HOW TO SHUTDOWN AND RESTART LINUX

How do I use shutdown command?

NOTE: Use commands without sudo for other linux distributions

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 computer 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