dotenv

Dotenv loads environment variables from .env into ENV.
Storing configuration in the environment is one of the tenets of a twelve-factor app. Anything that is likely to change between deployment environments–such as resource handles for databases or credentials for external services–should be extracted from the code into environment variables.
But it is not always practical to set environment variables on development machines or continuous integration servers where multiple projects are run. Dotenv load variables from a .env file into ENV when the environment is bootstrapped.

Installation

Rails

Add this line to the top of your application's Gemfile:

gem 'dotenv-rails', :groups => [:development, :test]

And then execute:

$ bundle

It should be listed in the Gemfile before any other gems that use environment variables, otherwise those gems will get initialized with the wrong values.

Sinatra or Plain ol' Ruby

Install the gem:

$ gem install dotenv

As early as possible in your application bootstrap process, load .env:

require 'dotenv'
Dotenv.load

To ensure .env is loaded in rake, load the tasks:

require 'dotenv/tasks'

task :mytask => :dotenv do
    # things that require .env
end

Usage

Add your application configuration to your .env file in the root of your project:

S3_BUCKET=YOURS3BUCKET
SECRET_KEY=YOURSECRETKEYGOESHERE

You can also create files per environment, such as .env.test.

S3_BUCKET=tests3bucket
SECRET_KEY=testsecretkey

An alternate yaml-like syntax is supported:

S3_BUCKET: yamlstyleforyours3bucket
SECRET_KEY: thisisalsoanokaysecret

Whenever your application loads, these variables will be available in ENV:

config.fog_directory  = ENV['S3_BUCKET']

Capistrano integration

If you use 2.x.x version:
In your config/deploy.rb file:

require "dotenv/capistrano"

It will symlink the .env located in /path/to/shared in the new release.
Remember to add :production group to the dotenv-rails gem in your application's Gemfile:

gem 'dotenv-rails', :groups => [:development, :test, :production]

---

If you use 3.x.x version:
In your config/deploy.rb file:
add .env to the list of linked files, similar to:

set :linked_files, %w{config/database.yml .env}

Should I commit my .env file?

It is recommended that you store development-only settings in your .env file, and commit it to your repository. Make sure that all your credentials for your development environment are different from your other deployments. This makes it easy for other developers to get started on your project, without compromising your credentials for other environments.

from https://github.com/bkeepers/dotenv