This question already has an answer here:
- su command + authentication failure 7 answers
ubuntu@ubuntu:~$ su Password: su: Authentication failure
I installed Ubuntu to a USB stick. I didn't set any password any time - why can't I use su?
There is normally no need to set a root password, you can do everything by using the
If you still want to change the root password, you will need to open a root shell by entering
in the terminal. If you're using the LiveCD version of Ubuntu, it won't ask you for password, otherwise it will ask you for the password of the user who is currently logged in.
The actual password for root account is generated (and thus unknown), if you want to set it to a known value, you can enter the following in a root shell:
This will prompt you for a new password without asking for the old one.