Execute Shell-Command over python using subprocess

Question

when I execute a command (while true; do date; sleep 1; done) over a Python-Skript nothing is displyed and no logging.

import logging
import sys
import subprocess as SP


logger = logging.getLogger('logging')
logger.setLevel(logging.INFO)

if not logger.handlers:
    log_handler = logging.FileHandler('test_logging.log')
    formatter = logging.Formatter('%(asctime)s %(message)s')
    log_handler.setFormatter(formatter)
    logger.addHandler(log_handler)
    log_handler.setLevel(logging.INFO)

command = 'while true; do date; sleep 1; done'
p = SP.Popen(command, shell=True, stdout=SP.PIPE, stderr=SP.PIPE)
print p.stdout.readlines()

for line in p.stdout.readlines():
    logger.info(line)
    print line

PM 2Ring · Accepted Answer · 2016-01-29 13:41:17Z

1

This works for me (on Python 2.6.6)

import subprocess as SP
command = 'while true; do date; sleep 1; done'
p = SP.Popen(command, shell=True, bufsize=1, stdout=SP.PIPE, stderr=SP.PIPE)
while True:
    print p.stdout.readline()

The bufsize=1 isn't essential, but it turns on line buffering, which should improve the efficiency a little.

answered Jan 29, 2016 at 13:41

PM 2Ring

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1 Comment

PM 2Ring Over a year ago

@eugeney: Then it's probably better to use one of the convenience functions, eg subprocess.check_output, rather than directly calling subprocess.Popen.

Eugene Yarmash · Accepted Answer · 2016-01-29 14:19:58Z

1

p.stdout.readlines() tries to read all the lines of the command's output in a list, which will never finish since the command is an infinite loop: 'while true; do date; sleep 1; done'.

If you're using Python 2, one way of iterating over the lines of the output could be:

for line in iter(p.stdout.readline, b''):
    print line.rstrip()

In Python 3 you can simply iterate over the p.stdout file object.

edited Jan 29, 2016 at 14:19

answered Jan 29, 2016 at 13:20

Eugene Yarmash

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2 Comments

J Richard Snape Over a year ago

This just hangs if using Python 2 (as implied by print syntax) on an Ubuntu 14.04 box. It works if I use Python 3. I don't know enough about the internals of subprocess.PIPE to speculate on why.

Eugene Yarmash Over a year ago

@J Richard: Indeed, iterating over the file object doesn't seem to work in Python 2. I've updated the post with a working version.

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