Check Changes using Git Post Merge hook

check-changes.sh

#!/usr/bin/env bash

# Check for things that are changed. A file or directory can be checked and a
# message is printed out for the user to perform an action on.  Emoji use
# encouraged for the echo message.
#
# You may copy and paste the `if changed` statement below and modify the string
# that you'd like to grep for. Make sure you escape any back-ticks "\`" in your
# echo statements in order to include them in your message.
#
# It's recommended that you do not add commands besides `echo` statements in
# the if-statement block. This is because this command will run on every
# machine whenever that performs a `git pull` and this can cause arbitrary side
# effects.  Also make sure you include a blank `echo` statement to allow for
# visual padding for the message, otherwise it can be difficult to visually
# discern the message from the previous output.

set -eo pipefail

function changed {
  # The following command is getting the difference between what's currently in
  # HEAD and comparing it with what's in the immediate prior value of the HEAD.
  #
  # This is not the same as "the previous commit" but rather "the previous
  # value" before the `git pull`.
  #
  # It's then using `grep` to determine if the file is at the beginning of the
  # line that the `--name-only` flag is outputting.
  #
  # For more infromation about this revision parameter, see the `git-rev-parse`
  # documentation here: https://git-scm.com/docs/git-rev-parse#_specifying_revisions
  git diff --name-only 'HEAD@{1}' HEAD | grep "^$1" > /dev/null 2>&1
}

# Check for changes in the `migrations/` directory.
if changed 'migrations/'; then
  echo
  echo "🗄  The migrations/ directory has changed. You may want to run \`make db_dev_migrate\`."
fi