Can you change a file content during git commit?

It turns out you can run "hooks" - they are actually handled by another mechanism - when staging files (at git add time) :

https://git-scm.com/book/en/v2/Customizing-Git-Git-Attributes#_keyword_expansion

(scroll down a bit to the "smudge" and "clean" diagrams)

Here is what I understood :

1. edit the .gitattributes, and create rules for the files which should trigger a dictionary update:

   novel.txt filter=updateDict

2. Then, tell Git what the updateDict filter does on smudge (git checkout) and clean (git add):

   $ git config --global filter.updateDict.clean countWords.script

   $ git config --global filter.updateDict.smudge cat

The actual commit stuck in by git commit is whatever is in the index once the pre-commit hook finishes. This means that you can change files in the pre-commit hook, as long as you git add them too.

Here's my example pre-commit hook, modified from the .sample:

#!/bin/sh
#
# An example hook script to verify what is about to be committed.
# [snipped much of what used to be in it, added this --
#  make sure you take out the exec of git diff-index!]

num=$(cat zorg)
num=$(expr 0$num + 1)
echo $num > zorg
git add zorg
echo "updated zorg to $num"
exit 0

and then:

$ git commit -m dink
updated zorg to 3
[master 76eeefc] dink
 1 file changed, 1 insertion(+), 1 deletion(-)

But note a minor flaw (won't apply to your case):

$ git commit
git commit
updated zorg to 4
# On branch master
# Untracked files:
[snip]
nothing added to commit but untracked files present (use "git add" to track)
$ git commit
updated zorg to 5
# Please enter the commit message for your changes. Lines starting
[snip - I quit editor without changing anything]
Aborting commit due to empty commit message.
$ git commit
updated zorg to 6
# Please enter the commit message for your changes. Lines starting

Basically, because the pre-commit hook updates and git adds, the file keeps incrementing even though I'm not actually doing the commit, here.

[Edit Aug 2021: I need to emphasize that I do not recommend this approach. Note that there are some oddball cases that can come up when using git commit -a, git commit --include, and git commit --only, including the implied --only that is inserted if you name files on the command line. This is due to the fact that this kind of git commit creates a second, and sometimes even a third, internal Git index. Any git add operations you do inside a hook can only affect one of these two or three index files.]