"which" command Ruby implementation

which.rb

#! /usr/bin/env ruby                                                           

def which( cmd, *additional )
  paths  = ENV['PATH'].split( File::PATH_SEPARATOR )
  paths += additional if additional
  path   = paths.map { |d|
    file = File.join( d, cmd )
    ( File.exist?( file ) ) ? file : nil
  }.compact.first

  return ( path ) ? path : "command not found : #{cmd}"
end

puts which( ARGV[0], *ARGV[1..ARGV.size-1] )