安装mysql5.7完成之后，执行service mysql start命令启动mysql，却报了如下错误：
[mysql@node113 ~]$ service mysql start
Starting MySQL.2018-01-11T07:59:43.669963Z mysqld_safe The file /usr/local/mysql/bin/mysqld
does not exist or is not executable. Please cd to the mysql installation
directory and restart this script from there as follows:
./bin/mysqld_safe&
See /doc/mysql/en/ for more information
ERROR! The server quit without updating PID file (/usr/local/mysqldb/data/).
这是因为mysqld_safe默认的会从从/usr/local/mysql/bin目录中去寻找启动脚本mysqld，但是我安装mysql的目录是/usr/local/mysqldb/mysql,因此要想解决上边这个报错的问题，可以在/usr/local/mysql/bin/目录下创建一个安装目录/usr/local/mysqldb/mysql/bin/mysqld的软连接：
ln -s /usr/local/mysqldb/mysql/bin/mysqld /usr/local/mysql/bin/mysqld
创建完软连接之后，再次执行service mysql start命令即可成功。