First or last symbols in string

Last 12 symbols from UUID:

uuidgen | tail -c 12

First 8 symbols from UUID:

uuidgen | head -c 8

Add user to group

sudo usermod -aG $GROUP $USER

Linux rescue

In grub menu select autoline and press e. Find the line which is started with linux and append

init=/bin/sh

to the end of this. After you will boot it (Ctrl-x), you will get a root shell immediately. But the filesystme will be in the readonly mode. To fix it execute:

mount -o remount, rw /