What is the difference between Halt and Shutdown commands?

Question

What is the difference between the halt and shutdown commands?

Answer 1

Generally, one uses the shutdown command. It allows a time delay and warning message before shutdown or reboot, which is important for system administration of multiuser shell servers; it can provide the users with advance notice of the downtime.

As such, the shutdown command has to be used like this to halt/switch off the computer immediately (on Linux and FreeBSD at least):

shutdown -h now

Or to reboot it with a custom, 30 minute advance warning:

shutdown -r +30 "Planned software upgrades"

After the delay, shutdown tells init to change to runlevel 0 (halt) or 6 (reboot). (Note that omitting -h or -r will cause the system to go into single-user mode (runlevel 1), which kills most system processes but does not actually halt the system; it still allows the administrator to remain logged in as root.)

Once system processes have been killed and filesystems have been unmounted, the system halts/powers off or reboots automatically. This is done using the halt or reboot command, which syncs changes to disks and then performs the actual halt/power off or reboot.

On Linux, if halt or reboot is run when the system has not already started the shutdown process, it will invoke the shutdown command automatically rather than directly performing its intended action. However, on systems such as FreeBSD, these commands first log the action in wtmp and then will immediately perform the halt/reboot themselves, without first killing processes or unmounting filesystems.

Answer 2

I suspect this is somewhat dependant on which version of UNIX/Linux you are using. On Centos (and I expec other modern Linux) halt calls shutdown (providing you're not at runlevel 0 or 6) so your system will be shutdown cleanly. On Solaris 10 halt is more brutal, it just flushes the disk caches and powers off the system - no attempt is made to run any scripts or shutdown smf facilities.

Answer 3

In linux, "halt" and "reboot" are aliases of the shutdown command -- shutdown -h and shutdown -r respectively. Bareword shutdown generally assumes -h.