Find a file in the current directory or in any of it's parents

find_config.rb

#! /usr/bin/env ruby

require 'pathname'

def find_config_file(file_name, path)
  path = Pathname(path) unless path.is_a? Pathname
  path = path.realpath

  path.ascend do |subpath|
    location = subpath + file_name
    return location if location.exist?
  end

  nil
end

ARGV.each do |filename|
  puts find_config_file filename, '.'
end