Ubuntu Server 18.04 LTSにMariaDBをインストールする

ぬにょす(挨拶)。

データベースとしてマリア様をインストールします。

$ sudo apt install mariadb-server mariadb-client
Code language: plaintext (plaintext)

初期設定をします。

$ sudo mysql_secure_installation

NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MariaDB
      SERVERS IN PRODUCTION USE!  PLEASE READ EACH STEP CAREFULLY!

In order to log into MariaDB to secure it, we'll need the current
password for the root user.  If you've just installed MariaDB, and
you haven't set the root password yet, the password will be blank,
so you should just press enter here.

Enter current password for root (enter for none):
OK, successfully used password, moving on...

Setting the root password ensures that nobody can log into the MariaDB
root user without the proper authorisation.

Set root password? [Y/n]
New password:
Re-enter new password:
Password updated successfully!
Reloading privilege tables..
 ... Success!

By default, a MariaDB installation has an anonymous user, allowing anyone
to log into MariaDB without having to have a user account created for
them.  This is intended only for testing, and to make the installation
go a bit smoother.  You should remove them before moving into a
production environment.

Remove anonymous users? [Y/n]
 ... Success!

Normally, root should only be allowed to connect from 'localhost'.  This
ensures that someone cannot guess at the root password from the network.

Disallow root login remotely? [Y/n]
 ... Success!

By default, MariaDB comes with a database named 'test' that anyone can
access.  This is also intended only for testing, and should be removed
before moving into a production environment.

Remove test database and access to it? [Y/n]
 - Dropping test database...
 ... Success!
 - Removing privileges on test database...
 ... Success!

Reloading the privilege tables will ensure that all changes made so far
will take effect immediately.

Reload privilege tables now?
 ... Success!

Cleaning up...

All done!  If you've completed all of the above steps, your MariaDB
installation should now be secure.

Thanks for using MariaDB!
Code language: plaintext (plaintext)

サービスを再起動します。

$ sudo systemctl restart mariadb
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MariaDBに接続して、データベースとユーザーを作成し、権限を付与します。

$ mysql -u root -p
Enter password:
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
Code language: plaintext (plaintext)

はじかれました(´･_･`)

私なりに色々と調べてみたんですけどね、はじかれる理由や直す設定が分からんのです。
分からんのですが、

$ sudo mysql
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 60
Server version: 10.1.38-MariaDB-0ubuntu0.18.04.1 Ubuntu 18.04

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)]>
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sudoを付けるとmysqlだけで接続できちゃう不思議(´･_･`)
rootパスワードは要らない子ですか、そうですか…。
正直、めっちゃ気持ち悪いんですけど、やることやっちゃいます。

create database db_name;
create user 'db_user'@'localhost' identified by 'password';
grant all on db_name.* to 'db_user'@'localhost';
Code language: SQL (Structured Query Language) (sql)

作業が終わったら \q でmysqlコマンドから抜けます。

ちなみにですが、新たに作成したデータベースユーザーとパスワードを指定すれば、sudoを付けなくてもmysqlコマンドで接続できます。

【※2019.03.24追記】直す方法が分かりました。