How to start a background process in Python?


I'm trying to port a shell script to the much more readable python version. The original shell script starts several processes (utilities, monitors, etc.) in the background with "&". How can I achieve the same effect in python? I'd like these processes not to die when the python scripts complete. I am sure it's related to the concept of a daemon somehow, but I couldn't find how to do this easily.

3/10/2016 9:55:45 AM

Accepted Answer

Note: This answer is less current than it was when posted in 2009. Using the subprocess module shown in other answers is now recommended in the docs

(Note that the subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using these functions.)

If you want your process to start in the background you can either use system() and call it in the same way your shell script did, or you can spawn it:

import os
os.spawnl(os.P_DETACH, 'some_long_running_command')

(or, alternatively, you may try the less portable os.P_NOWAIT flag).

See the documentation here.

11/19/2015 10:21:24 AM

While jkp's solution works, the newer way of doing things (and the way the documentation recommends) is to use the subprocess module. For simple commands its equivalent, but it offers more options if you want to do something complicated.

Example for your case:

import subprocess

This should run rm -r somefile in the background. But be wary: subprocess.Popen() only runs a process in the background if nothing in the python script depends on the output of the command being run:

For example, the following command will not run in the background:

import subprocess
ls_output=subprocess.Popen(["ls", "-a"], stdout=subprocess.PIPE)

See the documentation here.

Also, a point of clarification: "Background" purely a shell concept: what you probably want is to spawn a new process. I've used "background" here to refer to shell-background-like behavior, but don't mistake this for the process actually being in the background.

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