Heroku compiles your app without environment variables. This is all well and good, but how do you debug a problem that only shows up when an environment variable is missing? Not just set to an empty string "".

To do this bash has a great feature called --unset to remove an environment variable. For example if you wanted to run rake assets:precompile without the DATABASE_URL from your environment you could run:

$ env --unset=DATABASE_URL rake assets:precompile

This allows you to simulate missing any environment variables you want!