4.1.5 How to shut down
Just like any other modern OSs where the file operation involves caching data
in the memory, the Debian system needs the proper shutdown procedure before
power can safely be turned off to maintain the integrity of files. Use the
following command from the root command prompt to shutdown the system:
# shutdown -h now
This is for the normal multiuser mode. If you are in the single-user mode, use
following from the root command prompt:
# poweroff -i -f
Alternatively, you may type Ctrl-Alt-Delete to shutdown. 
Wait until the system displays "System halted" then shut off power.
If the APM or ACPI function has been turned on by the BIOS and Linux properly,
the system will power down by itself. See Large memory and auto power-off, Section
3.8.4 for the detail.