There are three steps.
1. Install Ruby.
It will be easier by using RVM. RVM is a ruby version manager.
1.1 Install RVM: https://rvm.io/rvm/install/
Install RVM with ruby:
$ \curl -L https://get.rvm.io | bash -s stable --ruby (By the way, curl is A command line tool for getting or sending files using URL syntax.)
1.2 Install ruby:
$ rvm install 1.9.3 (1.9.3 is the latest version of ruby)
1.3 Then do :
$ rvm list
You will see a list of ruby.
$ rvm --default use 1.9.3 (the version num depends on which version you installed)
error1 happens here : RVM is not a function, selecting rubies with 'rvm use ...' will not work.
Solutions For error1: http://stackoverflow.com/questions/9336596/rvm-installation-not-working-rvm-is-not-a-function
error2 happens here : Gemset '' does not exist, 'rvm gemset create ' first, or append '--create'.
Solutions For error2: rvm use 1.9.3 --create (it creates the gemset if not present, seems like the global gemset somehow got deleted)
$ ruby -v
You will see expected ruby version.
2. Install Gem: RubyGem is the standard Ruby Package Manager.
do: $ require 'rubygems'
but you don't need to do this in Ruby 1.9 or higher, it's already included.
3. Install Rails
$ sudo gem install rails
errors happen here:
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
$ gem install rails
another thing: need to install "Common Line Tools" in XCode->Component
important thing: $ bundle install