pre-commit.rb

#! /usr/bin/ruby

# Pre-commit hook for removing trailing whitespace, converting tabs to
# spaces and removing blank lines at the end of a file.
#
# Author: Clifton King <cliftonk@gmail.com>
# gist:   https://gist.github.com/3173611

if File.exists? '.git/merge-rebase'
  puts "Detected '.git/merge-rebase'! Skipping pre-commit hook..."
  exit 0
end

diff_errors =
  `git diff --staged --check HEAD`.each_line.reduce({}) do |memo, l|
    if l =~ /\A([^\+\-][^:]+):(\d+): (.*)\.\Z/
      file, line, message = $~[1..-1]

      puts "pre-commit -> #{message} in '#{file}' on line #{line}"

      memo[file] ||= []
      memo[file] << line
    end
    memo
  end

diff_errors.each do |file, lines|
  puts "\nFixing #{file}..."
  old_blob = `git rev-parse :0:#{file}`.strip

  tabs = lines.map {|l| "#{l} s/\t/  /g" }.join ';'
  trailing_whitespace = lines.map { |l| "#{l} s/[ \\t]*$//" }.join ';'
  blank_eof_line = ":a -e '/^\\n*$/{$d;N;ba' -e '}'"

  new_blob =
    `git cat-file blob #{old_blob}      |
     sed -e '#{tabs}'                   \
         -e '#{trailing_whitespace}'    |
     sed -e  #{blank_eof_line}          |
     git hash-object -w --stdin`.strip

  puts "  old blob: #{old_blob}\n  new blob: #{new_blob}"

  new_entry = `git ls-files --stage "#{file}"`.strip.
    sub(/ \w+ /, " #{new_blob} ")

  `git diff #{old_blob} #{new_blob} | patch "#{file}"`
  `echo "#{new_entry}" | git update-index --index-info`
end

if diff_errors.any?
  puts "\nCorrected formatting errors in your commit!"
end

You can use #!/usr/bin/env ruby as the hashbang and that will account for whatever version of ruby they are using with RVM or Rbenv

Yeah I did that on purpose. We use REE on RVM and there's a spin-up time that's annoying with it, so I'm preferring system ruby