Brett asked this question on the Heroku mailing list:

I want to let users purchase custom domain names to access my app on Heroku. When the purchase transaction takes place, I’d like to add the custom domain to my app in real time. I thought of using the Heroku gem/command line tool from within the app, analogous to the command-line expression: heroku domains:add Is this possible?

It really got me thinking about exploring all that the Heroku API has to offer.

I thought I remembered seeing someone using the Heroku gem inside their code before, but I couldn’t remember where, and I couldn’t seem to find any examples in the Heroku docs. Good! Time to get my hands dirty with some raw source code.

I opened up the Heroku client source code and it’s actually designed to be used inside your code. How about that. The first few lines after all the requires describe perfectly how to use it:

# A Ruby class to call the Heroku REST API.  You might use this if you want to
# manage your Heroku apps from within a Ruby program, such as Capistrano.
# Example:
#   require 'heroku'
#   heroku ='', 'mypass')
#   heroku.create('myapp')

As you scroll down through the functions, you find that you can do anything in your source code that you could do on the command line. To answer the original question, if you wanted to use this command-line task:

heroku domains:add

You would simply put these lines in your source code:

require 'heroku'
heroku ='','mypass')

The add_domain function is found on line #120 of client.rb.

UPDATE: Someone replied to the thread, pointing out that you should keep things such as your Heroku credentials out of your code and in an environment variable. Heroku already stores your app name by default in ENV["APP_NAME"]. So, to make the above code a little DRYer and safe to distribute. At the terminal:

heroku config:add HEROKU_PASS=mypass

And then in our code:

require 'heroku'
heroku.add_domain(ENV["APP_NAME"], '')

Now we won’t be distributing our Heroku credentials with our code.

UPDATE #2: Someone asked me if storing Heroku credentials as config variables would be secure. Ben Hartney had the same question and contacted Heroku support. Here is their response:

Storing this information in an environment variable is safe. We also store your database connection info in environment variables, so we consider it secure.