bot.rb

#!/bin/ruby
# coding: utf-8

require 'discordrb'

bot = Discordrb::Commands::CommandBot.new(
  token: ENV['DISCORD_TOKEN'],
  client_id: ENV['DISCORD_CLIENT_ID'],
  prefix: '!'
)

bot.command(:test, min_args: 0, max_args: 1) do |event, arg = 42|
  "Argument was: #{ arg }"
end

puts bot.invite_url
bot.run