How would I write a pre-merge hook in Git?

The question says it all. Is there a way to perform an action before a merge? I'm guessing there's a way to make use of a pre-commit hook, but I'm not quite sure.

You can try using the prepare-commit-msg hook. The second argument will be merge "if the commit is a merge or a .git/MERGE_MSG file exists". A non-zero exit status will abort the commit.

I don't think this will work with a fast-forward merge, since there won't be a commit message.

More info on hooks: https://www.kernel.org/pub/software/scm/git/docs/githooks.html#_prepare_commit_msg

With Git 2.24 (Q4 2019), no need for script wrapper, or prepare-message hook.

A new "pre-merge-commit" hook has been introduced.

See commit bc40ce4, commit 6098817, commit a1f3dd7 (07 Aug 2019) by Michael J Gruber (mjg).
See commit f78f6c7 (07 Aug 2019) by Josh Steadmon (steadmon).
^{(Merged by Junio C Hamano -- gitster -- in commit f76bd8c, 18 Sep 2019)}

> ## git-merge: honor pre-merge-commit hook

> git-merge does not honor the pre-commit hook when doing automatic merge commits, and for compatibility reasons this is going to stay. > > Introduce a pre-merge-commit hook which is called for an automatic merge commit just like pre-commit is called for a non-automatic merge commit (or any other commit).

The documentation now includes:

> ## pre-merge-commit > > This hook is invoked by git-merge.
It takes no parameters, and is invoked after the merge has been carried out successfully and before obtaining the proposed commit log message to make a commit.
Exiting with a non-zero status from this script causes the git merge command to abort before creating a commit. > > The default 'pre-merge-commit' hook, when enabled, runs the 'pre-commit' hook, if the latter is enabled. > > This hook is invoked with the environment variable GIT_EDITOR=: if the command will not bring up an editor to modify the commit message. > > If the merge cannot be carried out automatically, the conflicts need to be resolved and the result committed separately (see git-merge).
At that point, this hook will not be executed, but the 'pre-commit' hook will, if it is enabled.

---

With Git 2.36 (Q2 2022), the callers of run_commit_hook() learn if it got "success" because the hook succeeded or because there wasn't any hook.

See commit a8cc594, commit 9f6e63b (07 Mar 2022) by Ævar Arnfjörð Bjarmason (avar).
^{(Merged by Junio C Hamano -- gitster -- in commit 7431379, 16 Mar 2022)}

> ## merge: don't run post-hook logic on --no-verify
> ^{Signed-off-by: Ævar Arnfjörð Bjarmason}

> Fix a minor bug introduced in bc40ce4 ("merge: --no-verify to bypass pre-merge-commit hook", 2019-08-07, Git v2.24.0-rc0 -- merge listed in batch #3), when that change made the --no-verify option bypass the pre-merge-commit hook it didn't update the corresponding find_hook() (later hook_exists()) condition.
> > As can be seen in the preceding commit in 6098817 ("git-merge: honor pre-merge-commit hook", 2019-08-07, Git v2.24.0-rc0 -- merge listed in batch #3) the two should go hand in hand.
> There's no point in invoking discard_cache() here if the hook couldn't have possibly updated the index.
> > It's buggy that we use "hook_exist()" here, and as discussed in the subsequent commit it's subject to obscure race conditions that we're about to fix, but for now this change is a strict improvement that retains any caveats to do with the use of "hooks_exist()" as-is.

Another nice workaround would be to add a shell script, call it like you want, then add these lines to the script:

git() {
	if [ "$1" == "merge" ]; then
		echo "seems to work like a charme"
	fi
	command git "$@"
}

git "$@"

Then make an

alias git="./my-pre-merge-script.sh"

Then you are good to go. You just added your own pre-merge hook. I know, that you do not have access to whatever arguments git would pass to a real pre-merge hook, but you can prepare files or whatever you want to prepare for merge now; I personally am very happy with this approach: I spent 2 or 3 whole days to find something for pre-merge, then I had to go with the pre-commit-msg which I did not find accurate enough for my needs. This solves all my problems. Hope this helps anybody in the future.