This command will put your request for nodes in the queue. Since there is not script to run, and the system doesn't know what you intend to do with the nodes, you will have to take care yourself of starting whatever tasks you need. For that you will need to know (a) when the job starts, and (b) which nodes it gets.

By default, when cmgrab is run without the -w option, it will remain waiting until the job starts, at which point it will notify you, save the nodes in a file named <.nodes, and exit. Note that once cmgrab notifies you that the job had been submitted, killing it while it's waiting for the job to start will not cancel the job (use canceljob for that).

If you run cmgrab with -w, it will exit after submitting the job and making sure it's in the queue. You will have to monitor the queue (see section 2 above, particularly cmwait) to know when the job starts, and use cmgetnodes to get the list of allocated nodes once the job has become active.