GUI内终端执行如下指令，或者加入开机启动脚本内。
gsettings set org.gnome.settings-daemon.plugins.power button-power shutdown
will change your the behaviour of the power button of your system from interactive (default) to immediate normal shutdown.
The command is gsettings set org.gnome.settings-daemon.plugins.power button-power <option>
Available options are:
blank: does nothing
suspend: makes your system suspend
shutdown: starts a normal issued shutdown procedure
hibernate: hibernates your system
nothing: does nothing
log out: log your user out