code: def get_variable_name_from_bird(bird_conf):
      
      bird_variable_pattern = re.compile(
          r'''
          ^\s*
          define\s+
          (?P<name>\S+\b)
          \s+
          =
          ''', re.VERBOSE
      )
  
      with open(bird_conf, 'r') as content:
          for line in content.readlines():
              variable_match = bird_variable_pattern.search(line)
              if variable_match:
                  return variable_match.group('name')
  
      return None
docs: Return the variable name set in Bird configuration.
  
      The variable name in Bird configuration is set with the keyword 'define',
      here is an example:
  
          define ACAST_PS_ADVERTISE =
  
      and we exract the string between the word 'define' and the equals sign.
  
      Arguments:
          bird_conf (str): The absolute file name path of Bird configuration.
  
      Returns:
          The variable name as a string or None if it isn't found.