On Sun, Jun 14, 2020 at 6:19 PM Pete Biggs pete@biggs.org.uk wrote:
I'm quite sure that in original Berkeley Unix, as on the VAX 11/780, halt was an immediate halt of the CPU without any process cleanup or file
system
umounting or anything. Early SunOS (pre-Solaris) was like this, too.
The SunOS 4.1.2 man page for halt says
NAME halt - stop the processor SYNOPSIS /usr/etc/halt [ -oqy ] DESCRIPTION halt writes out any information pending to the disks and then stops the processor. halt normally logs the system shutdown to the system log daemon, syslogd(8), and places a shutdown record in the login accounting file Ivar/admlwtmp. These actions are inhibited if the -0 or -q options are present.
The BSD 4.3 (that ran on VAXen) man pages say largely similar things:
https://www.freebsd.org/cgi/man.cgi?query=halt&apropos=0&sektion=0&a...
ok, so it does a sync then hard halts, but it doesn't gracefully exit services, or unmount file systems.