Daryl,

Welcome to the community forums.

Which version of RHEL are you working with? In RHEL 5 the shutdown command was provided by SysVinit package, in RHEL 6 it is provided by the Upstart package and appears to have subtle differences.

RHEL 5 shutdown from SysVinit lists the options in --help that you have mentioned:

Usage:    shutdown [-akrhHPfnc] [-t secs] time [warning message]
                  -a:      use /etc/shutdown.allow
                  -k:      don't really shutdown, only warn.
                  -r:      reboot after shutdown.
                  -h:      halt after shutdown.
                  -P:      halt action is to turn off power.
                  -H:      halt action is to just halt.
                  -f:      do a 'fast' reboot (skip fsck).
                  -F:      Force fsck on reboot.
                  -n:      do not go through "init" but go down real fast.
                  -c:      cancel a running shutdown.
                  -t secs: delay between warning and kill signal.
                  ** the "time" argument is mandatory! (try "now") **

RHEL 6 shutdown from Upstart doesn't:

Usage: shutdown [OPTION]... TIME [MESSAGE]
Bring the system down.

Options:
  -r                          reboot after shutdown
  -h                          halt or power off after shutdown
  -H                          halt after shutdown (implies -h)
  -P                          power off after shutdown (implies -h)
  -c                          cancel a running shutdown
  -k                          only send warnings, don't shutdown
  -q, --quiet                 reduce output to errors only
  -v, --verbose               increase output to include informational messages
      --help                  display this help and exit
      --version               output version information and exit

I haven't checked RHEL 7 but I suspect shutdown will be provided by a systemd package which may have slight differences again.