Not understanding the simplest security scenario

[Jon D]

Hi,

I'm going crazy trying to understand why this command works (on my local test infrastructure):

----------- TEST START -----------

root:/etc/mysql# mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 23
Server version: 8.0.29-0ubuntu0.22.04.2 (Ubuntu)

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

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

mysql> select user, host, plugin from mysql.user;
+------------------+-----------+-----------------------+
| user | host | plugin |
+------------------+-----------+-----------------------+
| debian-sys-maint | localhost | caching_sha2_password |
| mysql.infoschema | localhost | caching_sha2_password |
| mysql.session | localhost | caching_sha2_password |
| mysql.sys | localhost | caching_sha2_password |
| root | localhost | auth_socket |
+------------------+-----------+-----------------------+
5 rows in set (0.00 sec)

----------- TEST END -----------

and this command fails:

----------- TEST START -----------

root:/etc/mysql# mysql -u root@localhost
ERROR 1045 (28000): Access denied for user 'root@localhost'@'localhost' (using password: NO)

----------- TEST END -----------

I've read many of the docs, but cannot find an explanation for this. Could anyone please help?

Thanks