当我没有使用系统 ruby​​ 时，这发生在我身上，感谢 RVM:

This happened to me when I wasn't using system ruby, thanks to RVM:

$ echo $GEM_HOME
/home/spiffytech/.rvm/gems/ruby-1.9.3-p448@rails3tutorial2ndEd

请注意，@rails3tutorial2ndEd 表示特殊的 Ruby 安装，而不是系统安装.

Note that @rails3tutorial2ndEd denotes a special Ruby install, and not the system install.

为了解决这个问题并让刀再次工作，我跑了:

To fix this and make knife work again, I ran:

rvm reset