slack.rb

#!/usr/bin/env ruby

require 'json'
require 'uri'
require 'net/http'

URL = '<YOUR INCOMING WEBHOOK URL HERE>'

MESSAGE = {
  username:   'monit',
  text:       ARGV * ' ',
  icon_emoji: ":rotating_light:"
}

uri = URI(URL)
https = Net::HTTP.new(uri.host, uri.port)
https.use_ssl = true

request = Net::HTTP::Post.new(uri.path)
request.body = JSON.unparse(MESSAGE)

p response = https.request(request)