open independent process python

Question

I'm running into a problem using the subprocess module to open completely independent processes through Python. I need the process spawned by the python script to live on even if the python program dies. I've noticed that subprocess creates a child process and nohup is not keeping it alive. It needs to work both on Windows and Linux, so I'm not sure if os.fork will work.

The structure I am aiming for is a main program running at all times that calls other programs, and if the main program dies I want the spawned programs to live on.

Main program

import subprocess
import time

subprocess.Popen(["/usr/bin/python", "/home/jchoinski/Desktop/Sentinel1.1/programTest.py"], stdin=subprocess.PIPE, stdout=PIPE, stderr=PIPE)
while True:
   time.sleep(1)

Spawned Program

import time

l = 1
while l < 100000:
    l += 1
    print(l)
    time.sleep(1)

The process tree looks like this:

MainProgram
|_CalledProgram

and I want it to look like:

MainProgram
CalledProgram

Any ideas? Should I even use subprocess for this?

http://stackoverflow.com/questions/8947711/call-a-python-subprocess-as-daemon-and-exit — Huazuo Gao, Sep 23 '15 at 14:40
http://stackoverflow.com/questions/13592219/launch-a-totally-independent-process-from-python?rq=1 — Psytho, Sep 23 '15 at 14:43
The first link does not spawn an independent process and the second link does not work for *nix systems. — Juliuszc, Sep 23 '15 at 15:10
Not sure how to do it platform independent, but https://en.wikipedia.org/wiki/Nohup and this http://stackoverflow.com/questions/285015/linux-prevent-a-background-process-from-being-stopped-after-closing-ssh-client have some general clues. — Robert E, Sep 23 '15 at 15:18
don't use `PIPE` unless you read from the pipes. [Use `subprocess.DEVNULL` instead](http://stackoverflow.com/a/11270665/4279). Also, use `close_fds=True` explicitly (Python 2 requires it). It should also fix "nohup is not keeping it alive" issue. — jfs, Sep 23 '15 at 15:32
I've managed to get the program to keep running on *nix system with nohup, but that option is not available for Windows. I guess I'll just have to check for os type and handle the cases accordingly. — Juliuszc, Sep 23 '15 at 15:36

score 0 · Answer 1 · answered Sep 24 '15 at 16:04

0

I couldn't find a true universal way to start the program independently as it's own process. I managed to get what I want by checking to see if it is a *nix OS and appending 'nohup' to the beginning if true. Windows does not kill child processes so it worked out.

answered Sep 24 '15 at 16:04

Juliuszc

377
2
4
16

[`nohup` is not enough](http://stackoverflow.com/q/6011235/4279), you might need `preexec_fn=os.setpgrp` too (and perhaps [other points from Unix daemon list](https://www.python.org/dev/peps/pep-3143/#correct-daemon-behaviour)). There are no process trees on Windows. There are "Job Objects" on Windows and [the default behavior is not to kill child processes](http://stackoverflow.com/q/53208/4279) – jfs Sep 24 '15 at 17:54
here's [code example that shows how to emulate `nohup` in pure Python](http://stackoverflow.com/a/27549535/4279) – jfs Sep 24 '15 at 17:57

score -1 · Answer 2 · answered Sep 23 '15 at 15:49

-1

Use Multiprocessing library instead and run the process as daemon.

answered Sep 23 '15 at 15:49

hspandher

15,934
2
32
45

3

it is the opposite of what OP wants. If the main process exits then all its daemonic child processes are killed. Note: [daemon process in `multiprocessing`](https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Process.daemon) has a _different_ meaning from [Unix daemon](https://www.python.org/dev/peps/pep-3143/#correct-daemon-behaviour) – jfs Sep 23 '15 at 20:58

open independent process python

2 Answers2