How to find the nearest parent of a Git branch

Let’s say I have the following local repository with a commit tree like this:

master --> a
            \
             \
      develop c --> d
               \
                \
         feature f --> g --> h

master is my this is the latest stable release code, develop is my this is the ‘next’ release code, and feature is a new feature being prepared for develop.

Using hooks, I want to be able to refuse pushes to feature to my remote repository, unless commit f is a direct descendant of develop HEAD. I.e., the commit tree looks like this, because feature has been git rebase on d.

master --> a
            \
             \
      develop c --> d
                     \
                      \
               feature f --> g --> h

So is it possible to:

• Identify the parent branch of feature?
• Identify the commit in parent branch which f is a descendant of?

From there I would check what HEAD of the parent branch is, and see if f predecessor matches the parent branch HEAD, to determine if the feature needs to be rebased.

A rephrasal

Another way to phrase the question is “What is the nearest commit that resides on a branch other than the current branch, and which branch is that?”

A solution

You can find it with a little bit of command-line magic

git show-branch \
| sed "s/].*//" \
| grep "\*" \
| grep -v "$(git rev-parse --abbrev-ref HEAD)" \
| head -n1 \
| sed "s/^.*\[//"

With AWK:

git show-branch -a \
| grep '\*' \
| grep -v `git rev-parse --abbrev-ref HEAD` \
| head -n1 \
| sed 's/[^\[]*//' \
| awk 'match($0, /\[[a-zA-Z0-9\/-]+\]/) { print substr( $0, RSTART+1, RLENGTH-2 )}'

Here’s how it works:

1. Display a textual history of all commits, including remote branches.
2. Ancestors of the current commit are indicated by a star. Filter out everything else.
3. Ignore all the commits in the current branch.
4. The first result will be the nearest ancestor branch. Ignore the other results.
5. Branch names are displayed [in brackets]. Ignore everything outside the brackets, and the brackets.
6. Sometimes the branch name will include a ~# or ^# to indicate how many commits are between the referenced commit and the branch tip. We don’t care. Ignore them.

And the result

Running the above code on

 A---B---D <-master
      \
       \
        C---E---I <-develop
             \
              \
               F---G---H <-topic

Will give you develop if you run it from H and master if you run it from I.

The code is available as a gist.