mysql5.7版本之后，与mariadb不同，在安装之后，在启动之时，会进行自动随机密码的设定，所以在systemctl start mysqld之后，会出现mysql -uroot -p无法登陆的情况
实际上mysqld在启动时，会自动设定root密码的，可以在其相关日志里面查看到，如下:
[root@bogon ~]# grep password /var/log/mysqld.log
2017-07-31T10:31:57.368883Z 1 [Note] A temporary password is generated for root@localhost: TjcrBm.j,7eU
2017-07-31T10:32:30.201882Z 3 [Note] Access denied for user 'root'@'localhost' (using password: NO)
2017-07-31T10:34:40.233539Z 0 [Note] Shutting down plugin 'validate_password'
2017-07-31T10:34:41.483983Z 0 [Note] Shutting down plugin 'sha256_password'
2017-07-31T10:34:41.483990Z 0 [Note] Shutting down plugin 'mysql_native_password'
mysql root的初始密码为TjcrBm.j,7eU(每人情况不一样，具体值在mysql日志中查看)
当然有些人可能是root密码真忘记了，那样可以通过mysql免密码登陆
在其配置文件/etc/my.cnf中加入skip-grant-tables=1即可
[root@bogon ~]# grep -v ^# /etc/my.cnf | grep -v ^$
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
symbolic-links=0
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
skip-grant-tables=1
然后重启mysql，使用mysql命令即可进入
[root@bogon ~]# systemctl restart mysqld
[root@bogon ~]# ss -tnl
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 127.0.0.1:9000 *:*
LISTEN 0 128 *:111 *:*
LISTEN 0 128 *:80 *:*
LISTEN 0 128 *:22 *:*
LISTEN 0 100 127.0.0.1:25 *:*
LISTEN 0 80 :::3306 :::*
LISTEN 0 128 :::111 :::*
LISTEN 0 128 :::22 :::*
LISTEN 0 100 ::1:25 :::*
[root@bogon ~]# mysql
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.19
Copyright (c) 2000, 2017, 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> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> update user set authentication_string = password("123456") where;
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 1
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
然后将/etc/my.cnf中的skip-grant-tables=1注释掉，重启mysql服务即可。
在此要注意的是，之前版本密码修改字段为password，在5.7版本之后字段为authentication_string
mysql密码修改为123456之后，有人发现使用123456能进入mysql，但是却不能使用mysql的任何功能。会出现如下情况:
[root@bogon ~]# mysql -uroot -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.19
Copyright (c) 2000, 2017, 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> show databases;
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.
实际上，在初始更改root密码时，并不能直接使用update来更改，需要使用alter user命令来更改
mysql> alter user 'root'@'localhost' identified by '123456';
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements
## mysql在5.7版本中加了密码安全等级，弱密码不能使用
mysql> alter user 'root'@'localhost' identified by 'QWEqwe123!@#';
Query OK, 0 rows affected (0.00 sec)
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
在上述修改密码之后，可查看mysql的密码安全策略
[root@bogon ~]# mysql -uroot -p'QWEqwe123!@#'
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.19 MySQL Community Server (GPL)
Copyright (c) 2000, 2017, 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> show variables like '%password%';
+---------------------------------------+--------+
| Variable_name | Value |
+---------------------------------------+--------+
| default_password_lifetime | 0 |
| disconnect_on_expired_password | ON |
| log_builtin_as_identified_by_password | OFF |
| mysql_native_password_proxy_users | OFF |
| old_passwords | 0 |
| report_password | |
| sha256_password_proxy_users | OFF |
| validate_password_check_user_name | OFF |
| validate_password_dictionary_file | |
| validate_password_length | 8 |
| validate_password_mixed_case_count | 1 |
| validate_password_number_count | 1 |
| validate_password_policy | MEDIUM |
| validate_password_special_char_count | 1 |
+---------------------------------------+--------+
14 rows in set (0.00 sec)
其中可以看到validate有很多参数:
validate_password_length：密码最小长度。
validate_password_mixed_case_count：密码至少要包含的小写字母个数和大写字母个数。
validate_password_number_count：密码至少要包含的数字个数。
validate_password_policy：密码强度检查等级，0/LOW、1/MEDIUM、2/STRONG。
validate_password_special_char_count：密码至少要包含的特殊字符数。
其中，关于validate_password_policy-密码强度检查等级：
1/MEDIUM：检查长度、数字、大小写、特殊字符。
2/STRONG：检查长度、数字、大小写、特殊字符字典文件。
可以直接在mysql中进行参数的修改
mysql> set global validate_password_policy=0;
Query OK, 0 rows affected (0.00 sec)
mysql> set global validate_password_special_char_count=0;
Query OK, 0 rows affected (0.00 sec)
mysql> set global validate_password_number_count=0;
Query OK, 0 rows affected (0.00 sec)
mysql> set global validate_password_mixed_case_count=0;
Query OK, 0 rows affected (0.00 sec)
mysql> set global validate_password_length=0;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%password%';
+---------------------------------------+-------+
| Variable_name | Value |
+---------------------------------------+-------+
| default_password_lifetime | 0 |
| disconnect_on_expired_password | ON |
| log_builtin_as_identified_by_password | OFF |
| mysql_native_password_proxy_users | OFF |
| old_passwords | 0 |
| report_password | |
| sha256_password_proxy_users | OFF |
| validate_password_check_user_name | OFF |
| validate_password_dictionary_file | |
| validate_password_length | 0 |
| validate_password_mixed_case_count | 0 |
| validate_password_number_count | 0 |
| validate_password_policy | LOW |
| validate_password_special_char_count | 0 |
+---------------------------------------+-------+
14 rows in set (0.00 sec)
当然，此种变更，只能在当前环境下生效，重启mysql后会失效
也可以直接在配置文件中进行修改禁用validate-password功能
编辑my.cnf配置文件，在mysqld下面加入“validate-password=0”,然后重启mysql即可。
[root@bogon ~]# grep -v ^# /etc/my.cnf | grep -v ^$
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
validate-password=0
symbolic-links=0
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid
将mysql的密码安全等级降低之后，可以将root密码修改回来
mysql> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> update user set authentication_string = password("") where;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql> exit
以上密码root即清除为空了。使用mysql即可直接进入mysql。当然生产环境中不推荐此种做法。