Run a program from python, and have it continue to run after the script is killed

I’ve tried running things like this:

subprocess.Popen(['nohup', 'my_command'],
                 stdout=open('/dev/null', 'w'),
                 stderr=open('logfile.log', 'a'))

This works if the parent script exits gracefully, but if I kill the script (Ctrl-C), all my child processes are killed too. Is there a way to avoid this?

The platforms I care about are OS X and Linux, using Python 2.6 and Python 2.7.

Answers:

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Method 1

The child process receives the same SIGINT as your parent process because it’s in the same process group. You can put the child in its own process group by calling os.setpgrp() in the child process. Popen‘s preexec_fn argument is useful here:

subprocess.Popen(['nohup', 'my_command'],
                 stdout=open('/dev/null', 'w'),
                 stderr=open('logfile.log', 'a'),
                 preexec_fn=os.setpgrp
                 )

(preexec_fn is for un*x-oids only. There appears to be a rough equivalent for Windows “creationflags=CREATE_NEW_PROCESS_GROUP“, but I’ve never tried it.)

Method 2

The usual way to do this on Unix systems is to fork and exit if you’re the parent. Have a look at os.fork() .

Here’s a function that does the job:

def spawnDaemon(func):
    # do the UNIX double-fork magic, see Stevens' "Advanced 
    # Programming in the UNIX Environment" for details (ISBN 0201563177)
    try: 
        pid = os.fork() 
        if pid > 0:
            # parent process, return and keep running
            return
    except OSError, e:
        print >>sys.stderr, "fork #1 failed: %d (%s)" % (e.errno, e.strerror) 
        sys.exit(1)

    os.setsid()

    # do second fork
    try: 
        pid = os.fork() 
        if pid > 0:
            # exit from second parent
            sys.exit(0) 
    except OSError, e: 
        print >>sys.stderr, "fork #2 failed: %d (%s)" % (e.errno, e.strerror) 
        sys.exit(1)

    # do stuff
    func()

    # all done
    os._exit(os.EX_OK)

Method 3

After an hour of various attempts, this works for me:

process = subprocess.Popen(["someprocess"], creationflags=subprocess.DETACHED_PROCESS | subprocess.CREATE_NEW_PROCESS_GROUP)

It’s solution for windows.

Method 4

Since 3.2 you can also use start_new_session flag (POSIX only).

import subprocess

p = subprocess.Popen(["sleep", "60"], start_new_session=True)
ret = p.wait()

See start_new_session in Popen constructor

Method 5

with open('/dev/null', 'w') as stdout, open('logfile.log', 'a') as stderr:
    subprocess.Popen(['my', 'command'], stdout=stdout, stderr=stderr)

class subprocess.Popen(…)

Execute a child program in a new process.
On POSIX, the class uses os.execvp()-like behavior to execute the
child program. On Windows, the class uses the Windows CreateProcess()
function.

os.execvpe(file, args, env)

These functions all execute a new program, replacing the current
process; they do not return. On Unix, the new executable is loaded
into the current process, and will have the same process id as the
caller. Errors will be reported as OSError exceptions.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0