Git pre-commit hook instructions

README.md

@@ -371,7 +371,46 @@ pip install pylint cardboardlint
 cardboardlinter --refspec master
 ```
 
-This will compare the code against master and run the linter on all the changes. We plan to introduce more linter checks (e.g. for C++) in the future.
+This will compare the code against master and run the linter on all the changes. We plan to introduce more linter checks (e.g. for C++) in the future. To run it automatically as a git pre-commit hook, do the following:
+
+```bash
+cat <<\EOF > .git/hooks/pre-commit
+#!/bin/bash
+if [ ! -x "$(command -v cardboardlinter)" ]; then
+    exit 0
+fi
+
+# First, stash index and work dir, keeping only the
+# to-be-committed changes in the working directory.
+echo "Stashing working tree changes..." 1>&2
+old_stash=$(git rev-parse -q --verify refs/stash)
+git stash save -q --keep-index
+new_stash=$(git rev-parse -q --verify refs/stash)
+
+# If there were no changes (e.g., `--amend` or `--allow-empty`)
+# then nothing was stashed, and we should skip everything,
+# including the tests themselves.  (Presumably the tests passed
+# on the previous commit, so there is no need to re-run them.)
+if [ "$old_stash" = "$new_stash" ]; then
+    echo "No changes, skipping lint." 1>&2
+    exit 0
+fi
+
+# Run tests
+cardboardlinter --refspec HEAD^ -n auto
+status=$?
+
+# Restore changes
+echo "Restoring working tree changes..." 1>&2
+git reset --hard -q && git stash apply --index -q && git stash drop -q
+
+# Exit with status from test-run: nonzero prevents commit
+exit $status
+EOF
+chmod +x .git/hooks/pre-commit
+```
+
+This will run the linters on just the changes made in your commit.
 
 ## Contact/Getting Help