System administration command. Close out filesystems, shut down the system, and power off. Because this command immediately stops all processes, it should be run only in single-user mode. If the system is not in runlevel 0 or 6, poweroff calls shutdown -h, then performs a poweroff.
Suppress writing to /var/log/wtmp.
Call reboot or halt and not shutdown, even when shutdown would normally be called. This option is used to force a hard halt or reboot.
Place hard drives in standby mode before halt or poweroff.
Shut down network interfaces before reboot.
Suppress normal call to sync.
Suppress normal execution; simply write to /var/log/wtmp.