ruby

CentOS

yum install ruby
yum install rubygems

rubygems

Windows

https://rubyinstaller.org/downloads/

Errors

ERROR:  Error installing jekyll:
        The last version of ffi (< 2, >= 0.5.0) to support your Ruby & RubyGems was 1.9.18. Try installing it with `gem install ffi -v 1.9.18` and then running the current command again
        ffi requires Ruby version < 2.5, >= 2.0. The current ruby version is 2.5.0.

fix

gem install ffi -f
gem install jekyll