先卸载homebrew：
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)"
再次安装：
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"解决OSX 10.13 High Sierra brew update问题
原文：https://www.cnblogs.com/shadowaix/p/9893404.html