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      libunique lets you send messages with commands to a running
      instance using unique_app_send_message(). The commands can be either
      predefined or custom. Some of the predefined libunique commands have
      equivalents in GApplication. Instead of sending the UNIQUE_ACTIVATE
      command, call g_application_activate(), instead of sending the
      UNIQUE_OPEN command, call g_application_open(). The
      UNIQUE_NEW and UNIQUE_CLOSE and user-defined commands don't
      have direct replacement at this time.
    
      As a replacement for custom commands, GApplication implements the
      GActionGroup interface and lets you add a group of actions with
      g_application_set_action_group(). The actions can then be invoked,
      either by using the D-Bus interface for GAction directly, or by
      calling g_action_group_activate_action() from another instance of
      the GApplication. The GApplication documentation contains an
      example for using GApplication with actions.
    
For more complex needs, GApplication supports passing entire commandlines to the running instance.