Using ERB

config.yml.erb

access_key: <%= access_key %>
secret_id: <%= secret_id %>

Rakefile

namespace :gsub do
  desc 'populate a configuration'
  task :populate do
    content = File.read('config.yml')
    content.gsub!('REPLACE_ME_KEY','REAL_KEY')
    content.gsub!('REPLACE_ME_ID','REAL_ID')
    puts content
  end
end

class SecretsConfig
  attr_reader :config

  def initialize(config = default_config)
    @config = config
  end

  def default_config
    'config.yml.erb'
  end

  def content
    File.read(config)
  end

  def to_yaml
    require 'erb'
    config_erb = ERB.new(content)
    config_erb.result(binding)
  end

  def access_key
    # File.read('')
    'my_key2'
  end

  def secret_id
    'my_secret2'
  end
end

namespace :erb do
  desc 'populate a configuration'
  task :populate do
    puts SecretsConfig.new.to_yaml
  end
end