How can I allow git merge commits to master but prevent non-merge commits?

Question

I have a Git pre-commit hook that prevents me from committing to master unless overridden, in order to encourage developing on branch.

However I would like to automatically allow merge commits to master. Is there a way to identify a merge commit from my pre-commit hook script? The script looks like this:

#!/bin/bash

BRANCH=`git branch --color=never| grep '^*'|cut -c3-`

if [ "${BRANCH}D" == "masterD" -a "${GIT_COMMIT_TO_MASTER}D" != "trueD" ]
then
  echo "Commit directly to master is discouraged."
  echo "If you want to do this, please set GIT_COMMIT_TO_MASTER=true and then commit."
  exit 1
fi

SOLVED: For anyone looking for a cut-and-paste, the working version of this hook script is:

#!/bin/bash

BRANCH=$(git rev-parse --abbrev-ref HEAD)

if [ "${BRANCH}" == "master" -a "${GIT_COMMIT_TO_MASTER}" != "true" ]
then
  if [ -e "${GIT_DIR}/MERGE_MODE" ]
  then
    echo "Merge to master is allowed."
    exit 0
  else
    echo "Commit directly to master is discouraged."
    echo "If you want to do this, please set GIT_COMMIT_TO_MASTER=true and then commit."
    exit 1
  fi
fi

Answer 1

I put a few comments in, but the important one here is that in a pre-commit hook, the commit you're about to test does not yet exist, so you can't count its parents.

Here's what you do get:

• If you're using git commit --amend to amend a merge commit, the pre-commit hook is run as usual, but it can't really detect that this is happening. The new commit will be a merge, but you can't tell.

• If you're using regular old git commit to create a non-merge commit, the file MERGE_HEAD will not exist in the git directory, and you can tell that this is not going to create a merge commit.

• If you're using git commit to finish off a conflicted merge, the file MERGE_HEAD will exist, and you can tell that this is going to create a merge commit.

• If you're running git merge and it succeeds on its own, it makes a new commit without using the pre-commit hook, so you don't even get invoked here.

Hence, if you're willing to allow git commit --amend on merges to misfire, you can get close to what you want: just test for the existence of $GIT_DIR/MERGE_HEAD to see if this is a git commit that is finishing off a conflicted merge. (The use of $GIT_DIR is a trick to make this work even if the commands are run outside the git tree. Git sets $GIT_DIR so that in-hook git commands will work right.)