shutdown: What is difference between "Power Off" and "Halt"?

With your typical Linux shutdown command, what is the difference between halting the system and powering off the system?

Solution 1:

Halting involves stopping all CPUs on the system. Powering off involves sending an ACPI command to signal the PSU to disconnect main power.

Solution 2:

IIRC

# shutdown -H now

will halt the system - meaning the system will shutdown and at the end stop at a screen with the last message beeing something like "System halted".

# shutdown -P now

will power off the system - meaning the system will shutdown and at the end power off (only possible if the system actually supports it but most systems I know have for quite a while now)

# shutdown -h now

will only halt or power off the system depending on what's the default on that system (can sometimes be changed in BIOS)

Solution 3:

If the machine in question does not have power management capabilities, there isn't a difference. If it does, the difference is power off will send the signal to power the machine off, Halt will shut everything down and it will wait at a "Power Off" prompt, meaning it is safe to hit the power switch.