Ruby style guide git pre-commit hook using Rubocop as the style guide checker. Only runs on staged ruby files that have been added and/or modified.

rubocop_pre_commit_hook

#!/usr/bin/env ruby

# http://ruby-doc.org/stdlib-2.0.0/libdoc/English/rdoc/English.html
require 'English'
require 'rubocop'

ADDED_OR_MODIFIED = /^\s*(A|AM|M)\s*/
GIT_COMMAND = "git status --porcelain".freeze

modified_files = `#{GIT_COMMAND}`.split(/\n/)
changed_files = modified_files
                .select { |file| file =~ ADDED_OR_MODIFIED }     # Check monitored files
                .map { |file| file.gsub(ADDED_OR_MODIFIED, '') } # collect modified files names
                .select { |file| File.extname(file) == '.rb' }   # Check to ruby files (.rb)
                .join(' ')                                       # joins files in string

system("rubocop --color #{changed_files}") unless changed_files.empty?
exit $CHILD_STATUS.to_s[-1].to_i

See https://gist.github.com/mpeteuil/6147292#gistcomment-1733607