set-the-default-development-port.md

Set The Default Development Port

For Rails 5+, Puma has been the default web server that gets installed with new Rails apps. Puma comes with some configuration in the config/puma.rb file.

If you open that file up, you'll see a number of settings that can be configured. One of them is the port that puma uses.

# Specifies the `port` that Puma will listen on to receive requests; default is 3000.
#
port        ENV.fetch('PORT') { 3000 }

It looks for a PORT value in the envionrment and uses that if it is present. That means you could run:

PORT=5005 rails s

and Puma would server the local development server at localhost:5005. If that value is not present, it will fallback to #fetch's block which contains 3000.

If you always want to local Rails development server to run at a port other than 3000, all you need to do is update that line.

port        ENV.fetch('PORT') { 5005 }

Now, running rails s on its own will start the dev server up at localhost:5005.

source