code: public function actionIndex($command = null)
      {
          if ($command !== null) {
              $result = Yii::$app->createController($command);
              if ($result === false) {
                  $name = $this->ansiFormat($command, Console::FG_YELLOW);
                  throw new Exception("No help for unknown command \"$name\".");
              }
  
              list($controller, $actionID) = $result;
  
              $actions = $this->getActions($controller);
              if ($actionID !== '' || count($actions) === 1 && $actions[0] === $controller->defaultAction) {
                  $this->getSubCommandHelp($controller, $actionID);
              } else {
                  $this->getCommandHelp($controller);
              }
          } else {
              $this->getDefaultHelp();
          }
      }
docs: Displays available commands or the detailed information
  about a particular command.
  
  @param string $command The name of the command to show help about.
  If not provided, all available commands will be displayed.
  @return int the exit status
  @throws Exception if the command for help is unknown