I can sudo, but I don't have the root password so I can't
su root. Using sudo, can I change the root password?
So you want to run something like
sudo passwd root?
You can also just use
and keep the root account without password.
Note that you also want to log in using
su - instead of
su (more info).
If you have your sudo password then try the following (this is on Ubuntu 14.04LTS):
ewan@user-sbc-ihsw:~$ sudo -i [sudo] password for ewan: root@user-sbc-ihsw:~# passwd Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully root@user-sbc-ihsw:~# whoami root
you can also use sudo with the "-i" option. this will log you in as root without using the su command.
You can change all these options in sudoers file, even make no password if you want( I would not recommend this though).