app.rb

require 'sinatra/base'

class Bacon < Sinatra::Base
  enable :logging, :clean_trace
  
  get '/' do
    'Hello from Sinatra!'
  end
end

config.ru

require 'sinatra'
require 'app.rb'

set :environment, (ENV["RACK_ENV"] || :production).to_sym
 
disable :run
 
log_dir = File.join(root_dir, "log")
FileUtils.mkdir_p(log_dir)
log = File.open(File.join(log_dir, "web-app.log"), "a")
# The Important Part iirc
$stdout.reopen(log)
$stderr.reopen(log)

run Bacon