Python subprocess.check_output stderr用法

Question

Hi I am newbie to Python and am trying to understand how stderr is used with subprocess check_output. I've read through the subprocess documentation and am having difficulty understanding how the stderr is used and what specifically subprocess.STDOUT actually accomplishes.

Can I please get some help with some examples or references that explains how stderr is used here?

I've tried these commands using both stderr and without and not seeing any real difference.

Code:

#!/usr/bin/python3
import subprocess

print(subprocess.check_output("echo Hello World!",
                              stderr=subprocess.STDOUT,
                              shell=True))

Output:

# ./ex.py
b'Hello World!\n'

Code:

#!/usr/bin/python3
import subprocess

print(subprocess.check_output("gecho Hello World!",
                              stderr=subprocess.STDOUT,
                              shell=True))

Output:

# ./ex.py
Traceback (most recent call last):
  File "./ex.py", line 6, in <module>
    shell=True))
  File "/usr/lib64/python3.3/subprocess.py", line 589, in check_output
    raise CalledProcessError(retcode, process.args, output=output)
subprocess.CalledProcessError: Command 'gecho Hello World!' returned non-zero exit status 127

Answer 1

subprocess.check_output will raise the exception you see if there's a non-zero exit code. It sound like what you're trying to get is a simple way of reading STDERR, which in that case the easiest thing is to use subprocess.run and pipe STDOUT and STDERR (examples are in Windows):

>>> p = subprocess.run(['cmd','/C','echo','hello','world'], stdout = subprocess.PIPE, stderr=subprocess.PIPE)
>>> p.stdout
b'hello world\r\n'

>>> p.stderr
b''

>>> p = subprocess.run(['cmd','/C','gecho','hello','world'], stdout = subprocess.PIPE, stderr=subprocess.PIPE)
>>> p.stdout
b''

>>> p.stderr
b"'gecho' is not recognized as an internal or external command,\r\noperable program or batch file.\r\n"

If you REALLY need to use check_output, the following will ignore the error code from the echo call and still print the error (example taken almost verbatim from the docs):

>>> print(subprocess.check_output('gecho hello& exit 0', stderr=subprocess.STDOUT, shell=True))
b"'gecho' is not recognized as an internal or external command,\r\noperable program or batch file.\r\n"

Or in Linux:

>>> print(subprocess.check_output('gecho hello; exit 0', stderr=subprocess.STDOUT, shell=True))
b'/bin/sh: 1: gecho: not found\n'

As a side note, it's almost never a good idea to use subprocess functions with the option shell=True . It's mostly due to security concerns; read the docs for full explanation.