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Do a bang-up job managing your environment variables
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Do a bang-up job managing your environment variables.

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ENV! provides a thin wrapper around ENV to accomplish a few things:

  • Provide a central place to specify all your app’s environment variables.
  • Fail loudly and helpfully if any environment variables are missing.
  • Prevent an application from starting up with missing environment variables. (This is especially helpful in environments like Heroku, as your app will continue running the old code until the server is configured for a new revision.)


Add this line to your application’s Gemfile:

gem 'env_bang'

Or for Rails apps, use env_bang-rails instead for more convenience:

gem 'env_bang-rails'

And then execute:

$ bundle


Basic Configuration

First, configure your environment variables somewhere in your app’s startup process. If you use the env_bang-rails gem, place this in config/env.rb to load before application configuration.

Example configuration:

ENV!.config do
  use :APP_HOST
  # ... etc.

Once a variable is specified with the use method, access it with


This will function just like accessing ENV directly, except that it will require the variable to have been specified, and be present in the current environment. If either of these conditions is not met, a KeyError will be raised with an explanation of what needs to be configured.

Adding a default value

For some variables, you’ll want to include a default value in your code, and allow each environment to ommit the variable for default behaviors. You can accomplish this with the :default option:

ENV!.config do
  # ...
  use :MAIL_DELIVERY_METHOD, default: 'smtp'
  # ...

Adding a description

When a new team member installs or deploys your project, they may run into a missing environment variable error. Save them time by including documentation along with the error that is raised. To accomplish this, provide a description (of any length) to the use method:

ENV!.config do
      'Generate a fresh one with `SecureRandom.urlsafe_base64(64)`; see'

Now if someone installs or deploys the app without setting the RAILS_SECRET_KEY_BASE variable, they will see these instructions immediately upon running the app.

Automatic type conversion

ENV! can convert your environment variables for you, keeping that tedium out of your application code. To specify a type, use the :class option:

ENV!.config do
  use :COPYRIGHT_YEAR,       class: Integer
  use :MEMCACHED_SERVERS,    class: Array
  use :MAIL_DELIVERY_METHOD, class: Symbol, default: :smtp
  use :DEFAULT_FRACTION,     class: Float
  use :ENABLE_SOUNDTRACK,    class: :boolean
  use :PUPPETMASTERS,        class: Hash

Note that arrays will be derived by splitting the value on commas (','). To get arrays of a specific type of value, use the :of option:

ENV!.config do
  use :YEARS_OF_INTEREST, class: Array, of: Integer

Hashes are split on commas (',') and key:value pairs are delimited by colon (':'). To get hashes of a specific type of value, use the :of option, and to use a different type for keys (default is Symbol), use the :keys option:

ENV!.config do
  use :BIRTHDAYS, class: Hash, of: Integer, keys: String

Default type conversion behavior

If you don’t specify a :class option for a variable, ENV! defaults to a special type conversion called :StringUnlessFalsey. This conversion returns a string, unless the value is a "falsey" string ('false', 'no', 'off', '0', 'disable', or 'disabled'). To turn off this magic for one variable, pass in class: String. To disable it globally, set

ENV!.config do
  default_class String

Custom type conversion

Suppose your app needs a special type conversion that doesn’t come with ENV_BANG. You can implement the conversion yourself with the add_class method in the ENV!.config block. For example, to convert one of your environment variables to type Set, you could write the following configuration:

# In your environment:
export NUMBER_SET=1,3,5,7,9
# In your env.rb configuration file:
require 'set'

ENV!.config do
  add_class Set do |value, options| self.Array(value, options || {})

  use :NUMBER_SET, class: Set, of: Integer
# Somewhere in your application:
#=> #<Set: {1, 3, 5, 7, 9}>

Implementation Notes

  1. ENV! is simply a method that returns ENV_BANG. In certain contexts (like defining a class), the exclamation mark notation is not allowed, so we use an alias to get this shorthand.

  2. Any method that can be run within an ENV!.config block can also be run as a method directly on ENV!. For instance, instead of

    ENV!.config do
       add_class Set do
       use :NUMBER_SET, class: Set

    It would also work to run

    ENV!.add_class Set do
    ENV!.use :NUMBER_SET, class: Set

    While the config block is designed to provide a cleaner configuration file, calling the methods directly can occasionally be handy, such as when trying things out in an IRB/Pry session.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request
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