6)
重置root用户密碼
從MySQL 5.7後，第一次啟動MySQL，會生成root用户的隨機密碼，可以執行以下命令查看
grep 'temporary password' /var/log/mysqld.log 
example:
[root@60-250-98-236 yum.repos.d]# grep 'temporary password' /var/log/mysqld.log
2018-03-16T15:19:38.975505Z 1 [Note] A temporary password is generated for root@localhost: yPdTsk(jk1Zu
2018-03-19T07:27:21.313947Z 1 [Note] A temporary password is generated for root@localhost: Nn&j?Ty.y1_X

這個密碼是臨時密碼，必須修改為正式密碼，否則使用這個密碼登錄MySQL Server後，執行任何命令，都會顯示以下錯誤
登入MySQL Server:
mysql -u root -p

以上若發生
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

原因是mysql这个文件的权限问题，需要把这个文件的权限改为当前用户
 sudo chown -R root:root /var/lib/mysql

我們可以執行以下命令，修改root用户密碼
mysql> alter user user() identified by 'pass0'; 
但是會顯示以下錯誤
...

這是因為MySQL默認使用的validate_password_policy是MEDIUM的緣故，MySQL的官網介紹validate_password_policy有以下三種:
Policy,      Tests Performed 
0 or LOW,    Length //只檢查密碼長度
1 or MEDIUM, Length; numeric, lowercase/uppercase, and special characters //檢查密碼長度，是否包含數字，大小寫字母，特殊字符
2 or STRONG, Length; numeric, lowercase/uppercase, and special characters; dictionary file //檢查密碼長度，是否包含數字，大小寫字母，特殊字符，以及是否包含字典文檔

引用地址
https://dev.mysql.com/doc/refman/5.7/en/validate-password-options-variables.html#sysvar_validate_password_policy
MySQL安裝後默認的validate_password_policy為MEDIUM，我們重新設置的密碼不符合這個policy的要求，所以命令執行失敗。
如果要修改密碼成功，我們需要修改validate_password_policy為0
mysql> set global validate_password_poilcy=0; 

默認的validate_password_length是8，我們可以修改為5
mysql> set global validate_password_length=5; 

完成這兩步後我們可以修改root用户的密碼為admin，命令和前面一樣。
mysql> alter user user() identified by 'pass0'; 
root/pass0




----------
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

解决：
1、找到密码[root@localhost opt]#  cat /var/log/mysqld.log  | grep password
上面的結果太多, 可改用下面這個
grep 'temporary password' /var/log/mysqld.log

2016-07-16T05:01:47.771362Z 1 [Note] A temporary password is generated for root@localhost: q.)aq!YaH6y-
2016-07-16T05:02:32.000199Z 2 [Note] Access denied for user 'root'@'localhost' (using password: NO)
2016-07-16T05:02:39.063852Z 3 [Note] Access denied for user 'root'@'localhost' (using password: NO)

2、重新连接mysql：[root@zzs171 opt]# /usr/bin/mysql -u root -p
Enter password: 输入上面找到的密码，即可进入mysql客户端连接。
3、修改mysql的root的密码：
进入mysql客户端之后，show databases; 会提醒先修改密码：
mysql> show databases;
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.设置密码，以下两种方式均可：
SET PASSWORD FOR 'root'@'localhost' = PASSWORD('Mysqlpassw0rd.');

alter user 'root'@'localhost' identified by 'Mysqlpassw0rd.';

OK，至此设置成功。注意mysql有密码复杂度的限制。密码太简单会提示：mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('123');
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements


----------
20190109

密碼重置:
systemd is now used to look after mySQL instead of mysqld_safe (which is why you get the -bash: mysqld_safe: command not found error - it’s not installed)
The user table structure has changed.

1. 先關閉 3306 防火牆. ----> 參考"firewall-cmd防火牆.txt"


2. Stop mysql: 
systemctl stop mysqld

3. Set the mySQL environment option 
systemctl set-environment MYSQLD_OPTS=”–skip-grant-tables”

Start mysql usig the options you just set 
4. systemctl start mysqld

Login as root 
5. mysql -u root
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

mysql -u root mysql
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

Update the root user password with these mysql commands 
mysql> UPDATE mysql.user SET authentication_string = PASSWORD(‘MyNewPassword’) 
-> WHERE User = ‘root’ AND Host = ‘localhost’; 
mysql> FLUSH PRIVILEGES; 
mysql> quit

Stop mysql 
systemctl stop mysqld

Unset the mySQL envitroment option so it starts normally next time 
systemctl unset-environment MYSQLD_OPTS

Start mysql normally: 
systemctl start mysqld

Try to login using your new password: 
mysql -u root -p

重新開啟 3306 防火牆. ----> 參考"firewall-cmd防火牆.txt"


Reference
As it says at http://dev.mysql.com/doc/refman/5.7/en/mysqld-safe.html,
Note
As of MySQL 5.7.6, for MySQL installation using an RPM distribution, server startup and shutdown is managed by systemd on several Linux platforms. On these platforms, mysqld_safe is no longer installed because it is unnecessary. For more information, see Section 2.5.10, “Managing MySQL Server with systemd”.Which takes you to http://dev.mysql.com/doc/refman/5.7/en/server-management-using-systemd.html where it mentions the systemctl set-environment MYSQLD_OPTS= towards the bottom of the page
The password reset commands are at the bottom of 
http://dev.mysql.com/doc/refman/5.7/en/resetting-permissions.html


----------
以下 mysqld_safe 已經變更為以上的 systemd


忘記密碼重設

先關閉 3306 防火牆. ----> 參考"firewall-cmd防火牆.txt"

1. 停止 MySql
systemctl stop mariadb
systemctl stop mysql

2.
在啟動 MySQL 或 MariaDB 時，若跳過 trant table 的話，就可以在不用密碼的情況下以資料庫 root 權限存取資料庫裡面的資料，這樣我們就可以重設密碼。
不過以這種模式啟動資料庫會有風險，所以建議關閉網路功能（networking），以防止外部的使用者連入。
sudo mysqld_safe --skip-grant-tables --skip-networking &

3.
使用 root 帳號連線至 MySQL/MariaDB 資料庫：
# 以 root 連線至 MySQL/MariaDB 資料庫
mysql -u root

4.
-- 載入 grant tables
FLUSH PRIVILEGES;

5
變更 root 密碼：
-- 重設 root 密碼
ALTER USER 'root'@'localhost' IDENTIFIED BY 'new_password';

若是 MySQL 5.7.5 或 MariaDB 10.1.20 以前的版本，則要改用舊的指令：
-- 重設 root 密碼（舊版指令）
SET PASSWORD FOR 'root'@'localhost' = PASSWORD('new_password');

更改完成後，按下 Ctrl + d（或執行 exit 指令）離開。

6.
停止剛剛手動啟動的 MariaDB（或 MySQL）資料庫。
# 停止 MariaDB
sudo kill `cat /var/run/mariadb/mariadb.pid`

# 停止 MySQL
sudo kill `cat /var/run/mysqld/mysqld.pid`

7.
啟動系統的 MariaDB（或 MySQL）服務。
# 啟動 MariaDB 服務
sudo systemctl start mariadb

# 啟動 MySQL 服務
sudo systemctl start mysql


8.
使用 root 與新密碼進入 MySQL/MariaDB 資料庫。
# 使用新密碼進入資料庫
mysql -u root -p

重新開啟 3306 防火牆. ----> 參考"firewall-cmd防火牆.txt"