As I am currently working on a rather large software development project (we are still in the planning phase) we have been thinking about a proper VCS. For some features we want to have for collaborating with the whole team it will be needed that we can execute a shell script whenever something is added to the staging area.
I have read through different websites and the Git Book as well as the man pages for Git, but could not find a solution.
There are all kinds of hooks; Pre-Commit, Prepare-commitmsg etc. but unfortunately there's nothing (AFAIK) that responds to changes in the staging area.
Thanks in advance for any help provided.
EDIT: From the comment I've got (and I've tried out) I now know that a git clean filter would be what I want to use. It lets me do the operations I want to perform and cancel the stage by supplying a non-zero return value. This doesn't really help, though, because I would need something that's interactive or at least that provides output to the user which seems impossible by using filters.
Use git filters as you already found out yourself.
To output a message to the user you can redirect stdout to stderr like echo "message to user" >&2 in a shell script.