After executing grails run-app, except using Ctrl + C", is there a command to shutdown it?
No. grails run-app is intended to be run for development, interactively.
If you want to control grails as a service, you should deploy it to a web application container such as tomcat. The tomcat plugin allows you to easily deploy your app to tomcat, for example. Add lines like
tomcat.deploy.username="manager"
tomcat.deploy.password="secret"
tomcat.deploy.url="http://myserver.com/manager"
to Config.groovy and then you can use
grails tomcat deploy
grails tomcat undeploy
to start and stop your application. Alternatively, you can use grails war to bundle your app into a war archive which all java app servers should be able to use.
If you really want to stop grails run-app without Ctrl+C, write a small controller that calls System.exit(0). Then browse to that URL, or write a small shell script or batch file that invokes it with e.g. wget or curl.