Issue with subprocess.check_output printing stderr, looking to implement error checking

Question

I am trying to beautify the output of different system commands, but since I am using a non Redhat based OS my program snags executing the rpm feature. Could someone give a nudge in the right direction, I would like to make this more Pythonic and implement error checking against the rpm function if not a RH based OS.

def kernVer(kern1, kern2, kern3, kern4):
    if subprocess.check_output(["uname", "-a"]).decode('ascii').strip():
        print ('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern1) + '\x1b[0m')
    if subprocess.check_output(["uname", "-mrs"]).decode('ascii').strip():
        print ('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern2) + '\x1b[0m')
    if subprocess.check_output(["cat", "/proc/version"]).decode('ascii').strip():
        print ('\x1b[6;30;42m' + '[✔] %s' % (kern3) + '\x1b[0m')
    if subprocess.check_output(["rpm", "-q","kernel"]).decode('ascii').strip():
        print ('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern4) + '\x1b[0m')
    else:
        print ('\x1b[1;32;41m' + '[✘] Unable To Obtain Kernel Version! [✘]' + '\x1b[0m')
        return

traceback from Atto Allas’s code:

   Traceback (most recent call last):
  File "Kern_Func_2.py", line 40, in <module>
    main()
  File "Kern_Func_2.py", line 35, in main
    kernel_info = get_kern_info()
  File "Kern_Func_2.py", line 20, in get_kern_info
    return_val = subprocess.check_output(command)
  File "/usr/lib/python3.5/subprocess.py", line 316, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.5/subprocess.py", line 383, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.5/subprocess.py", line 676, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1289, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'cat /proc/version'

I have also included an image of what is currently working in my program and to better illustrate desire results, including the kernels. I hope this better helps illustrate the dilemma.

Atto Allas · Accepted Answer · 2017-12-03 02:32:54Z

1

In my opinion, using a specific try: and except: is the most 'pythonic' way to do it. See the following example:

Replace:

if subprocess.check_output(["rpm", "-q","kernel"]).decode('ascii').strip():
    print ('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern4) + '\x1b[0m')
else:
    print ('\x1b[1;32;41m' + '[✘] Unable To Obtain Kernel Version! [✘]' + '\x1b[0m')
    return

With:

try:
    subprocess.check_output(["rpm", "-q","kernel"]).decode('ascii').strip()
    print('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern4) + '\x1b[0m')
except subprocess.CalledProcessError:
    print('\x1b[1;32;41m' + '[✘] Unable To Obtain Kernel Version! [✘]' + '\x1b[0m')
    return

or alternatively,

popen_check = subprocess.Popen(["rpm", "-q","kernel"])
popen_check.communicate()

if popen_check.returncode == 127: #127 is the command not found code
    print('\x1b[1;32;41m' + '[✘] Unable To Obtain Kernel Version! [✘]' + '\x1b[0m')
else:
    print('\x1b[6;30;42m' + '[✔] %s [✔]' % (kern4) + '\x1b[0m')

Note: I do not think your code will work, but this is how you would write your code, if it were correct. Also, I cannot write this code for you, because I don't actually know what you are trying to achieve.

An example of what your code might look like is this:

import subprocess

def get_kern_info():
    command_lists = [["uname", "-a"], ["uname", "-mrs"], ["cat /proc/version"], ["rpm", "-q","kernel"]]
    # cat is only one command because on some implementations, it breaks if the filepath is not directly in the same string as cat

    return_values = []

    for command in command_lists:
        try:
            return_val = subprocess.check_output(command)
            formatted_val = return_val.decode('ascii').strip()

            if formatted_val != '':
                return_values.append(formatted_val)
            else:
                return_values.append(str(command)+’ outputted nothing!’)

        except subprocess.CalledProcessError:
            return_values.append(str(command)+’ could not be called!’)
            continue

    return return_values

def main():
    kernel_info = get_kern_info()

    print("The data I got was: \n" + '\n'.join(kernel_info))

main()

Sample output from Windows bash terminal:

Hope this helps!

edited Dec 3, 2017 at 2:32

answered Dec 2, 2017 at 21:15

Atto Allas

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15 Comments

th0r Over a year ago

Atto Allas, I appreciate the help, but like you mentioned I believe my approach is flawed and I am still unable to get the desired results. I am attempting to use the subprocess call to check the kernel version, if present and output it with print using hex to color code version(s) found, but since I am using a debian based OS at the moment, it will obviously error out on the rpm request. I was hoping to make it a universal script, by skipping kernels not present on the machine and reporting only versions found. Also, I thought calling it through a class would be best to keep it tidy

Atto Allas Over a year ago

@th0r Just checking, do you want the function to output ALL of the data it can find (i.e. if it's a standard debian system, it would print 3 things) or just the first one it can find?

th0r Over a year ago

I would like it to print ALL output it can find, yes. So, as you stated Debian based OS would find the first three, 'uname -a', 'uname -mrs', 'cat /proc/version' and skip RPM if not found on the system. Trying to make the script universal as possible, as I add upon it. Thanks for any guidance.

Atto Allas Over a year ago

@th0r Updated my answer. I hope it helps (I have not been able to thoroughly test it as at the moment I am on my Windows partition)

th0r Over a year ago

I really appreciate the help, but the code executes(no errors), but returns no output. I will tinker with the previous examples and the latest provided example and hopefully figure it out or just take out the rpm call. Thanks again for taking time to help.

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