Ubuntu安裝Mysql很簡單，但是也會遇到一些問題。
我的Ubuntu版本是18.04.1，安裝mysql版本是5.7。
Ubuntu安裝MySQL的步驟如下：
1. sudo apt-get install mysql-server
2. apt-get install mysql-client
3. sudo apt-get install libmysqlclient-dev
安裝以後，通過mysql --version命令查看版本信息：
mysql Ver 14.14 Distrib 5.7.26, for Linux (x86_64) using EditLine wrapper
mysql是安裝成功了，但是直接輸入mysql，卻顯示沒有權限：
ERROR 1045 (28000): Access denied for user 'qingyuan'@'localhost' (using password: NO)
根據網上的建議，查看mysql到底有沒有啓動：
sudo netstat -tap |grep mysql
輸出顯示是成功的：
tcp 0 0 localhost:mysql 0.0.0.0:* LISTEN 4892/mysqld
繼續跟隨網上建議，關閉mysql，然後重啓
systemctl restart mysqld
輸出如下：
Failed to start mysqld.service: Unit mysqld.service not found.
輸出顯示：沒有mysqld.service這個單元。
輸入mysqld命令，輸出如下：
mysqld: Can't change dir to '/var/lib/mysql/' (Errcode: 13 - Permission denied)
2019-07-01T13:44:44.228936Z 0 [Warning] Changed limits: max_open_files: 1024 (requested 5000)
2019-07-01T13:44:44.229209Z 0 [Warning] Changed limits: table_open_cache: 431 (requested 2000)
2019-07-01T13:44:44.479228Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2019-07-01T13:44:44.596275Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.597278Z 0 [Note] mysqld (mysqld 5.7.26-0ubuntu0.18.04.1) starting as process 3561 ...
2019-07-01T13:44:44.905374Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.905433Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.936710Z 0 [Warning] One can only use the --user switch if running as root
2019-07-01T13:44:44.937074Z 0 [ERROR] failed to set datadir to /var/lib/mysql/
2019-07-01T13:44:44.937156Z 0 [ERROR] Aborting
2019-07-01T13:44:44.939133Z 0 [Note] Binlog end
2019-07-01T13:44:44.958845Z 0 [Note] mysqld: Shutdown complete
但是，切換到root後，卻可以直接訪問mysql了
這到底是爲什麼？
這得從Ubuntu下安裝高版本的MySql有關。在Ubuntu上安裝MySQL時，對於root用戶，可以設置空密碼。
輸入如下指令查看mysql賬戶信息：
SELECT User, Host, HEX(authentication_string) FROM mysql.user;
輸出爲：
+------------------+-----------+------------------------------------------------------------------------------------+
| User | Host | HEX(authentication_string) |
+------------------+-----------+------------------------------------------------------------------------------------+
| root | localhost | |
| mysql.session | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 |
| mysql.sys | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 |
| debian-sys-maint | localhost | 2A33383636414146424141463045393833383341423435353142373433363739333531383934423043 |
+------------------+-----------+------------------------------------------------------------------------------------+
我們看到，root的密碼確實爲空。
試着改變root的密碼，輸入如下指令：
ALTER USER 'root'@'localhost' IDENTIFIED BY 'test';
輸出爲：
Query OK, 0 rows affected (0.00 sec)
然後發現，root的密碼仍然爲空，繼續嘗試：
SET PASSWORD FOR 'root'@'localhost' = PASSWORD('test');
輸出：
Query OK, 0 rows affected, 2 warnings (0.02 sec)
仍然沒有成功，並且有兩個警告。我們看看警告是什麼：
show warnings;
輸出：
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1287 | 'SET PASSWORD FOR <user> = PASSWORD('<plaintext_password>')' is deprecated and will be removed in a future release. Please use SET PASSWORD FOR <user> = '<plaintext_password>' instead |
| Note | 1699 | SET PASSWORD has no significance for user 'root'@'localhost' as authentication plugin does not support it. |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.04 sec)
輸出顯示當前使用的授權插件不支持修改密碼，那我們看一下當前的授權插件是什麼：
SELECT user, plugin FROM mysql.user;
輸出：
+------------------+-----------------------+
| user | plugin |
+------------------+-----------------------+
| root | auth_socket |
| mysql.session | mysql_native_password |
| mysql.sys | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+
我們看到，root採用的是授權插件是auth_socket，也即通過套接字進行鑑權。如果你安裝的版本是5.7，並且沒有爲root用戶提供密碼（安裝時並不需要），那麼就會使用auth_socket插件，這個插件不關心也不需要密碼，它只是通過UNIX套接字的用戶名進行驗證，如果驗證通過，那麼就鑑權成功。這就是root用戶可以正常使用mysql的原因，因爲root用戶在mysql的user表中。
如果我們想配置密碼，那麼就需要同時改變插件並且設置密碼。如果是先改變插件，然後設置密碼就會不起作用。插件也會回滾到auth_socket。
輸入如下命令：
update mysql.user set authentication_string=PASSWORD('123456'), plugin='mysql_native_password' where user='root';
輸出如下：
Query OK, 1 row affected, 1 warning (0.09 sec)
Rows matched: 1 Changed: 1 Warnings: 1
查看操作後的用戶信息
mysql> SELECT User, Host, HEX(authentication_string) ,plugin FROM mysql.user;
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+
| User | Host | HEX(authentication_string) | plugin |
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+
| root | localhost | 2A36424234383337454237343332393130354545343536384444413744433637454432434132414439 | mysql_native_password |
| mysql.session | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 | mysql_native_password |
| mysql.sys | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 | mysql_native_password |
| debian-sys-maint | localhost | 2A33383636414146424141463045393833383341423435353142373433363739333531383934423043 | mysql_native_password |
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+
此時root用戶確實有了密碼 。我們查看一下warning是什麼：
mysql> show warnings;
+---------+------+-------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------+
| Warning | 1681 | 'PASSWORD' is deprecated and will be removed in a future release. |
+---------+------+-------------------------------------------------------------------+
1 row in set (0.00 sec)
警告顯示，PASSWORD被廢棄並且在未來版本中要被移除。可以通過如下命令
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '123456';
退出mysql，重啓mysql，退出root用戶，切換到普通用戶，用設置的密碼123456，root用戶登錄mysql。
mysql -u root -p
然後輸入密碼：123456，登錄成功：
qingyuan@ubuntu:~$ mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.26-0ubuntu0.18.04.1 (Ubuntu)
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
參考資料
1. Ubuntu18.04安裝MySQL後普通用戶無法登錄
2. Change user password in MySQL 5.7 with “plugin: auth_socket”