在使用brew安装mysql的时候，报了如下错误：

mkdir: /usr/local/var/homebrew/locks: Permission denied
Error: Can't create update lock in /usr/local/var/homebrew/locks!
Fix permissions by running:
  sudo chown -R $(whoami) /usr/local/var/homebrew
Warning: mysql 8.0.12 is already installed, it's just not linked
You can use `brew link mysql` to link this version.

提示我们mysql已经安装过了，但是没有链接，需要运行brew link mysql进行链接。
运行brew link mysql，又报了这样一个错：

Error: Permission denied @ dir_s_mkdir - /usr/local/var

解决方法：
运行brew doctor，它会提示这样一段话：

Warning: The following directories do not exist:
/usr/local/var/homebrew/linked

You should create these directories and change their ownership to your account.
  sudo mkdir -p /usr/local/var/homebrew/linked
  sudo chown -R $(whoami) /usr/local/var/homebrew/linked

运行里面的命令

sudo mkdir -p /usr/local/var/homebrew/linked
sudo chown -R $(whoami) /usr/local/var/homebrew/linked

然后在运行brew link mysql。