mysql 启动失败：su: warning: cannot change directory to /nonexistent: No such file or directory

这种错误一般是 mysql 服务器异常关机导致的，解决方案如下：

# Ubuntu
sudo service mysql stop
sudo usermod -d /var/lib/mysql/ mysql
sudo service mysql start

 

# CentOS
sudo systemctl stop mysql.service
sudo usermod -d /var/lib/mysql/ mysql
sudo systemctl start mysql.service