首次安裝看這裏: here,
安裝完成後會download 一個 git repository of homebrew-core 到 `/usr/local/Homebrew`
repository 裏放的都是 ruby 腳本.
叫Formulae 的, 裏面有你用 brew 命令安裝 package 的邏輯. 當你 run brew install <package_name> 的時候,homebrew 就會去找到 repository 裏相應的 Formulae 去運行它.
brew update and upgrade
brew update # update repository
brew upgrade # update actural packages
refer: https://itssiva.medium.com/difference-between-update-and-upgrade-on-brew-apt-get-etc-8a227fcbbd45