最近使用二进制安装的方式安装了一台MySQL5.7，发现使用之前（源码安装）的连接方式报错。
[root@oracle_db ~]# mysql -uroot -p Enter password: ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)
我的配置文件（/etc/my.cnf）：
[mysqld] log_bin = server-130-bin gtid_mode = ON log_slave_updates = 1 enforce_gtid_consistency = ON binlog_format=row #read_only=1 relay_log_purge=0 basedir = /usr/local/mysql/ #datadir = /home/mysql/data/ datadir = /usr/local/mysql/data/ port = 3306 server_id = 130 socket = /usr/local/mysql/mysql.sock # join_buffer_size = 128M # sort_buffer_size = 2M # read_rnd_buffer_size = 2M # sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES [client]
当MySQL处于关闭状态时，连接MySQL也会报这种错。所以要先确保mysql确实已经是在运行。用ps检查一下就可以了。
[root@oracle_db ~]# ps -ef|grep mysql root 94151 94019 0 01:28 pts/2 00:00:00 /bin/sh /usr/local/mysql/bin/mysqld_safe --user=mysql mysql 94364 94151 16 01:28 pts/2 00:00:00 /usr/local/mysql/bin/mysqld --basedir=/usr/local/mysql/ --datadir=/usr/local/mysql/data --plugin-dir=/usr/local/mysql//lib/plugin --user=mysql --log-error=oracle_db.err --pid-file=oracle_db.pid --socket=/usr/local/mysql/mysql.sock --port=3306 root 94393 94019 0 01:28 pts/2 00:00:00 grep mysql
从mysqld的运行参数可以看到，MySQL实际的socket文件在/usr/local/mysql/mysql.sock，而使用mysql客户程序使用的默认socket 在/tmp/mysql.sock，这个当然是不对的。解决的办法有下面几种。
第一种方法：加 --socket/-S 参数
[root@oracle_db ~]# mysql -uroot -p -S/usr/local/mysql/mysql.sock
Enter password:
Welcome to the MySQL monitor. Commands end with ; or g.
Your MySQL connection id is 5
Server version: 5.7.25-log 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>
这种方法每次登陆都需要指定socket参数，稍显麻烦，于是有了第二种方法。
第二种方法：在配置文件指定socket参数
[client] socket = /usr/local/mysql/mysql.sock
第三种方法：指定--host/-h参数
[root@oracle_db ~]# mysql -uroot -p -h127.0.0.1 Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 6 Server version: 5.7.25-log 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>
指定host参数登陆时，使用tcp/ip 连接MySQL，而不是socket连接。
注：-h localhost ,也是使用socket这种方式连接MySQL。
第四种方法：
在配置文件中注释socket参数，然后重启MySQL
[mysqld] #socket = /usr/local/mysql/mysql.sock
再回到最开始的问题，为什么我用源码安装，然后直接使用mysql -uroot -p就可以连接呢？
我在编译MySQL的时候加上了-DMYSQL_UNIX_ADDR=/usr/local/mysql/mysql.sock 这个参数，这可能会修改mysql程序soket参数的默认值。即将默认值/tmp/mysql.sock 修改为 /usr/local/mysql/mysql.sock，于是直接使用mysql -uroot -p登陆，socket参数的值是正确的。
cmake . -DCMAKE_INSTALL_PREFIX=/usr/local/mysql -DMYSQL_DATADIR=/usr/local/mysql/data -DMYSQL_UNIX_ADDR=/usr/local/mysql/mysql.sock -DDEFAULT_CHARSET=utf8 -DDEFAULT_COLLATION=utf8_general_ci -DEXTRA_CHARSETS=gbk,gb2312,utf8,ascii -DWITH_INNOBASE_STORAGE_ENGINE=1 -DWITH_FEDERATED_STORAGE_ENGINE=1 -DWITH_BLACKHOLE_STORAGE_ENGINE=1 -DWITHOUT_EXAMPLE_STORAGE_ENGINE=1 -DWITHOUT_PARTITION_STORAGE_ENGINE=1 -DWITH_FAST_MUTEXES=1 -DWITH_ZLIB=bundled -DENABLED_LOCAL_INFILE=1 -DWITH_READLINE=1 -DWITH_EMBEDDED_SERVER=1 -DWITH_DEBUG=0