A pre-commit hook to run Rubocop. Adapted from another gist (see comment on line 3), but updated to run autocorrect for you and commit the changes.

pre-commit

#!/usr/bin/env ruby

# adapted from https://gist.github.com/stevenharman/f5fece7fd39356e57ce3c710911f714f

RED = "\033[0;31m"
GREEN = "\033[0;32m"
NO_COLOR = "\033[0m"

if `command -v bundle exec rubocop` == ""
  puts "💀#{RED}   Install Rubocop and be sure it is available on your PATH#{NO_COLOR}"
  puts "ℹ️   Try 'gem install rubocop'"
  exit 1
end

suspects = `git diff --staged --name-only --diff-filter=AMC *.rb`.split("\n").map(&:strip).join(" ")
exit 0 if suspects.empty?

changes_to_stash = !`git diff --name-only`.empty?
if changes_to_stash
  changed_files = `git diff --staged --name-only`.split("\n").map(&:strip).join(" ")

  `git commit --no-verify --message='temporary commit to create a clean stash'`
  `git stash push --quiet`
  `git reset HEAD^`
  `git add #{changed_files}`
end

puts "🚔   Linting files: #{suspects}"

if system("bundle exec rubocop --auto-correct #{suspects}")
  `git add #{suspects}`
  exit_status = 0
  puts "🎉#{GREEN}   Rubocop is appeased.#{NO_COLOR}"
else
  puts "#{RED}💀   Rubocop found some issues it wasn't able to auto-correct. Fix, stage, and commit again#{NO_COLOR}\n"
  exit_status = 1
end

`git stash apply --quiet && git stash drop --quiet` if changes_to_stash

exit exit_status

It will stash changes that are not staged before running and unstash them after running. But because autocorrect must run on whole files, it will correct and commit all violations in a staged file instead of just staged violations.