-r
Requests that the system be rebooted after it has been brought down.

-h
Requests that the system be either halted or powered off after it has been
brought down with the choice as to which left up to the system.

-H
Requests that the system be halted after it has been brought down.

-P
Requests that the system be powered off after it has been brought down.

-c
Cancels a running shutdown. TIME is not specified with this option, the
first argument is MESSAGE.

-k
Only send out the warning messages and disable logins, do not actually bring
the system down.
