A small helper method that temporarily changes environment variables in specs. It's an alternative to the ClimateControl rubygem that feels more like RSpec.

let_env.rb

# A tiny RSpec helper 
#
# spec/support/helpers/let_env.rb
module LetEnv
  def let_env(name)
    around do |example|
      @original_environment_vars ||= {}
      @original_environment_vars[name] ||= ENV[name.to_s]

      ENV[name.to_s] = yield
      example.run
      ENV[name.to_s] = @original_environment_vars[name]
    end
  end
end

# Setup
require_relative 'support/helpers/let_env.rb'

Rspec.configure do |config|
  config.extend LetEnv
end

# Usage
let_env(:ENV_VAR) { 'temporary value' }

after(:all) do
  # Back to original value.
  puts ENV['ENV_VAR']
end

it 'will read temporary_value' do
  expect(ENV['ENV_VAR']).to eq('temporary value')
end