post-checkout git hook to intelligently run `bundle` if Gemfile was changed

post-checkout

#!/bin/sh

# Via http://tbaggery.com/2011/02/07/bundle-while-you-git.html
# Save as http://.git/hooks/post-checkout in your project and `chmod +x`
# See the post for how to tell Git you want this in new/newly cloned repos

if [ $1 = 0000000000000000000000000000000000000000 ]; then
  # Special case for initial clone: compare to empty directory.
  old=4b825dc642cb6eb9a060e54bf8d69288fbee4904
else
  old=$1
fi
if [ -f Gemfile ] && command -v bundle >/dev/null &&
  git diff --name-only $old $2 | egrep -q '^Gemfile|\.gemspec$'
then
  (unset GIT_DIR; exec bundle) | grep -v '^Using ' | grep -v ' is complete'
  true
fi