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

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.

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.

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