twitter_app_only_auth.rb

require 'yaml'
require 'httparty'

def get_secret
  secret_file = File.expand_path('../.secret.yml', __FILE__)
  YAML.load_file(secret_file)
end

secret = get_secret

consumer_key_secret = "#{secret["CONSUMER_KEY"]}:#{secret["CONSUMER_SECRET"]}"
encoded_key_secret  = Base64.strict_encode64(consumer_key_secret)

headers = {
  'Authorization' => "Basic #{encoded_key_secret}",
  'Content-Type' => "application/x-www-form-urlencoded;charset=UTF-8"
}

body = 'grant_type=client_credentials'

response = HTTParty.post('https://api.twitter.com/oauth2/token', body: body, headers: headers)

puts response["access_token"] if response.code == 200