Launch a completely independent process

Question

I want to initiate a process from my python script main.py. Specifically, I want to run the below command:

`nohup python ./myfile.py &`

and the file myfile.py should continue running, even after the main.py script exits.

I also wish to get the pid of the new process.

I tried:

• os.spawnl*
• os.exec*
• subprocess.Popen

and all are terminating the myfile.py when the main.py script exits.

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Update: Can I use os.startfile with xdg-open? Is it the right approach?

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Example

a = subprocess.Popen([sys.executable, "nohup /usr/bin/python25 /long_process.py &"],\
     stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
print a.pid

If I check ps aux | grep long_process, there is no process running.

long_process.py which keeps on printing some text: no exit.

Am I doing anything wrong here?

Answer 1

You open your long-running process and keep a pipe to it. So you expect to talk to it. When yor launcher script exits, you can no longer talk to it. The long-running process receives a SIGPIPE and exits.

The following just worked for me (Linux, Python 2.7).

Create a long-running executable:

$ echo "sleep 100" > ~/tmp/sleeper.sh

Run Python REPL:

$ python
>>>

import subprocess
import os
p = subprocess.Popen(['/bin/sh', os.path.expanduser('~/tmp/sleeper.sh')])
# look ma, no pipes!
print p.pid
# prints 29893

Exit the REPL and see the process still running:

>>> ^D
$ ps ax | grep sleeper
29893 pts/0    S      0:00 /bin/sh .../tmp/sleeper.sh
29917 pts/0    S+     0:00 grep --color=auto sleeper

If you want to first communicate to the started process and then leave it alone to run further, you have a few options:

• Handle SIGPIPE in your long-running process, do not die on it. Live without stdin after the launcher process exits.
• Pass whatever you wanted using arguments, environment, or a temporary file.
• If you want bidirectional communication, consider using a named pipe (man mkfifo) or a socket, or writing a proper server.
• Make the long-running process fork after the initial bi-direcional communication phase is done.