Where does RACK log to?

The line of LOGGER = Logger.new("sinatra.log") in @include's answer above did not work for me.

However, an alternative listed here (along with some helpful explanations) worked for me, tested with ruby 2.5.3 and sinatra 2.0.1.

For additional info, that alternative is based on the structure presented in a Sinatra recipe.

It depends. Many developers define their app log file to app/servername.log or just to the current path where the Rack app is loaded.

Yes you can change it's path.

Usually you get a config.ru file with something like:

    log = File.new("sinatra.log", "a+")
    $stdout.reopen(log)
    $stderr.reopen(log)

    # optionally to sync logs while the server is running
    $stderr.sync = true
    $stdout.sync = true

and/or

    configure do
      LOGGER = Logger.new("sinatra.log")
      enable :logging, :dump_errors
      set :raise_errors, true
    end

in this case the log file is located under appdir/sinatra.log. But remember this code can be anywhere in your Rack app, so please seek for "log" in your application directory.

    $ cd projectname
    $ grep -ri 'log' *

have fun and post here your config.ru and/or the mainprojectfile.rb.