About halt in Linux in these places:

Indicate the following:

halt is usually to get to a state where you can perform low level maintenance.


You can use it to get the system to a state where you can perform low level maintenance.

Therefore: What does "you can perform low level maintenance" mean?

Some examples/scenarios to understand would be valuable. I want to know what is possible to do without the OS running and the PC with running (therefore electricity is available)

1 Answer 1


A quick history lesson: before the ATX form factor and associated power supply connections were introduced in 1995 or so, there was no standard way for desktop PCs to power themselves off. On laptops and some name-brand systems, there might have been vendor-specific power-off solutions, requiring a vendor-specific power supply. Also, the only standard software control interface for power-off function was APM, which (I think) used 16-bit BIOS calls, and so was awkward to use with new 32-bit OSs.

To power off the system, the user had to first shut down the OS, and operate a mechanical power switch only after the OS shutdown process had indicated that it was safe to do so.

After the ATX power supplies and the ACPI power management interface became common, it became the norm for any PC computers to automatically power themselves off at the end of the OS shutdown process.

On modern systems, automatic power-off is the norm. Even server-type systems started using automatic power-off after it was realized that combined with Wake-on-LAN or more advanced remote management technologies, it allowed remotely powering down unneeded servers in datacenters, enabling the possibility of power and cooling cost savings.

Today, the halt state is mostly a historical remnant that has very little use.

But if you need to physically poke at the system's innards while it's powered on (e.g. you wish to use a multimeter to check the voltage output levels of your desktop PC's power supply without disconnecting the power supply), it would be safest to do it while the system is in a halted state, so no data can be lost even if the system or any part of it loses power without warning.

Also, some systems may have diagnostic LEDs that indicate the location of a faulty component. If your system has something like that, and you need to open the chassis to e.g. identify a failing memory DIMM, you might halt the system to safeguard your data, but avoid powering down and potentially turning the diagnostic LEDs off before you know which component to replace.

Within a common desktop PC chassis, essentially only SATA/SAS disk connectors (both power and data) are guaranteed to be hot-pluggable at the hardware level. To be safe, I would assume that any other chassis-internal components are neither removable nor insertable while the system has power in any way unless I had documentation telling me the proper hot-plug/unplug procedures for that specific hardware model.

(Well, the chassis-internal USB cables might be relatively safe too... but only if you are absolutely sure you're plugging in the connectors in the correct orientation.)

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