ERROR: Could not find 'which

配置ruby环境的时候，安装时候出现这样的报错ERROR: Could not find 'which' command, make sure it's available first before continuing installation.

然后直接google这个错误，发现貌似是环境变量问题？然后找到这篇大概了解了下各个目录
http://www.jb51.net/LINUXjishu/32192.html

后来发现直接输入

[root@localhost usr]# which

也是没有
其实which这命令也没有，于是在这篇文章中找到解决办法
https://www.question-defense.com/2009/12/13/linux-bash-which-command-not-found

其实就是

yum install which

嗯........md