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

Question

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.

Answer 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.)

Answer 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)