which_on_windows

which.cmd

@echo off
ruby -S -x "%0.cmd" %*
goto end
#!ruby -Ks

paths = ENV['PATH'].split(';')
pathexts = ENV['PATHEXT'].split(';')
cmd, = ARGV.shift

re = Regexp.union(*pathexts.map{|ext| Regexp.new("^#{cmd}#{ext}$", Regexp::IGNORECASE)})

paths.each do |path|
  begin
    puts Dir.entries(path).select{|item|re.match(item)}.map{|item|"#{path}/#{item}"}
  rescue
  end
end
__END__
:end

which.rb

#!/usr/bin/ruby 

paths = ENV['PATH'].split(';')
pathexts = ENV['PATHEXT'].split(';')
cmd, = ARGV.shift

re = Regexp.union(*pathexts.map{|ext| Regexp.new("^#{cmd}#{ext}$", Regexp::IGNORECASE)})

paths.each do |path|
  puts Dir.entries(path).select{|item|re.match(item)}.map{|item|"#{path}/item"}
end