There is the following in the Sage FAQ

QUESTION: How do I run sage in daemon mode, i.e. as a service?

ANSWER: We currently do not have a
ready-to-go solution. There are
several possibilities: Use screen,
nohup or disown. We are tracking the
issue at
[http://trac.sagemath.org/sage_trac/ti...]
- so stay tuned.

Hmm, this ticket is 5 years old, so I guess there is no full daemon mode available anywhere soon. But I guess you are satisfied with something simpler.

1) Make a startscript sage_bgd.sh (assuming $SAGE_ROOT is in your path, else use full path) which just contains:

```
sage -notebook
```

make it executable and klick on it in the GUI - the notebook server should start without terminal emulator.

2) Depending which Linux variant you use there should be alternative terminals available. E.g I can switch with Ctr-Alt-F1/F2/F3 between 3 different terminals.
(On my system F1 has the default X session and F4 switches to the GUI, but that might be different for your system). But I can press Ctrl-Alt-F2, then log in with username/password, then start the sage notebook server (simplest with sage -notebook), and then go back with Ctrl-Alt-F4 to the GUI. The sage server is then running on tty2, I can use localhost:8000 in the X-session to log in to the notebook without any open terminal there.

there will be more sophisticated variants of course...