I downloaded the rails along with the article
When you press rails server

Rails is not currently installed on this system.To get the latest version simply type
$sudo gem install rails
You can then return your "rails" command


$sudo gem install rails

Successfully installed rails-5.2.1
parsing documentation rails-5.2.1
Done installing documentation for rails after 0 seconds
1 gem installed

And the above
It will be the same as rails server. . .
Why is this? . .