The script look dangerous to me.
You might stop a running instance by accident, I would use an explicit start/stop/restart parameter.
Also you can not run several JBoss instances at the same host as the script is not able to divide and stop one existing instance or try to stop a non JBoss process if there is a process found with the jboss.server.base.dir.
You might use the JBOSS_HOME to store a file with the PID if you start, with that file you can check the running instance.
I probably should have clarified that this script will execute on a server dedicated to only running JBOSS and we only run a single instance of JBOSS - so this on/off toggle concept is not dangerous in my opinion. That's not to say this is a good concept for all situations.
I like the suggestion to use parameters - I think I might introduce that to the script.
Thanks allot for your feedback.