         # longer a session leader, preventing the daemon from ever acquiring
         # a controlling terminal.
         pid = os.fork()    # Fork a second child.
      except OSError, e:
         raise Exception, "%s [%d]" % (e.strerror, e.errno)

      if (pid == 0):    # The second child.
         # Since the current working directory may be a mounted filesystem, we
         # avoid the issue of not being able to unmount the filesystem at
         # shutdown time by changing it to the root directory.
         os.chdir(WORKDIR)
         # We probably don't want the file mode creation mask inherited from
         # the parent, so we give the child complete control over permissions.
         os.umask(UMASK)
      else:
         # exit() or _exit()?  See below.
         os._exit(0)    # Exit parent (the first child) of the second child.
   else:
      # exit() or _exit()?
      # _exit is like exit(), but it doesn't call any functions registered
      # with atexit (and on_exit) or any registered signal handlers.  It also
      # closes any open file descriptors.  Using exit() may cause all stdio
      # streams to be flushed twice and any temporary files may be unexpectedly
      # removed.  It's therefore recommended that child branches of a fork()
      # and the parent branch(es) of a daemon use _exit().
      os._exit(0)   # Exit parent of the first child.

Answer 2

Update: To properly daemonize your process, use daemonize.

Original answer: Since the shell is waiting for your process to finish, the main process must terminate. You can use os.fork() to fork off a child process and sys.exit() from the parent process:

import os
import sys

if os.fork():
    sys.exit()
# Child code goes here