When I run the gem command it gives error.

while executing gem (Gem::Exception)
  Unable to require openssl. install openSSL and rebuilt ruby (preferred) or use non HTTPs sources

首先：

刪除 https 源 $gem source -r https://rubygems.org/ to remove新增 http 源 $gem source -a http://rubygems.org/ to read

其次：

在 usr/local/src 路徑下保證有 ruby2.3.1 在src路徑下安裝openssl和ruby、rails

rvm get stable
brew update
brew doctor
brew install openssl
rvm install ruby-2.1 (or whatever version)
rvm use (ruby version here)  i.e. rvm use ruby-2.1
rvm gemset create NAME   i.e. rvm gemset create rails41
gem install rails -v 4.1

  這樣就可以保證openssl安裝成功，

即可安裝$sudo gem install cocoapods --pre