If you work with the new gnome 3 as your desktop you probably noticed
that clicking the power button doesn’t shut the computer , in my case it just restart it .
in order to set the power of there is a small tweak to be made :
open dconf-editor and navigate to : org -> gnome -> settings–daemon -> plugins ->power
change power-button from none to shutdown
now when you press power button , the system will shut it self down