run RuboCop on staged files with pre-commit hook

pre-commit

#!/usr/bin/env ruby
# frozen-string-literal: true

# SEE: https://gist.github.com/mpeteuil/6147292

require 'rubocop'

VALID_FILE_CODES = %w[A M].freeze

# staged files have both a valid code and a `git status --porcelain` line that doesn't start with a space
staged_files = `git status --porcelain`
               .split(/\n/) # one file per line
               .filter { |status_and_file| status_and_file[0] != ' ' }
               .map { |status_and_file| status_and_file.split(' ', 2) } # each line has a status and a filename
               .select { |status, filename| VALID_FILE_CODES.include?(status[0]) && File.extname(filename) == '.rb' }
               .map { |_status, filename| filename.delete('"') } # --porcelain may add "

system("bundle exec rubocop #{staged_files.shelljoin}") unless staged_files.empty?

exit $CHILD_STATUS.to_s[-1].to_i

git diff-index --name-status --relative --cached @ might be a bit easier to parse (and only includes staged files so you don't have to do an extra step to filter them).

Also, I couldn't use git status --porcelain because my Rails app is in a sub-folder so I needed the list of files to be relative to the Rails root instead of relative to the git repo root (although git status in general seems to respect the --relative option, git status --porcelain seems to not).

Never mind, it looks like git diff-index --name-status --relative --cached @ from within the pre-commit hook (not sure why).

But since I'm using https://github.com/rycus86/githooks, I can just get the list from:

changed = ENV['STAGED_FILES'].split("\n")