How do I use shutdown command?
The shutdown command can be used to turn off or reboot a computer. Type the command as follows to shutdown server / computer immediately:
$ sudo shutdown -h now
OR
$ sudo shutdown -h 0
How do I shutdown compute at specific time?
To shutdown computer at 6:45pm, enter:
$ sudo shutdown -h 18:45 "Server is going down for maintenance"
At 6:30pm message will go out to all user and 6:45 system will shutdown.
Please note that you can also use halt or poweroff or reboot command for stopping and restarting the system:
$
sudo halt
OR
$
sudo poweroff
How do I reboot computer?
Simply use reboot command:
$
sudo reboot
OR
$
sudo shutdown -r 0
source: http://www.cyberciti.biz/faq/shutdown-ubuntu-linux-computer/