bash - Git hook, modify commit files

Question

I'm trying to write git pre-commit hook script, it should write date of commit at the begining of modified files. My problem is that i can't add modified files to previous commit. When i trying invoke git commit again it runs recursive. How i can write script, which append time of modification at the end of modified files?

My code:

#!/bin/bash

files_modified=`git diff-index --name-only HEAD`

for f in $files_modified; do
    if [[ $f == *.groovy ]]; then
        $line = $(head -1 f)
        if [[ $line == "/%%*" ]];
        then
            sed -i 1d
        fi
        echo "/%% " + $(date +"%m_%d_%Y") + " %%" >> f
        git add f
    fi
done 
git commit --amend #recursive
exit

Answer 1

You cannot amend a commit in a pre commit hook.
And what you are doing is similar to the keyword expansion mechanism, which is not a best practice with Git (or any DVCS), as explained in "To put the prefix ?<revision-number> to codes by Git/Svn".

Other approaches include:

• generating a separate file with the information you want in it (and then commit).
  See for instance "Expanding Git SHA1 information into a checkin without archiving?".
• storing that information in git note (that are separates from commits and don't change the SHA1).
  See "Adding Git notes to a blob".