One of the changes that were seen in the new Ubuntu Unity is the default function of Ctrl+Alt+Del. Ctrl Alt Del earlier used to give option for restart and shut down but now it just gives option for Log Out.
Lets see what we need to do in order to use Ctrl Alt Del for Shut Down instead of Log Out in Ubuntu 12.04. Go in Unity Dash and look for Keyboard:
Go for Shortcut tab and look for Custom Shortcut option in left side bar. Click on the + sign on the lower bottom panel to add a new shortcut. The pic below might be helpful:
Now name the custom shortcut. I have named it Shutdown. Choice is all yours. Put the following command in the command field:
That is it. Restart your computer and it is ready to work: