There is no specific command to logout in an Ubuntu system.

Why ?

Here’s how it works. The desktop you’re seeing is a separate software. They are called Desktop Environments (DE). The default DE of Ubuntu is Unity from Ubuntu’s 11.04 versions. Before it was GNOME. There are many DEs for Ubuntu. Some of them are :

• Openbox
• XFCE
• LXDE

and many more.. There are also separate Login Managers to provide the login screen while Ubuntu is loaded. When you log in, the Login Manager starts the default DE set up on your system and when this DE program is closed or terminated, it will go back to the Login Screen or Login Manager.