### parse_modified_files_from_git.rb

#!/usr/bin/env ruby -wn

modified_file_pattern = /^#\s+(?:modified|new file):\s+(.*)$/
puts $1  if modified_file_pattern =~ $_

### rm_trailing_whitespace.rb

#!/usr/bin/env ruby -wn

$:.unshift(File.dirname(__FILE__))

require "trailing_whitespace_eraser"
TrailingWhiteSpaceEraser.rm_trailing_whitespace!($_)

### trailing_whitespace_eraser.rb

class TrailingWhiteSpaceEraser
  FILE_TYPES = ["rb", "feature", "yml", "erb", "haml"]

  def self.rm_trailing_whitespace_from_file!(file)
    trimmed = File.readlines(file).map do |line|
      line.gsub(/[\t ]+$/, "")
    end
    open(file, "w") { |f| f.write(trimmed) }
  end

  def self.rm_trailing_whitespace!(root)
    root = File.expand_path(root)
    files = File.directory?(root) ? Dir.glob("#{root}/**/*.{#{FILE_TYPES.join(',')}}") : [root]
    files.each { |file| rm_trailing_whitespace_from_file!(file.chomp) }
  end
end

### run it like so:

#!/bin/sh
parse_modified_files_from_git_status.rb | rm_trailing_whitespace.rb