How to get command's standard output with subprocess?

Question

For some reason, I want to gather the help message of some commands. In order to do it, I use the subprocess module in Python3. My code works fine for linux commands but not when I use it on BASH commands. Typically I want it to work on the cd BASH command.

Here is the snippet of code I use for now:

import subprocess

instruction = ["cat", "--help"]
proc = subprocess.run(instruction, stdout=subprocess.PIPE,stderr=subprocess.PIPE, universal_newlines=True)
return proc.stdout

As said before, it works fine and it returns the help message of the command cat.

Here is what it returns when I try to adapt my code in order to handle BASH commands:

>>> import subprocess
>>> subprocess.run("cd --help", shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE).stdout
b''

My question is simple, is it possible to get BASH commands' help message using Python3. If so, how to do that ?

Answer 1

You can take a look at what subprocess.run returns:

>>> import subprocess
>>> result = subprocess.run("cd --help", shell=True, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
>>> result
CompletedProcess(args='cd --help', returncode=1, stdout=b'', stderr=b'/bin/sh: line 0: cd: --: invalid option\ncd: usage: cd [-L|-P] [dir]\n')

Turns out, cd --help is an error:

$ cd --help
-bash: cd: --: invalid option
cd: usage: cd [-L|-P] [dir]

So you should look for it in result.stderr.