修改mysql参数文件：
[root@test ~]# vi /etc/my.cnf
在
[mysqld]后面加入：
skip-external-locking

保存。

[root@test ~]# service mysqld restart
MySQL server PID file could not be found!                  [FAILED]

不需要密码就能登录：
[root@test ~]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.19-log 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 [(none)]> USE mysql ;
Database changed

MySQL [mysql]> update user set authentication_string=password('test') where user='root' and Host = '127.0.0.1'; 
Query OK, 1 row affected, 1 warning (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 1


MySQL [mysql]> update user set authentication_string=password('test') where user='root' and Host = '%'; 
Query OK, 0 rows affected, 1 warning (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 1

修改完毕。
再把原来在/etc/my.cnf里面[mysqld]后面的
skip-external-locking
去掉。

需要输入密码登录：
[root@test ~]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.19-log 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 [(none)]> 

完成。