How can I find out my user name ( system login username in ubuntu 20.04 )?
How do i find my system login username in Ubuntu 20.04.3 LTS ?
Use this solution if you didn't encrypt your home directory.
Do you still have the installation media(LiveCD/USB) from which you installed Ubuntu? Boot from it and select Try Ubuntu.
Go to Files and then, you will see you already installed Ubuntu partition. Click on it, it will mount.
Go to /home. Here you will see a list of all users on the system that you have created.
To jog your memory, however, what is your name? Did you enter the same name while installing Ubuntu? Do you recall the name that was displayed on the login prompt where you entered your password? Ubuntu by default sets the username as your first name in lowercase.
Example; name= John Doe, automatically set username= john
souce : https://askubuntu.com/a/704873
