Why does 'halt' require '-p' to turn off the computer?
 

This is more of a curiosity question than an actual problem.

If you run
the OS shuts down, but the computer's power stays on, and to turn it off you have to press and hold the power button. I don't see why this is useful. Maybe someone has a different view on it?

When I shut down from a terminal, I use:
The system shuts down and the machine powers off. In fact, I have never used the -P option. If the command you are using is simply halt, perhaps it puts the systems into hibernation? I do not know, just guessing.

you can find a lot of threads on the net about it, like these: http://www.computerhope.com/unix/uhalt.htm and http://serverfault.com/questions/191...r-off-and-halt.
The most important difference is that halt will not send ACPI commands to the power supply. Long time ago those power supplies could not be set to off by ACPI, just by hand. Nowadays you are allowed to stop your box this way or automatically switch power off too.
So you can say, historical reason...

I love history. Thanks.

As an alternative, I tend to use I believe does just that... It stops further processing and the everything is in a halted state. I think that at this point would resume any processes which were halted.

BTW, In the words of Egon Spengler, I believe that then powering down via the big grey button is a BAD thing, as you could lose data. You should use instead to write any cached info back to disk before powering off.

IBM, (doncha love 'em; dasd = Disk, ROS = ROM) have to be different with used to shut down AIX boxes as I recall. :D

Play Bonny!

:hattip:

as far as I know you should first sync (twice), before halt. You cannot do anything if system was halted. That continue does not exist (or it is called warm boot probably).

@pan64
You are, of course, absolutely correct. Is there not a way out of a halt situation? I thought there was. Maybe I'm confusing it with a on a Sun box which drops you to the OK prompt where will resume.

Play Bonny!

:hattip:

Thanks for the insights!

On the computers I've used, 'halt' runs the computer through its normal shutdown routine (at least, I see all the same messages on the screen) and when it's done it sits there. I can't type 'sync' or 'continue' because the cursor is frozen.

After I posted this thread, I started to think about it, and my thoughts were like what pan64 said, that omitting the '-p' doesn't send the 'power off' command to the PSU.

I normally, to turn my computer off as root just type
Or
in a sudo base distro like Ubuntu