Get only stderr output from subprocess.check_output

Question

I want to run an external process in python, and process its stderr only.

I know I can use subprocess.check_output, but how can I redirect the stdout to /dev/null (or ignore it in any other way), and receive only the stderr?

Answer 1

Unfortunately you have tagged this python-2.7, as in python 3.5 and up this would be simple using run():

import subprocess

output = subprocess.run(..., stdout=subprocess.DEVNULL,
                        stderr=subprocess.PIPE).stderr

With check_output() stdout simply cannot be redirected:

>>> subprocess.check_output(('ls', 'asdfqwer'), stdout=open(os.devnull, 'wb'), stderr=subprocess.STDOUT)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
    raise ValueError('stdout argument not allowed, it will be overridden.')
ValueError: stdout argument not allowed, it will be overridden.

Use Popen objects and communicate() with python versions less than 3.5. Open /dev/null using os.devnull in python 2.7:

>>> import subprocess
>>> import os
>>> with open(os.devnull, 'wb') as devnull:
...     proc = subprocess.Popen(('ls', 'asdfqwer'),
...                             stdout=devnull,
...                             stderr=subprocess.PIPE)
...     proc.communicate()
...     proc.returncode
... 
(None, "ls: cannot access 'asdfqwer': No such file or directory\n")
2

Communicate sends input to stdin, if piped, and reads from stdout and stderr until end-of-file is reached.

Answer 2

I found a simple trick:

import subprocess
stderr_str = subprocess.check_output('command 2>&1 >/dev/null')

This will filter out the stdout, and keeps only the stderr.