forked pre-commit hook for changed ruby files. (whitespace sensitivity in the regex was borking selection)

rubocop_pre_commit_hook

#!/usr/bin/env ruby

require 'english'
require 'rubocop'

ADDED_OR_MODIFIED = /^\s*(A|AM|M)/.freeze

changed_files = `git status --porcelain`.split(/\n/).
    select { |file_name_with_status|
      file_name_with_status =~ ADDED_OR_MODIFIED
    }.
    map { |file_name_with_status|
      file_name_with_status.split(' ')[1]
    }.
    select { |file_name|
      File.extname(file_name) == '.rb'
    }.join(' ')

system("rubocop #{changed_files}") unless changed_files.empty?

exit $CHILD_STATUS.to_s[-1].to_i

throw it into the hooks folder in the .git folder:

.git/hooks/pre-commit