man <command>

OPTIONS
   -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.