If you updated to High Sierra from an earlier version of mac OS (sometimes called an "over the top" installation), and you previously installed a Rails development environment, your earlier installation remains intact.You will need to install the new version of Xcode Command Line Tools (full instructions below). Up-to-date and detailed instructions, plus troubleshooting, for the Rails newest release.How to set up and install Rails 5.2, the newest version of Rails, on mac OS 10.13 High Sierra.For more on Cloud9, see the article Ruby on Rails with Cloud9.Cloud9 is a good option if you have trouble installing Ruby on Rails on your Mac.You should update to Ruby 2.5.0 by using RVM, the Ruby Version Manager. If you are maintaining older Rails applications, you will likely need to install older Ruby versions.
Hosted development, using a service such as Cloud9, means you get a computer "in the cloud" that you use from your web browser.
If you are an experienced Unix administrator, you can consider alternatives such as Chruby or Sam Stephenson’s rbenv. You may hear about one-click installation programs such as Rails Installer, Cinderella, and Bit Nami Ruby Stack.
These installation programs are often outdated and they are no longer needed as at the system level creating potential conflicts.
Likewise, if you frequently work on more than one computer, a hosted environment eliminates the difficulty of maintaining duplicate development environments.
For these reasons some developers prefer to "work in the cloud" using Cloud9.