just installed mysql password
$ sudo cat /etc/mysql/debian.cnf
Note the lines which read:
user = debian-sys-maint
password = blahblahblah
Then:
$ mysql -u debian-sys-maint -p
Enter password: // type 'blahblahblah', ie. password from debian.cnf
mysql> USE mysql
mysql> SELECT User, Host, plugin FROM mysql.user;
+------------------+-----------+-----------------------+
| User | Host | plugin |
+------------------+-----------+-----------------------+
| root | localhost | auth_socket |
| mysql.session | localhost | mysql_native_password |
| mysql.sys | localhost | mysql_native_password |
| debian-sys-maint | localhost | mysql_native_password |
+------------------+-----------+-----------------------+
4 rows in set (0.00 sec)
mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> COMMIT; // When you don't have auto-commit switched on
Either:
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY 'new_password';
Or:
// For MySQL 5.7+
UPDATE mysql.user SET authentication_string=PASSWORD('new_password') where user='root';
Then:
mysql> FLUSH PRIVILEGES;
mysql> COMMIT; // When you don't have auto-commit switched on
mysql> EXIT
$ sudo service mysql restart
$ mysql -u root -p
Enter password: // Yay! 'new_password' now works!