Stop the mysq database service.
service mysql stop
Create needed unix socket directory first.
mkdir -p /var/run/mysqld && chown mysql:mysql /var/run/mysqld
Start the server in safe mode in the background.
mysqld_safe --skip-grant-tables --skip-networking &
Open the MySQL client as root.
mysql -u root
Change the root user.
UPDATE `mysql`.`user`
SET `authentication_string`=PASSWORD('the-root-password'), `password_expired`='N'
WHERE `User`='root' AND `Host`='localhost';
Shutdown mysql in safe mode.
mysqladmin -S /var/run/mysqld/mysqld.sock shutdown
Start the service again.
service mysql start
This happens when an older version backup is restored on a newer version.
Create needed /var/run/mysqld directory for the socket.
mkdir -p /var/run/mysqld && chown mysql:mysql /var/run/mysqld
Start mysqld_safe.
mysqld_safe --skip-grant-tables &
Login to MySQL as root.
mysql -u root
Use mysql database.
use mysql
Update password to nothing.
update user set authentication_string=PASSWORD("") where User='root';
Set password resolving to default mechanism for root user
update user set plugin="mysql_native_password" where User='root';
Stop the mysqld_safe appliaction.
mysqladmin shutdown