subprocess confusion

[Tim Arnold]

Hi,
Just discovered that my subprocess call with the preexec_fn wasn't doing
what I thought.
If 'machine' value is different than the current machine name, I want to
remsh the command to that machine, but obviously I misunderstood the
preexec_fn arg.

Should I just put the remsh in the actual command instead of preexec_fn?
thanks,
--Tim Arnold
-------------------------------
if machine == socket.gethostname():
shname = None
else:
shname = lambda :'/bin/remsh %s ' % (machine)
p = subprocess.Popen(preexec_fn = shname,
shell = True,
args = command,
stderr = subprocess.STDOUT,
stdout = log,
env = env,
)
try:
p.wait()
if log:
log.close()
except:
pass

-------------------------------

[Michael Hoffman]

Tim Arnold wrote:

If 'machine' value is different than the current machine name, I want to
remsh the command to that machine, but obviously I misunderstood the
preexec_fn arg.

I don't think the return value of preexec_fn is used for anything. You
can use to do things like set process group IDs in the child. For your
use, everything needs to be args.

If you are using Python 2.5, and log is a file, you want:

from __future__ import with_statement

with log:
if machine != socket.gethostname():
command = "/bin/remsh %s %s" % (machine, command)

subprocess.check_call(command, shell=True, env=env,
stderr=subprocess.STDOUT, stdout=log)

The semantics are slightly different, since log will always close this
way, while in the other example, you have left it open if there is an
exception.
--
Michael Hoffman

[Nick Craig-Wood]

Tim Arnold <ti****@sas.comwrote:

Just discovered that my subprocess call with the preexec_fn wasn't doing
what I thought.
If 'machine' value is different than the current machine name, I want to
remsh the command to that machine, but obviously I misunderstood the
preexec_fn arg.

Should I just put the remsh in the actual command instead of
preexec_fn?

Yes.

The preexec_fn is run after the fork() but before the exec(). Ie a
new process has been made, but it hasn't started your task yet.

For example a classic use of preexec_fn is

preexec_fn=os.setsid

You seem to be thinking it is pre-pending something to your command
line which isn't how it works.

--
Nick Craig-Wood <ni**@craig-wood.com-- http://www.craig-wood.com/nick

[Tim Arnold]

"Nick Craig-Wood" <ni**@craig-wood.comwrote in message
news:sl*****************@irishsea.home.craig-wood.com...

Tim Arnold <ti****@sas.comwrote:

<snip>

> Should I just put the remsh in the actual command instead of
preexec_fn?

Yes.

The preexec_fn is run after the fork() but before the exec(). Ie a
new process has been made, but it hasn't started your task yet.

For example a classic use of preexec_fn is

preexec_fn=os.setsid

You seem to be thinking it is pre-pending something to your command
line which isn't how it works.

--
Nick Craig-Wood <ni**@craig-wood.com-- http://www.craig-wood.com/nick

Thanks much to you and Michael H. for the great explanations.
Now everything is working fine, and I understand subprocess a little better!

--Tim Arnold