cocoapods 的使用，不知道怎么会出现问题

为什么我输入sudo gem install cocoapods会输出这样的情况？求大神指点：
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.

输入gem install cocoapods为什么出现这样的情况：
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.