Can a Git hook automatically add files to the commit?

Question

I'd like to add an automatically generated file to the same commit using a pre- or post-commit hook in Git, dependent on the files that were modified in that commit. How would I go about this?

I've tried this as a pre-commit hook, but no luck:

#!/bin/sh
files=`git diff --cached --name-status`
re="<files of importance>"
if [[ $files =~ $re ]]
then
  echo "Creating files"
  exec bundle exec create_my_files
  exec git add my_files
  exec git commit --amend -C HEAD
fi

This successfully adds them to the repository, but does not add them to the commit. I've also tried using the last two exec lines in a post-commit hook along with the pre-commit inspection, but no good either.

Answer 1

What happens when you try this hook? Do you get an error? Are the files created? What is the output of git status afterwards?

Seems to me you shouldn't need the last line since the commit hasn't actually happened yet (pre-commit :)).

Answer 2

If the files are automatically generated, and they can be generated anywhere (implicit in your desire to build them in the Git pre-commit hook) then you shouldn't be putting them under source control in the first place. You should only control source files -- generated files should be generated as part of the build scripts.

The only reason to put a generated file under source control is when it requires unique/privileged resources to generate (such as a licensed program) or it requires a significant amount of time to generate.

Added

From http://www.kernel.org/pub/software/scm/git/docs/githooks.html :

pre-commit This hook is invoked by git commit, and can be bypassed with --no-verify option. It takes no parameter, and is invoked before obtaining the proposed commit log message and making a commit. Exiting with non-zero status from this script causes the git commit to abort.

The default pre-commit hook, when enabled, catches introduction of lines with trailing whitespaces and aborts the commit when such a line is found.

All the git commit hooks are invoked with the environment variable GIT_EDITOR=: if the command will not bring up an editor to modify the commit message.

The intent of the pre-commit hook is to be a pass-fail check on the state of the workspace and the contents of the commit, prior to making the commit. Attempting to change the contents of the commit won't work.

My recommendation would be add two steps to your build scripts: (1) a step that will build all of the out-of-date files that needs to be generated (and adds them to the workspace), and (2) a step that will check to ensure that all of the generated files are up-to-date, and return a non-zero status code. Your Git pre-commit hook should run the second step. Your developers should be trained to run the first step as necessary.

Answer 3

How about writing a post-commit script instead which generates your files, and then have that do (something along the lines of) git add my_files; git commit --amend.

Answer 4

It's possible to do what you want using pre-commit hooks. We do something similar for a heroku deployment (compiling coffeescript to javascript). The reason your script isn't working is because you used the exec command improperly.

From the man page:

The exec builtin is used to replace the currently running shells process image with a new command. On successful completion, exec never returns. exec can not be used inside a pipeline.

Only your first exec command is running. After that your script is basically terminated.

Give something like this a try (as a pre-commit hook):

#!/bin/sh
files=`git diff --cached --name-status`
re="<files of importance>"
if [[ $files =~ $re ]]
then
  echo "Creating files"
  bundle exec create_my_files
  git add my_files
fi