sinatraでログ出力

さくっとログはくには

get '/' do
  logger.info "loading data"
  # ...
end

ファイルにログはくには

require 'sinatra'

configure do
  # logging is enabled by default in classic style applications,
  # so `enable :logging` is not needed
  file = File.new("#{settings.root}/log/#{settings.environment}.log", 'a+')
  file.sync = true
  use Rack::CommonLogger, file
end

get '/' do
  'Hello World'
end

参考
http://www.sinatrarb.com/intro.html#Logging
Sinatra Recipes - Middleware - Rack Commonlogger