where.rb

#!/usr/bin/ruby
# encoding: utf-8

module Where
  extend self
  def where(cmd)
    return enum_for(:where, cmd) unless block_given?
    exts = ENV['PATHEXT'] ? ENV['PATHEXT'].split(';') : ['']
    ENV['PATH'].split(File::PATH_SEPARATOR).each do |path|
      exts.each do |ext|
        exe = File.join(path, "#{cmd}#{ext}")
        yield exe if File.executable? exe
      end
    end
  end

  def which(cmd)
    where(cmd).first
  end
end

raise "Usage: \`#{$0}\` executable-name" unless ARGV.size == 1
puts (Where::where(ARGV.first).to_a.tap { |result| exit 1 if result.empty? })