The haltsys utility uses uadmin(ADM) to flush out pending disk I/O, mark the filesystems as clean, and halt the processor. haltsys takes effect immediately, so user processes should be killed beforehand. shutdown(ADM) is recommended for normal system shutdown, since it warns users, terminates processes, then calls haltsys. Use haltsys directly only if you cannot run shutdown; for example, because of some system problem.
haltsys displays a prompt indicating that the system has been shut down and can be rebooted or powered down. If the -d option is used, the system will remain down and you are not given the option to reboot.
The reboot command performs the same function as haltsys, except that the system is rebooted automatically afterwards.
Only the super user can execute haltsys or reboot.
reboot is not part of any currently supported standard; it was developed at the University of California, Berkeley and is used with permission.