simple example for git hooks

Does anyone have a simple example of a git pre-commit hook

I have difficulties finding understandable tutorals about git hooks.

What I am looking at is basic examples about

precommit hooks ================

- how get the commit comment and check it's contents
- how to get list of modified files

The issue I'm currently blocked with is rather simple.

I'd like to get a list aof all new or modified file names, such, that I can check, that for example all .h files contain a project specific header.

As soon as I have the file names I should be able to proceed.

How could I do this best from a shell script.

Is there any clear documentation about hich git commands I'm allowed to use during a trigger script and which ones I can't

Lateron I would be interested to implement a small server script, that refuses a git push in case, that the most recent commit in a branch would contain .h files without a certain header text

thanks for your help