reading data from command line

[mcmxl22]

I want to automate updating python packages through pip. I think I have most of it. I just don't know how to evaluate the output from results = os.system('pip list --outdated'). I thought putting 0 for none would be sufficient but it doesn't work. It gives me a list of outdated packages but says they are all up to date. It never runs lines 12-21.

def pkg_update():
    options = ['Check for updates?', '1 Yes', '2 Exit']
    for option in options:
        print(option)
    choice = input('> ')
    if choice == '1':
        print('Checking for updates.')
        results = os.system('pip list --outdated')
        if results == 0: # What do I put here?
            print('All packages are up to date.\n')
            pkg_update()
        else:
            print(results)
            while True:
                update_list = list(results)
                current = update_list.pop(0)
                print(f'Updating {current}.')
                os.system(f'''pip install --upgrade
				             {current}''')
                print('Done')
            pkg_update()

    elif choice == '2':
        raise SystemExit

    else:
        pkg_update()


if __name__ == "__main__":
    pkg_update(