Can you change a file content during git commit?

Question

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?

Answer 1

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.]