Check if a subprocess is running in Linux using Python

Question

I have an environment variable that allows a suite of applications to run under certain conditions, and the applications cannot run with the environment variable off.

My python script uses

p = subprocess.Popen(cmdStr3,stdout=subprocess.PIPE,stderr=subprocess.STDOUT)

to open the subprocess.

To test if the application is running, I have tried

try:
    os.kill(pid, 0)
    return True
except OSError:
    return False

and also checking p.returncode. However, these always return true, because even if the application doesn't pull up on the screen, there are mini processes of ~1 MB that still run without the application fully running, so the os sees these and returns true. Is there a way around this?

Another issue is that os.kill doesn't work, the only way I have found to terminate the application is os.killpg at the end.

What I've learned from the comments is that what actually happens is that the subprocess I call is a starter that calls a child which is another application. My subprocess always runs, but with the environment variable off, the child application does not run. Is there a way to see if the child is running?

What do you mean by 'miniprocesses'? If pid can be found, the program exists and is running, right? — user707650
– user707650, Commented May 21, 2014 at 13:08
You can use p.terminate() to stop the child process and p.poll() to check if it is still running. — Hans Then
– Hans Then, Commented May 21, 2014 at 13:09
@Evert you're right. I guess what actually happens is that the subprocess I call is a starter that calls a child which is another application. My subprocess always runs, but with the environment variable off, the child application does not run. Is there a way to see if the child is running? — user3632833
– user3632833, Commented May 21, 2014 at 13:15
Sounds like you want something like ps, assuming those sub-child processes have a unique name. Otherwise, if the starter process doesn't keep track of the child, I think all bets are off (you could search for orphan processes, but there may be plenty). — user707650
– user707650, Commented May 21, 2014 at 13:21
I will look into ps. For all the cases I've tested, the child application always has a process ID that is one more than the subprocess. Will this always be the case or is this a foolish way to check — user3632833
– user3632833, Commented May 21, 2014 at 13:25

zmo · Accepted Answer · 2014-05-21 13:13:57Z

2

you can use p.poll() to check if the process is still running.

Another issue is that os.kill doesn't work, the only way I have found to terminate the application is os.killpg at the end.

If you want to kill a process launched using subprocess, you can use p.terminate().

Finally, if you want to wait until the process child dies, you can use p.wait().

answered May 21, 2014 at 13:13

zmo

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2 Comments

Don Smith Over a year ago

Can you please add an example of how to use p.poll?

zmo Over a year ago

you can find an example on this answer: stackoverflow.com/questions/12057794/… ☺

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