When you are going to install MySql in your ubuntu (Linux machine) 

First Step:

run command sudo apt install mysql-server

this command will install mysql-server your machine. After successful installation. Type

sudo mysql -u root

This will allow to login into mysql without entering password. now set the password for root user.

Now set password for user ‘root’ 

alter user root@localhost identified by ‘Pass@123’;

this will set the password for user root@localhost.

now you can sign out by typing exit. and again login with root user and password. 

sudo mysql -u root -p 

You can run any command like show databases;