So how do you shutdown or reboot Ubuntu Linux from a terminal or a shell prompt? If GUI is working you can always click on a Quit button. If GUI is not working or if you are working remotely over ssh type the following command:

To shutdown / poweroff Ubuntu Linux

Type the command:

sudo halt

OR

sudo shutdown -h now

OR

sudo shutdown -r now

OR

sudo init 6

To reboot Ubuntu Linux

Type the command:

sudo reboot

More information can be found about these two commands by typing following commands (man page):

man reboot
man shutdown