One way to check if it's still working is using top command, and check the top cpu usage. If it contains tracker, I think it's working on that.

It'll be slow if you have many sms to backup, and background apps may also slow it down. So it's better to close all applications before running, especially the Message app.

closing the terminal should be fine to close it, while the better way is to get the pid by ps and kill it, and check again, so it's surely closed