MariaDB 10.4.13 でのルートのパスワードを忘れた時に、再設定する方法です。
次のページを参考にしました。
[How To Reset Your MySQL or MariaDB Root Password](https://www.digitalocean.com/community/tutorials/how-to-reset-your-mysql-or-mari
adb-root-password)

mariadb の停止

sudo systemctl stop mysqld

セーフモードで mariadb を立ち上げる

sudo mysqld_safe --skip-grant-tables --skip-networking &

root でアクセス

$ mysql -u root
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 8
Server version: 10.4.13-MariaDB Arch Linux

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]>

グラントテーブルのロード

MariaDB [(none)]> flush privileges;
Query OK, 0 rows affected (0.003 sec)

root パスワードの変更

> ALTER USER 'root'@'localhost' IDENTIFIED BY 'tiger123';

結果を反映して、接続の遮断

MariaDB [(none)]> flush privileges;
Query OK, 0 rows affected (0.001 sec)

MariaDB [(none)]> exit;
Bye

セーフモードで動いている　mariadb の停止

$ ps ax | grep mysql
   5284 pts/1    S      0:00 sudo mysqld_safe --skip-grant-tables --skip-networking
   5285 pts/1    S      0:00 /bin/sh /usr/bin/mysqld_safe --skip-grant-tables --skip-networking
   5370 pts/1    Sl     0:00 /usr/bin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --user=mysql --skip-grant-tables --skip-networking --log-error=/var/lib/mysql/iwata.err --pid-file=iwata.pid
   5479 pts/1    S+     0:00 grep mysql
$ sudo kill -9 5284 5285 5370
[1]+  Killed                  sudo mysqld_safe --skip-grant-tables --skip-networking

mariadb を起動

sudo systemctl start mysqld

root で接続

$ mysql -uroot -ptiger123
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 10
Server version: 10.4.13-MariaDB Arch Linux

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]>