ubuntu 下安装 mysql 启动报错：ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

环境：

ubuntu LTS 18.04.1
mysql  Ver 14.14 Distrib 5.7.29, for Linux (x86_64) 

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初探 linux，按照如下安装完mysql

sudo apt install mysql-server
sudo apt install mysql-client

sudo service mysql start

启动报错：

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

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放狗搜索，找了老半天也没解决。

最后，解决办法是：

# 切换到root
sudo -i

# 还是上面的启动 mysql 命令
service mysql start

# 终于出现了喜人的：
 * Starting MySQL database server mysqld