Run rubocop on only staged files in git pre-commit

pre-commit

#!/bin/sh

STAGED_FILES=$(git diff-index HEAD --name-only --cached)
if [[ -z $STAGED_FILES ]]
then
  exit # no staged files, no need to run rubocop
fi

# Checks if any staged files have unstaged changes
# otherwise rubocop isn't running on what is actually
# going to be committed.
WARN_FILES=$(git diff-files --stat -- $STAGED_FILES)
if [[ -n $WARN_FILES ]]
then
  echo 'There are unstaged changes to files (git stash --keep-index to commit safely):'
  echo "$WARN_FILES\n"
  exit 1
fi

echo "Running rubocop..."
exec git diff --cached --name-only --relative --diff-filter=ACMR | xargs \
  bundle exec rubocop --force-exclusion \
                      --autocorrect-all --fail-level A \
                      --format simple \
                      --parallel --cache true

Note that this applies RuboCop's unsafe autocorrect (-A) option, but still prevents the commit if all the offenses were corrected. The changes it makes won't be staged in git yet, so it should be easy to undo them if it isn't what you wanted.

Change bundle exec rubocop to bin/rubocop or just rubocop if you don't want to use bundle exec.