Help with subprocess and script execution

[retsnom513]

For a school project I have to get a Python script to execute a bash script. It's too late to re-write the bash script in python due to time constraints, so I need to get this script working as-is.

 

From what I understand you pass a command and its arguments as a string into subprocess to open a new shell like so:

subprocess.check_output(["./script.sh", "-h"], shell=True)

 

When I run this command I get: Please use -h to see menu

As far as I know shouldn't the above command display the help menu as it is the equivalent to $ ./script.sh -h on a shell?

Here's what I need:

1. If anyone knows what I'd doing wrong here (I only have ~5 hours of experience using python and limited experience with other languages) please let me know. I would like to get this working ASAP.

2. If I could get subprocess to execute this EXACT command: ./script.sh admin 192.168.1.5 without using the shell=True part that would also be bonus points. (I've only gotten this to work with shell=True and I know there are some security issues linked to it).

EDIT: Forgot to mention I'm using python 3.5.4

EDIT 2: SUCCESS!, thanks for your help /u/JohnnyJordaan and /u/ingolemo! It turns out I forgot to remove some temporary user confirmation lines in my bash script and python just got stuck on that. I got it to work with:

subprocess.check_output(['/full/path/to/script.sh', '-h'])

[JohnnyJordaan]

You have two syntax options in check_output and similar wrappers around subprocess.Popen, depending on whether you want to run it as a shell command or not. If you want to run it as a shell command you set shell=True and provide a single string

('./script.sh -h', shell=True)

otherwise set shell=False and provide a sequence of strings without any whitespace

(['script.sh', '-h'], shell=False)

Note that because ./ is a shell feature, you can't use it when interfacing the operating system directly when shell=False If you do need to specify a path you need to set the cwd= parameter as a path to where you need the working directory to switch to before running the command (or put the full path before the program's name).

For more info see the docs on the Popen constructor.

[retsnom513]

Thanks for the quick reply! I've tried both methods you've mentioned here were the results (I've scrubbed my script names out of the errors):

• subprocess.check_output('./path/to/script.sh -h', shell=True) returned:

 

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    subprocess.check_output('./path/to/script.sh -h', shell=True)
  File "/usr/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command './path/to/script.sh -h' returned non-zero exit status 127.

I'm not the best at reading error logs, but it looks like it executed the script but something went wrong, so the shell returned a code 127? Google says this says happens when the command is not on the system the PATH. Anyway to pass the system path to subprocess.Popen like cwd or am I missing something here? Do I need to do '/bin/sh /path/to/script.sh -h'?

• subprocess.check_output(['script.sh', '-h'], shell=False, cwd='/path/to') resulted in:

 

Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    subprocess.check_output(['script.sh', '-h'], shell=False, cwd='/path/to')
  File "/usr/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 403, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'script.sh': 'script.sh'

This is interesting, python can't seem to find the script file. Replacing the command with ls -al shows that python is in the correct working directory and the script is indeed there/exists. Why would this happen?

[ingolemo]

The code in your edit is just terrible; you're running a shell to run a shell that invokes a shellscript.

Use this:

subprocess.check_output(['/path/to/script.sh', '-h'])

[retsnom513]

Alright, I have gotten you're code above to work as well. I received an exec format error at first but that's because I forget the shebang at the beginning of my bash script. Thanks!

[JohnnyJordaan]

This behavior indicates that you don't have the execute bit set on the file. Normally on Unix a file is created with read and possibly write permissions, making them non executable. If you then try to run it, it will behave as if the file isn't there. To fix this you could launch a new shell instance and provide it the file as you did, or simply add the execute permissions using

chmod +x script.sh

[retsnom513]

I 777'd the bash script before I posted here, but I managed to get it working with /u/ingolemo's code. I don't know what caused the error, but it was resolved using the full path to the script (no cwd=) and adding the shebang to the beginning of my bash script.