Basic Ruby+git test-before-commit workflow, testing only the changes being committed

pre-commit

#!/bin/sh

exec bundle exec rake pre_commit

Rakefile

task(default: [:link_hooks, :test])

task :link_hooks do
  sh 'ln', '-nsf', '../../hooks/pre-commit', '.git/hooks/pre-commit'
end

task :pre_commit do
  require 'open3'

  Dir.mktmpdir do |tmpdir|
    tree = "#{tmpdir}/tree"
    sh 'git', 'clone', Dir.getwd, tree
    diff, status = Open3.capture2(*%w(git diff --cached))
    raise "git diff failed: #{status}" unless status.success?

    Dir.chdir(tree) do
      unless diff.empty?
        output, status = Open3.capture2(*%w(git apply), stdin_data: diff)
        raise "git apply failed: #{status}" unless status.success?
      end
      sh *%w(bundle exec rake test)
    end
  end
end