Subprocess returns an empty string

Question

I would like to check the result of a Bash command in a Python script. I'm using subprocess.check_output().

Before that, I checked manually the result of two commands in a shell:

user@something:~$ hostname
> something

user@something:~$ command -v apt
> /usr/bin/apt

It works as expected. Now I'm trying to run the subprocess.check_output() function in the Python interpreter:

>>> import subprocess
>>> subprocess.check_output(['hostname'], shell=True, stderr=subprocess.STDOUT)
b'something\n'
>>> subprocess.check_output(['command', '-v', 'apt'], shell=True, stderr=subprocess.STDOUT)
b''

As you can see, the first command is working as expected, but not the second (as it returns an empty string). Why is that?

EDIT

I already tried removing shell=True, it returns an error:

>>> subprocess.check_output(['command', '-v', 'apt'], stderr=subprocess.STDOUT)
Traceback (most recent call last):
[...]
FileNotFoundError: [Errno 2] No such file or directory: 'command'

Removing `shell=True` will fix your issue. Also, you could use `shutil.which('apt')` instead of second call — Yaroslav Surzhikov, Oct 17 '17 at 08:42
I didn't set `shell=True` by default, I added it because I was getting an error without it. — , Oct 17 '17 at 08:46
@Eldy that's easier to understand that way. `command` is built-in, so `shell=True` is necessary. But what does "command" do? — Jean-François Fabre, Oct 17 '17 at 08:49
`echo $PATH` in terminal and with `subprocess.check_output(['echo', ' $PATH'], stderr=subprocess.STDOUT)` will show you why. Also, to clarify - exec `which command` in terminal. I bet that it's location is not in $PATH using subprocess — Yaroslav Surzhikov, Oct 17 '17 at 08:49

Moses Koledoye · Answer 1 · 2017-10-17T08:49:10.737

0

Drop shell=True from the parameters:

>>> subprocess.check_output(['command', '-v', 'apt'], stderr=subprocess.STDOUT) 
'/usr/bin/apt\n'

See Actual meaning of 'shell=True' in subprocess.

If shell is True, it is recommended to pass args as a string rather than as a sequence.

This ensures the command is properly formatted as it would if you had typed it directly in your shell:

>>> subprocess.check_output('command -v apt', shell=True, stderr=subprocess.STDOUT)
'/usr/bin/apt\n'

However, using shell is generally discouraged.

edited Oct 17 '17 at 08:49

answered Oct 17 '17 at 08:42

Moses Koledoye

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can you explain why it's required? – Jean-François Fabre Oct 17 '17 at 08:43
I already tried that, and it's not working. I'll update my question. – Oct 17 '17 at 08:44

Subprocess returns an empty string

1 Answers1