hostcheck.rb

#!/usr/bin/env ruby

require 'curb'
require 'yaml'

hosts = %w(
  example.com
  google.com
)

@sid = 'YOUR TWILIO SID'
@key = 'YOUR TWILIO KEY'

def send_sms text
  params = {
    From: 'YOUR TWILIO NUMBER',
    To: 'YOUR CELL PHONE NUMBER',
    Body: text
  }
  url = "https://#{@sid}:#{@key}@api.twilio.com/2010-04-01/Accounts/#{@sid}/SMS/Messages"
  Curl.post url, params
end

file = '/tmp/last_responses'
last_responses = YAML.load_file(file) if File.exist?(file)
last_responses ||= {}

hosts.each do |host|
  last_responses[host] ||= 200
  result = Curl.get(host) { |c| c.follow_location = true }

  code = result.response_code
  send_sms "#{Time.now.utc}: #{host} response code #{code}" if code != last_responses[host]
  last_responses[host] = code
end

File.open(file, 'w') { |f| f.write last_responses.to_yaml }

setup.sh

# Assuming debian/ubuntu.

# Change this path to wherever you put the hostcheck.rb file
chmod u+x /usr/local/bin/hostcheck.rb

gem install curb

# This will run every 5m. Change the 5 to whatever frequency you want the checks run.
cat << EOF | sudo tee /etc/cron.d/hostcheck
*/5 * * * * `whoami` /usr/local/bin/hostcheck.rb
EOF