docs: def get_user_id(username):
      """
      Returns the user id to a given user name. Returns ``None`` if the user does not exist.
  
      :param username: User name.
      :type username: unicode
      :return: User id.
      :rtype: int
      """
      uid = single_line_stdout('id -u {0}'.format(username), expected_errors=(1,), shell=False)
      return check_int(uid)