Can you change a file content during git commit?

One of the things I keep in my open novel in GitHub is a list of words I would like to set automatically the first line, which is the number of words in the dictionary. My first option is to write a pre-commit hook that reads the file, counts the words, rewrites the first line and writes it back again. Here's the code

PRE_COMMIT {
  my ($git) = @_;
  my $branch =  $git->command(qw/rev-parse --abbrev-ref HEAD/);
  say "Pre-commit hook in $branch";
  if ( $branch =~ /master/ ) {
     my $changed = $git->command(qw/show --name-status/);
     my @changed_files = ($changed =~ /\s\w\s+(\S+)/g);
     if ( $words ~~ @changed_files ) {
       my @words_content = read_file( $words );
       say "I have $#words_content words";
       $words_content[0] = "$#words_content\n";
       write_file( $words, @words_content );
     }
   }
};

However, since the file has already been staged, I get this error

error: Your local changes to the following files would be overwritten by checkout: text/words.dic Please, commit your changes or stash them before you can switch branches. Aborting

Might it be better to do it as a post-commit hook and have it changed for the next commit? Or do something completely different altogether? The general question is: if you want to process and change the contents of a file during commit, what's the proper way of doing it?

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.]

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 :

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

novel.txt filter=updateDict
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

Can you change a file content during git commit?

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