Comments - Upgrading from MySQL to MariaDB
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See Troubleshooting Connection Issues. You may want to consider whether you actually need to run mysql_secure_installation, and whether you actually have a root password, but instead are running unix socket authentication. See Authentication from MariaDB 10.4.
Not knowing any way to get access other than "$ sudo mysql", and knowing any other way into it, that was the only path I knew. I solved the immediate problem by running mysql_secure_installation as root, and set a MySQL root password; problem solved. But now I'll go to that link you gave because I need to educate myself on this topic, too, Actual SQL will have to wait.