[SOLVED] Get MERGE_HEAD in pre-merge-commit hook

Get MERGE_HEAD in pre-merge-commit hook

In pre-merge-commit, I need the hash of the merge head to verify a few things about the commits that are about to be merged to the current branch.

However, it seems that neither the reference MERGE_HEAD nor the file .git/MERGE_HEAD exist at the time when the hook is running.

How can I get the merge head revision in my hook?

Solution

Internally Git has a function write_merge_heads that finds the correct object and writes it to the file .git/MERGE_HEAD. Unfortunately, this function is called after the pre-merge-hook was already executed. (It's called regardless if the hook succeeds or fails.)

If you're willing to have a tiny bit of extra typing each merge, there is a really simple fix. Just exit from the pre-merge-commit hook with non-0 code if the MERGE_HEAD is not present. After the merge is resumed, MERGE_HEAD will be there.

Here is a template for such pre-merge-commit hook:

#!/bin/sh

if [ ! -f .git/MERGE_HEAD ]
then
    printf 'Cannot find the MERGE_HEAD!\n' 1>&2
    printf 'Git tends to not pass this information for the first call of the `pre-merge-commit` hook.\n' 1>&2
    printf 'Please, run `git merge --continue` or just `git commit` and it should work this time.\n' 1>&2
    exit 1
fi

# The actual hook's body...