code: def do_execute(self):
          
          directory = str(self.resolve_option("dir"))
          if not os.path.exists(directory):
              return "Directory '" + directory + "' does not exist!"
          if not os.path.isdir(directory):
              return "Location '" + directory + "' is not a directory!"
          collected = []
          result = self._list(directory, collected)
          if result is None:
              for c in collected:
                  self._output.append(Token(c))
          return result
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str