Keeping Child Branch up to Date with Parent when using commit --amend

Question

My current Git repo has three branches: mainline, feature_a, and feature_b. The history is as follows:

1. mainline was cloned from a remote repo
2. feature_a branch was checked out from mainline
3. Changes were made on feature_a and committed as commit_a
4. feature_b branch was checked out from feature_a
5. Changes were made on feature_b and committed as commit_b
6. Changes were made to feature_a and amended to commit_a

How do I update the feature_b branch with the changes amended to commit_a?

Answer 1

Your history probably looks something like this:

           A' [feature_a]
          /
*--*--*--* [mainline]
          \
           A [original feature_a]
            \
             B [feature_b]

Commit A is the original commit you made on feature_a. When you amended the commit with git commit --amend, it created a new commit, A'.

You'll need to rebase feature_b to attach it to the new commit A'. Do:

git rebase --onto feature_a A feature_b

(You might also be able to get away with simply doing git rebase feature_a feature_b, but I'm not sure how this will work since you amended commit A.)

Now your history will look like this:

             B' [feature_b]
            /
           A' [feature_a]
          /
*--*--*--* [mainline]

Technically commits A and B are still in the repo, but since they aren't being used, Git will garbage-collect them later.