mini DSL to validate environment variables presence and value

env_vars_checker.rb

# Usage :
#   EnvVarsChecker.new do 
#     env_includes 'USER'
#     env_includes 'USER', in: -> { `echo $USER`.chomp}
#     env_includes 'USER', in: %w(deployer deploy rails)
#   end

class EnvVarsChecker
  class Error < RuntimeError ; end
  
  def initialize(&block)
    instance_eval(&block)
  end

  def env_includes(var_name, options={})
    detect_unset_variable!(var_name)
    detect_invalid_value!(var_name, options[:in]) if options[:in]
  end

private

  def detect_unset_variable!(var_name)
    raise( Error, "ENV['#{var_name}'] not found") unless ENV[var_name]
  end

  def detect_invalid_value!(var_name, possible_values)
    possible_values = possible_values.call if possible_values.is_a?(Proc)
    return if possible_values.include?(ENV[var_name])
    raise( Error, "ENV['#{var_name}'] invalid value (must be one of #{possible_values.inspect})")
  end
end

test_env_vars_checker.rb

require_relative 'env_vars_checker'

def assert_failure_detected(expected_message)
  begin
    yield
    raise "***TEST FAILURE : an EnvVarsChecker::Error should have been raised"
  rescue EnvVarsChecker::Error => e
    raise "***TEST FAILURE : invalid message (#{e.message})" unless e.message == expected_message
  end
end

#------------------------------------------------------------
# Success : var is set and value is valid
EnvVarsChecker.new do 
  env_includes 'USER'
  env_includes 'USER', in: -> { `echo $USER`.chomp}
  env_includes 'USER', in: Array(`echo $USER`.chomp)
end

#------------------------------------------------------------
# Failure : variable is not set
assert_failure_detected("ENV['an-unknown-variable'] not found") do
  EnvVarsChecker.new do 
    env_includes 'an-unknown-variable'
  end
end

# Failure : variable is set with invalid value
assert_failure_detected(%{ENV['USER'] invalid value (must be one of ["foobarbuz"])}) do
  EnvVarsChecker.new do 
    env_includes 'USER', in: ["foobarbuz"]
  end
end

# Failure : variable is set with invalid value (lambda)
assert_failure_detected(%{ENV['USER'] invalid value (must be one of ["foobarbuz"])}) do
  EnvVarsChecker.new do 
    env_includes 'USER', in: (-> { ["foobarbuz"] })
  end
end