yarn-post-merge-and-checkout-hooks.sh

#/usr/bin/env bash

git config --global init.templatedir '~/.git-templates' && \
mkdir -p ~/.git-templates/hooks && \
cd ~/.git-templates/hooks

cat >post-checkout <<'EOL'
changed_files="$(git diff-tree -r --name-only --no-commit-id ORIG_HEAD HEAD)"
check_run() {
  echo "$changed_files" | grep -E --quiet "$1" && eval "$2" || true
}
# yarn all the things if yarn.lock has changed
check_run "web/yarn.lock" "cd web && yarn"
EOL

cp post-checkout ./post-merge && cp post-checkout ./post-fetch
chmod -R a+x .

This will set up git hooks that ensure you yarn when you ought to.

To install,

curl -L https://gist.githubusercontent.com/realyze/f2a53a4119f955ff63337af72ebbc5cf/raw/5892159f63e5c7e0248ca0e395abd86088f15494/yarn-composer-post-merge-and-checkout-hooks.sh | bash
git init 

Important notes:

• Don't run this if you already have your own ~/.git-templates as this would overwrite them.
• If you already have hooks configured in .git/hooks/, this won't change them so you'll want to manually add the section in EOL