Here is a quick tip to kill a crashed Linux / UNIX X desktop system. Many new user do not know this simple tip and end up hitting computer reboot button. Press CTRL + ALT + Backspace to kill GUI and get back to login screen. There are more ways to kill a crashed desktop without restarting your computer.
If CTRL + ALT + Backspace refused to work, try to login to console by pressing CTRL +ALT + F1. To kill GDM (Gnome Desktop) manger, enter:
You can also run the following:
To start GDM again, enter:
To kill KDM (KDE Desktop), enter:
To start KDM again, enter:
This is useful for killing desktop session. You can always kill indidual process such as a web server or firefox using kill / killall command line option. Under X windows you can use xkill command kill a client by its X resource and not by process ID.