System administration command. Insert a note in the file /var/log/wtmp; if the system is in runlevel 0 or 6, stop all processes; otherwise, call shutdown -h.
Suppress writing to /var/log/wtmp.
Call halt even when shutdown -nf would normally be called (i.e., force a call to halt, even when not in runlevel 0 or 6).
Place hard drives in standby mode before halt or power off.
Shut down network interfaces before halt.
No sync before reboot or halt.
Perform power off when halting system.
Suppress normal call to sync.
Suppress normal execution; simply write to /var/log/wtmp.