“ -bash：curl：未找到命令”以及“ -bash：ruby：未找到命令”

Question

我尝试使用本手册http://brysonpayne.com/2015/01/10/setting-up-pygame-on-a-mac/将pygame安装到我的Mac上，但是当我在终端上运行：ruby -e “$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)”时，会得到以下信息：

-bash: curl: command not found
-bash: ruby: command not found

有人可以帮忙吗？