What happens to environment variables that were already set?
We will never modify any environment variables that have already been set. In particular, if there is a variable in your .env file which collides with one that already exists in your environment, then that variable will be skipped. This behavior allows you to override all .env configurations with a machine-specific environment, although it is not recommended.

Add an option to ignore environment vars and set the variables from a file.

For example a user has NODE_ENV=development set in his system. Jest will fail on that. I want to be able to ignore system vars and set explicitly all variables that I need for my script/command (NODE_ENV=test)