Create new user, to use a public key

create a new user

sudo useradd fred

and set password:

sudo passwd fred

(tip - You can list all users by looking in the file /etc/passwd, with cat /etc/passwd)

give that user sudo privileges

Add a user to the sudo group with:

gpasswd -a fred sudo

(here's another way... usermod -aG sudo fred ...?)

You must be root or have sudo privileges in order to do this.

test that the new user can use sudo privileges

use su (switch user) to switch to the new user...

su - fred

(you will need to use their password to do this)

Try and do this... (it shouldn't let you...)

ls -la /root

Now sudo do it!

sudo ls -la /root

(and enter fred's password...)

test that the new user can connect through SSH