忘记了root用户密码， 首先使用如下重启mysqld服务

[root@mysql8-01 ~]# systemctl set-environment MYSQLD_OPTS="--skip-grant-tables"

[root@mysql8-01 ~]# systemctl stop mysqld

[root@mysql8-01 ~]# systemctl start mysqld

如果需要使用简单密码，需要修改系统参数:

set global validate_password.policy=0;

set global validate_password.length=4;

接下来登录:

[root@mysql8-01 ~]# mysql

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 7

Server version: 8.0.14 MySQL Community Server - GPL

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.

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

mysql>

mysql> ALTER USER 'root'@'*' IDENTIFIED BY '123456';

ERROR 1290 (HY000): The MySQL server is running with the --skip-grant-tables option so it cannot execute this statement

mysql> flush privileges;

Query OK, 0 rows affected (0.02 sec)

mysql> ALTER USER 'root'@'*' IDENTIFIED BY '123456';

ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

mysql> set global validate_password.policy=0;

Query OK, 0 rows affected (0.00 sec)

mysql> set global validate_password.length=4;

Query OK, 0 rows affected (0.00 sec)

mysql> flush privileges;

Query OK, 0 rows affected (0.01 sec)

mysql> ALTER USER 'root'@'*' IDENTIFIED BY '123456';

ERROR 1396 (HY000): Operation ALTER USER failed for 'root'@'*'

mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY '123456';

Query OK, 0 rows affected (0.00 sec)

mysql> flush privileges;

Query OK, 0 rows affected (0.00 sec)

mysql>

mysql> exit