execute : shutdown
 
shutdown
Shut down the FortiGate unit now. You will be prompted to confirm this command.
 
Abruptly powering off your FortiGate unit may corrupt its configuration. Using the reboot and shutdown options here or in the web-based manager ensure proper shutdown procedures are followed to prevent any loss of configuration.
Syntax
execute shutdown [comment <comment_string>]
comment is optional but you can use it to add a message that will appear in the event log message that records the shutdown. The comment message of the does not appear on the Alert Message console. If the message is more than one word it must be enclosed in quotes.
Example
This example shows the reboot command with a message included.
execute shutdown comment “emergency facility shutdown”
An event log message similar to the following is recorded:
2009-09-08 11:12:31 critical admin 41986 ssh(172.20.120.11) shutdown User admin shutdown the device from ssh(172.20.120.11). The reason is 'emergency facility shutdown'