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Forking into the background (Linux)

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  Forking into the background (Linux) Olive <not0read0765@yopmail.com> - 2012-12-24 01:50 +0100
    Re: Forking into the background (Linux) Hans Mulder <hansmu@xs4all.nl> - 2012-12-24 02:56 +0100
      Making a Unix daemon process (was: Forking into the background (Linux)) Ben Finney <ben+python@benfinney.id.au> - 2012-12-24 18:17 +1100

#35422 — Forking into the background (Linux)

My goal is to write a script that 1) write something to stdout; then 
fork into the background, closing the stdout (and stderr, stdin) pipe.

I have found this answer (forking -> setsid -> forking) 
http://stackoverflow.com/a/3356154

However the standard output of the child is still connected to the 
terminal. I would like that if we execute a subprocess.checkprocess on 
this program,  only "I would like to see this" is captured and that the 
program terminates when the parent exits.

#! /usr/bin/python2
import os,sys,time

print "I would like to see this"
pid = os.fork()
if (pid == 0): # The first child.
     # os.chdir("/")
    os.setsid()
    # os.umask(0)
    pid2 = os.fork()
    if (pid2 == 0):  # Second child
      print "I would like not see this"
      time.sleep(5)
    else:
      sys.exit()    #First child exists
else:           # Parent Code
   sys.exit()   # Parent exists

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#35429

On 24/12/12 01:50:24, Olive wrote:
> My goal is to write a script that 1) write something to stdout; then
> fork into the background, closing the stdout (and stderr, stdin) pipe.
> 
> I have found this answer (forking -> setsid -> forking)
> http://stackoverflow.com/a/3356154
> 
> However the standard output of the child is still connected to the
> terminal. I would like that if we execute a subprocess.checkprocess on
> this program,  only "I would like to see this" is captured and that the
> program terminates when the parent exits.
> 
> #! /usr/bin/python2
> import os,sys,time
> 
> print "I would like to see this"
> pid = os.fork()
> if (pid == 0): # The first child.
>     # os.chdir("/")
>    os.setsid()
>    # os.umask(0)
>    pid2 = os.fork()
>    if (pid2 == 0):  # Second child
>      print "I would like not see this"
>      time.sleep(5)
>    else:
>      sys.exit()    #First child exists
> else:           # Parent Code
>   sys.exit()   # Parent exists

You could do this before forking:

	sys.stdin.close()
	sys.stdin = open('/dev/null', 'r')
	sys.stdout.close()
	sys.stdout = open('/dev/null', 'w')
	sys.stderr.close()
	sys.stderr = open('/dev/null', 'w')


You may want to look at the python-daemon module on Pypy, which appears
to do what you need, including some features you haven't asked for, yet.


Hope this helps,

-- HansM





	

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#35444 — Making a Unix daemon process (was: Forking into the background (Linux))

Hans Mulder <hansmu@xs4all.nl> writes:

> On 24/12/12 01:50:24, Olive wrote:
> > My goal is to write a script that 1) write something to stdout; then
> > fork into the background, closing the stdout (and stderr, stdin) pipe.
> > 
> > I have found this answer (forking -> setsid -> forking)
> > http://stackoverflow.com/a/3356154

You're following a path that leads to the desire for a “daemon”
<URL:http://stackoverflow.com/questions/473620/how-do-you-create-a-daemon-in-python/688448#688448>.

> You may want to look at the python-daemon module on Pypy, which
> appears to do what you need, including some features you haven't asked
> for, yet.

It's even better when you look at it on PyPI
<URL:http://pypi.python.org/pypi/python-daemon/> (note that PyPy is a
Python implementation, PyPI is an index of Python packages).

The discussion forum for ‘python-daemon’ development is at
<URL:http://lists.alioth.debian.org/mailman/listinfo/python-daemon-devel>.

-- 
 \         “Faith may be defined briefly as an illogical belief in the |
  `\                  occurrence of the improbable.” —Henry L. Mencken |
_o__)                                                                  |
Ben Finney

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