Как назначить номер приоритета для «update-alternatives --config gdm3.css» в одной командной строке?

На основе комментария @PRATAP я разработал это python3.6 решение своего вопроса. Это работало над моей системой Ubuntu 18.04. Я надеюсь, что это может принести пользу другим с той же потребностью.

#!/usr/bin/env python3.6
# -*- coding: utf-8 -*-

from subprocess import run, PIPE
from pathlib import Path
import mimetypes

class CSSFileTypeError(Exception):
    pass

class GDM3_alternatives:
    '''Class to query and configure gdm3.css.

    Argument:
      mytheme - path to my gnome-shell theme .css file.

    Attributes:
      mytheme - path to my gnome-shell theme .css file.
      query   - stdout from "update-alternatives --query gdm3.css" store in a list
      link    - gdm3.css path
      best    - gdm3.css alternative path selected by auto mode 
      value   - current gdm3.css alternative path 
      status  - whether gdm3.css is selected by manual or automatic mode 
      max     - maximum Priority value of all the installed gdm3.css alternatives

    Methods:
      exist()     - determines whether "mytheme" is installed as a gdm3.css alternative.
      configure() - configure "mytheme" file as a gdm3.css alternative.
    '''
    def __init__( self, mytheme=None ):       
        def _get( qvalue ):
            return [ line[ line.index('/') : ] for line in self.query if qvalue in line ][0]
        self.mytheme = mytheme
        self.query = run( [ 'update-alternatives', '--query', 'gdm3.css' ],
                          stdout=PIPE, encoding="utf-8" ).stdout.splitlines()
        self.link   = _get( 'Link:' )
        self.best   = _get( 'Best:' )
        self.value  = _get( 'Value:' )
        self.status = [ line[ line.index(':')+2 : ] for line in self.query if 'Status:' in line ][0]
        self.max = max( [ int( line[ line.index(':')+1 : ] ) for line in self.query if 'Priority:' in line ] )
        #print( f'self.query = {self.query}' )  #For debugging
        #print( f'self.link  = {self.link}' )   #For debugging
        #print( f'self.best  = {self.best}' )   #For debugging
        #print( f'self.value = {self.value}' )  #For debugging
        #print( f'self.status= {self.status}' ) #For debugging
        #print( f'self.max   = {self.max}' )    #For debugging

    def exist( self ):
        '''Method that determines whether "mytheme.css" is installed as a gdm3.css alternative. '''
        if self.mytheme == None:
            raise TypeError( '.css file was not defined.' ) 
        if not Path( self.mytheme ).exists():
            raise FileNotFoundError( f'{self.mytheme} does not exist.' )
        if 'css' not in mimetypes.guess_type( self.mytheme )[0] :
            raise CSSFileTypeError( f'{self.mytheme} is not a css file.' )
        return True in [ True for line in self.query if self.mytheme in line ]

    def configure( self ):
        '''Method to configure my theme ".css" file as a gdm3.css alternative.'''
        def _config():
            if 'auto' not in self.status:
                run( [ 'update-alternatives', '--auto', 'gdm3.css' ] ) #Ensure auto mode is used
            run( [ 'update-alternatives', '--install', self.link, 'gdm3.css', self.mytheme, str(self.max + 1) ] )
            print( f'Configured {self.mytheme} as gdm3.css alternative.' )

        if not self.exist():
            _config()
        elif self.value in self.mytheme:
            print( f'{self.mytheme} is already gdm3.css alternative.' )
        else:
            run( [ 'update-alternatives', '--remove', 'gdm3.css', self.mytheme ] )
            self.__init__( self.mytheme )
            _config()

if __name__ == '__main__':
    #mytheme = '/usr/share/gnome-shell/theme/mytheme/mytheme.css' 
    mytheme = '/usr/share/gnome-shell/theme/mytheme/mytheme.css' #Change this to your theme
    gdm3 = GDM3_alternatives( mytheme )
    gdm3.configure()

## This script needs to be executed by sudo. ## 

, Как использовать этот сценарий:

1. Сохраняют его в файл с .py расширение, например, myscript.py.
2. строка Изменения mytheme = '/usr/share/gnome-shell/theme/mytheme/mytheme.css' для показа темы .css путь.
3. Запущенный скрипт на терминале с sudo разрешением , т.е. тип sudo python3.6 myscript.py. Или откройте свой Python НЕАКТИВНОЕ использование sudo разрешение и запустите этот скрипт Python.

версия update-alternatives в моей системе Ubuntu 18.04:

$ update-alternatives --version
Debian update-alternatives version 1.19.0.5.

This is free software; see the GNU General Public License version 2 or
later for copying conditions. There is NO warranty.