Git pre-commit hook for `crystal tool format`

pre-commit

#!/bin/sh
#
# This script ensures Crystal code is correctly formatted before committing it.
# It won't apply any format changes automatically.
#
# Only staged files (the ones to be committed) are being processed, but each file is checked
# entirely as it is stored on disc, even parts that are not staged.
#
# To use this script, it needs to be installed in the local git repository. For example by running
#   curl https://gist.githubusercontent.com/straight-shoota/fdaf4cf1954e084cd5abccf8f77975f6/raw/pre-commit > .git/hooks/pre-commit && chmod +x .git/hooks/pre-commit
#
# Called by "git commit" with no arguments. The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.

 changed_cr_files=$(git diff --cached --name-only --diff-filter=ACM | grep '\.cr$')

 [ -z "$changed_cr_files" ] && exit 0

 if [ -x bin/crystal ]; then
   # use bin/crystal wrapper when available to run local compiler build
   exec bin/crystal tool format --check $changed_cr_files >&2
 else
   exec crystal tool format --check $changed_cr_files >&2
 fi