[CentOS] halt versus shutdown

On Sun, Jun 14, 2020 at 5:20 PM Pete Biggs <pete at biggs.org.uk> wrote:

>
> > fwiw, i've always used 'init 0' to shut down all sorts of unix/linux
> > systems.
>
> In EL7/EL8, init is now a symlink as well because everything is
> controlled by systemd.
>
> >   On old school unix, and I think even early Linux, halt was an
> > /immediate/ halt, as in catch fire.   might as well hit the power switch.
> >
> Not quite. Shutdown is a timed thing so you can tell it to shutdown or
> reboot at a certain time or after a certain delay and it can broadcast
> messages to the users - it's useful on multi-user systems to be able to
> warn users that the system is about to go down. Halt is an immediate
> thing without any broadcast messages or delay but it does do the halt
> cleanly.  There is an option to halt to not sync the disks - this is
> not a wise thing to do and is an emergency option - certainly the
> original man pages for halt said something like "only do this if your
> disks are on fire".



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.




-- 
-john r pierce
  recycling used bits in santa cruz