How to detect commit --amend by pre-commit hook ?

Question

When I do commit --amend, it is unsafe commit if the commit already has been pushed to remote repository.

I want to detect unsafe commit --amend by pre-commit hook and a

Answer 1

A quick way to detect a "pure" amend in the pre-commit hook:

if git diff --cached --quiet ; then
  echo "This is a pure amend"
else
  echo "This is a commit with changes"
fi

By "pure" I mean you're only rewriting the commit message and not any of the changes in the commit. If there are any changes in your index when you call git commit --amend, you're rewriting more than the commit message and this will behave as if you're doing a conventional git commit.