It may sometime happen that our Linux powered system hangs completely and the only option available is to shut down the system the crude way via the power button.

Using the  power button to shut down a frozen system can severely affect your Linux installation, specially if you initiated the shut down when data was being written into the hard disk.

Linux has an option for a safe restart or shutdown.

Hold down Alt+Sytem requirement. If you are using a laptop with the system requirement accessible through the function keys, use the Alt+Fn+Sys Req combination.

To restart, type REISUB in one go.

To shut down, type REISUO in one go.

What it does:

With every keypress of REISUB or REISUO, you initiate an action.

R: Switch the keyboard from raw mode to XLATE mode

E: Send the SIGTERM signal to all processes except init

I: Send the SIGKILL signal to all processes except init

S: Sync all mounted filesystems

U: Remount all mounted filesystems in read-only mode

B/O: Immediately reboot/shut down the system, without unmounting partitions or syncing

This comes in handy as even the ctrl+alt+backspace  combination fails to respond in some situations.