Running a nohup command through SSH with Python's Popen then logging out

Question

I'm trying to use this answer to issue a long-running process with nohup using subprocess.Popen, then exit the connection.

From the login shell, if I run

$ nohup sleep 20 &
$ exit

I get the expected behavior, namely that I can log out while the sleep process is still running, and connect back later to check on its status. If I try to the same through Python, however, it seems as though the exit command does not get executed until the sleeping time is over.

import subprocess

HOST=<host>
sshProcess = subprocess.Popen(['ssh', HOST],
                                stdin = subprocess.PIPE,
                                stdout = subprocess.PIPE,
                                universal_newlines = True,
                                bufsize = 0)
sshProcess.stdin.write("nohup sleep 20 &\n")
sshProcess.stdin.write("exit\n")
sshProcess.stdin.close()

What am I missing?

Why not just `subprocess.Popen(['ssh', HOST, 'nohup', 'sleep', '20'], stdin = subprocess.PIPE, stdout = subprocess.PIPE)` ? — Samuel, Feb 28 '17 at 11:29
Because then my Python process will keep running until sleep is done, whereas I want to issue the command (which will be a long-running Python process) and then exit and do other things. — Pietro Marchesi, Feb 28 '17 at 12:18
You can just remove pipes from `Popen` to run in background. More info here http://stackoverflow.com/a/7224186/524743 — Samuel, Feb 28 '17 at 15:31

score 1 · Accepted Answer · edited May 23 '17 at 12:00

1

From the docs: Python Docs

Warning Use communicate() rather than .stdin.write, .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Alright pretty sure i got it now:

About communicate(): [...]Wait for process to terminate So i guess your earlier Solution was better. But if you call it at the end if shouldn't be a problem or just don't call it at all if you don't need stdin or stderr as output

However according to this:StackOverflow Comment If you set preexec_fn=os.setpgrp in your Popen call it should work.

edited May 23 '17 at 12:00

Community

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answered Feb 28 '17 at 11:16

ShuzZzle

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I have tried `communicate()` but I get the same behavior. I still have to wait for the sleep process to be done before I can run anything else. – Pietro Marchesi Feb 28 '17 at 11:22
Indeed, the (to me) mysteryous `preexec_fn=os.setpgrp` does the trick. Thanks for pointing that out. – Pietro Marchesi Feb 28 '17 at 12:26

Running a nohup command through SSH with Python's Popen then logging out

1 Answers1