git hook to run a command after `git pull` if a specified file was changed. Run `chmod +x post-merge` to make it executable then put it into `.git/hooks/`.

post-merge

#!/usr/bin/env bash
# MIT © Sindre Sorhus - sindresorhus.com

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"
}

# `npm install` and `npm prune` if the `package.json` file gets changed
# to update all the nodejs ( grunt ) dependencies deleting the unused packages (not listed into the  `package.json` file)
check_run package.json "npm install && npm prune"

# `bower install` and `bower prune` if the `bower.json` file gets changed
# to install all the frontend dependencies removing the unused packages ( not listed into the  `bower.json` file )
check_run bower.json "bower install && bower prune"

# `composer install` if the `composer.json` file gets changed
# to update all the php dependencies
check_run composer "composer install"