How to get Sinatra to auto-reload the file after each change?

See the Sinatra FAQ,

“How do I make my Sinatra app reload on changes?”

First off, in-process code reloading in Ruby is hard and having a
solution that works for every scenario is technically impossible.

Which is why we recommend you to do out-of-process reloading.

First you need to install rerun if you haven’t already:

 $ gem install rerun

Now if you start your Sinatra app like this:

$ ruby app.rb

All you have to do for reloading is instead do this:

$ rerun 'ruby app.rb'

If you are for instance using rackup, instead do
the following:

$ rerun ‘rackup’

You get the idea.

If you still want in-process reloading, check out Sinatra::Reloader.