Actually I don't use red hat, I sue fc2, forgot to upgrade my info...sorry...
anyway, I tried switcdesk and startx, the following came out,
[root@cc]# switchdesk blackbox
Red Hat Linux switchdesk 4.0
Copyright (C) 1999-2004 Red Hat, Inc
Redistributable under the terms of the GNU General Public License
Unknown desktop requested: blackbox
[root@cc /]# startx
Fatal server error:
Server is already active for display 0
If this server is no longer running, remove /tmp/.X0-lock
and start again.
Please consult the The X.Org Foundation support
thanks a lot for any help!