Check if subprocess is still running while reading its output line by line [duplicate]

Question

I am starting a subprocess in Python and trying to read each line of output. Unfortunately I can't find a good way of testing if my processes is still alive. The standard method seems to be checking poll(), but that seems to always return None. Here is my code.

proc = Popen(sys.argv[1:], stdin=PIPE, stdout=PIPE, stderr=STDOUT)

while True:
    process_line(proc.stdout.readline().decode('utf-8'))

    if not proc.poll():
        break

for line in proc.communicate()[0].splitlines():
    process_line(line.decode('utf-8'))

I've also tried using os.kill(proc.pid, 0), which works for non-spawned processes, but it seems that Python keeps a handle on processes it starts so os.kill(proc.pid, 0) always returns.

What am I doing wrong?

You opened a pipe to the process's stdin. Is it waiting to read data? — Chad S.
– Chad S., Commented Mar 7, 2016 at 22:25
No the process only outputs data. In the future I want to add support for both directions, which is why I have that in there. — Kris Harper
– Kris Harper, Commented Mar 7, 2016 at 22:26

Corey Goldberg · Accepted Answer · 2016-03-07 22:48:49Z

3

to process subprocess output line by line, try this:

p = subprocess.Popen(cmd, stdout=subprocess.PIPE, bufsize=1)
while p.poll() is None:
    line = p.stdout.readline()
    ...do something with the line here

notice I set the buffer size to 1.

also, os.kill(proc.pid, 0) is not needed. Just call .kill() on the subprocess you spawned. You can also call .wait() to wait for termination instead of immediately killing the process.

answered Mar 7, 2016 at 22:48

Corey Goldberg

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4 Comments

Kris Harper Over a year ago

This seems to work. Thanks.

jfs Over a year ago

@KrisHarper: there is no need to check whether the process is alive if you want to capture its output line by line.

Kris Harper Over a year ago

@J.F.Sebastian Hmm so in your Python 3 answer here stackoverflow.com/a/17698359/817630, there is no risk of the process ending while lines are still being sent to stdout? Before I was using this answer stackoverflow.com/a/2716032/817630 to the same question, but I see now that it recommends yours instead.

jfs Over a year ago

@KrisHarper if the process is dead then (obviously) it doesn't produce any output and — as soon as already buffered data is read — the loop ends.

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