Running Sinatra on port 80

Question

I installed Sinatra and it works but it uses port 4567 by default. I want it to run on port 80.

In an effort to get it to work on port 80, I tried this:

Answer 1

An alternate way to accepted answer

rvmsudo rackup -p 80

In case one is using RVM to manage Ruby versions, you may not be able to use sudo that easily (or else would need to setup ruby in path).