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参考mysql 连接方式：socket和tcp/ip
unix系统上，mysql的登陆方式有两种，分别是socket和tcp/ip方式登陆。
当server和client在同一台服务器上的时候，我们可以直接用mysql命令登陆。
[root@mysql ~]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.5.46-log MySQL Community Server (GPL)
Copyright (c) 2000, 2015, 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.
root@[(none)]>当什么参数都没有指定的时候，mysql默认使用socket方式登陆，如果my.cnf的[client]没有指定socket文件路径时，mysql默认会去寻找/tmp/mysql.sock，所以如果mysql服务启动的时候，生成的socket文件不是默认路径的话，登陆会报错。
[root@mysql ~]# mysql
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)mysql服务启动的时候，socket文件的生成目录可以在[mysqld]上指定，如果没有指定，默认是/tmp/mysql.sock；
mysql登陆的时候，指定参数-h，会使用tcp/ip的方式连接，如果没有指定端口的话，默认是使用3306端口
[root@mysql ~]# mysql -h192.1.1.20
ERROR 2003 (HY000): Can't connect to MySQL server on '192.1.1.20' (111)
[root@mysql ~]# mysql -h192.1.1.20 -P3307
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.5.46-log MySQL Community Server (GPL)
Copyright (c) 2000, 2015, 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.
root@[(none)]>
当mysql登陆时，同时指定-h和-S，mysql会默认使用tcp/ip的方式连接。
[root@mysql ~]# mysql -h192.1.1.20 -P3307 -S /ddd/sdfs.scok
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.5.46-log MySQL Community Server (GPL)
Copyright (c) 2000, 2015, 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.
root@[(none)]>随便指定一个socket文件都可以登陆。
可以查看简洁版的：MySQL 连接方式
更多细节可以查看 ：http://blog.itpub.net/15480802/viewspace-1755100/