How to determine pid of process started via os.system

Question

I want to start several subprocesses with a programm, i.e. a module foo.py starts several instances of bar.py.

Since I sometimes have to terminate the process manually, I need the process id to perform a kill command.

Even though the whole setup is pretty “dirty”, is there a good pythonic way to obtain a process’ pid, if the process is started via os.system?

foo.py:

import os
import time
os.system("python bar.py \"{0}\ &".format(str(argument)))
time.sleep(3)
pid = ???
os.system("kill -9 {0}".format(pid))

bar.py:

import time
print("bla")
time.sleep(10) % within this time, the process should be killed
print("blubb")

Solution 1

os.system return exit code. It does not provide pid of the child process.

Use subprocess module.

import subprocess
import time
argument = '...'
proc = subprocess.Popen(['python', 'bar.py', argument], shell=True)
time.sleep(3) # <-- There's no time.wait, but time.sleep.
pid = proc.pid # <--- access `pid` attribute to get the pid of the child process.

To terminate the process, you can use terminate method or kill. (No need to use external kill program)

proc.terminate()

Solution 2

Sharing my solution in case it can help others:

I took the info from this page to run a fortran exe in the background. I tried to use os.forkpty to get the pid of it, but it didnt give the pid of my process. I cant use subprocess, because I didnt find out how it would let me run my process on the background.

With help of a colleague I found this:

exec_cmd = 'nohup ./FPEXE & echo $! > /tmp/pid'

os.system(exec_cmd)

In case of wanting to append pids to the same file, use double arrow.

Solution 3

You could use os.forkpty() instead, which, as result code, gives you the pid and fd for the pseudo terminal. More documentation here: http://docs.python.org/2/library/os.html#os.forkpty

Comments

[Sebastian Werk]

Thanks, I will use this as a fallback, if os.spawnl(...) fails

[falsetru]

@SebastianWerk, According to os.spawn* documentation, "Note that the subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using these functions. ..."

[Sebastian Werk]

I know, therefor I used it in first place, but it takes around 20ms to start one process on my computer, and this is a bit too long ):

[falsetru]

@SebastianWerk, How about posting EDIT part of your question as answer and accepting it instead of trying to close your question?

[Tommy]

does this work if you want the main process (the code above) to go down after spawning the new one and writing a pid file?

[sai harsha vardhan]

how to capture the stdout to a file if we use this approach? nohup ./FPEXE | tee my_log.log & echo $! > /tmp/pid . Becuase the pid capture is the proccess id of tee and not the pid of the actual command.