enable password in mysql root user in mysql 8

use mysql;

update user set authentication_string=PASSWORD("mynewpassword") where User='root';

flush privileges;

quit

1. If you in skip-grant-tables mode in mysqld_safe:

mysql> UPDATE mysql.user SET authentication_string=null WHERE User='root';
mysql> FLUSH PRIVILEGES;
mysql> exit;

and then, in terminal:

$ mysql -u root

in mysql:
mysql> ALTER USER 'root'@'localhost' IDENTIFIED WITH caching_sha2_password BY 'yourpasswd';


2. Not in skip-grant-tables mode just in mysql:
mysql> ALTER USER 'root'@'localhost' IDENTIFIED WITH caching_sha2_password BY 'yourpasswd';