If you don’t use the mysql root account very often and change passwords reasonably frequently you may (like me) find that you no longer know the mysql root password on a dev box.
The mysql site has a reasonable guide here https://dev.mysql.com/doc/refman/5.0/en/resetting-permissions.html#resetting-permissions-unix
But there always seems to be something additional, this time for me it was apparmor preventing mysql from reading my reset file
to reset I had to look in /etc/apparmor.d/usr.sbin.mysqld and identify /etc/mysql/conf.d/ as a good place to place a reset init file
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