SlackのIncoming Webhooksにポストするだけの簡単なRubyスクリプト

post_to_slack.rb

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

uri = URI.parse("https://hooks.slack.com/services/...(あたなのWebhook URL)")
payload = {
  text: "テストメッセージ",
  channel: "#random",
  username: "bot",
  icon_emoji: ":ghost:"
}

Net::HTTP.post_form(uri, { payload: payload.to_json })