Using git rebase --onto to attach a branch to an earlier commit

February 20, 2022  [git] 

Let’s say you a working on a feature branch feature that is “attached” to a commit X. The latter was at some point the latest commit in the main branch, for instance when you started working on feature, or when you rebased your branch against main to keep up with what the others have added to main. Futher you see that there is some weird stuff happening due to certain changes intoructed in some commits preceding X, and you want to check how your code in the feature branch works as if it was based on some earlier commit, say Y. The original configuration looks like that:

...-(Y)-( )-( )-...-(X)-...-(main)
                       ( )-( )-(feature) 

To “re-attach” your feature branch, you apply the full version of git rebase, namely the one with the --onto option. For out situation, the command is as follows (where X and Y denote SHA-1 hashes on the respective commits of interest):

$ git rebase --onto Y X feature

Or, if we are already on the feature branch:

$ git rebase --onto Y X

The configuration changes to the following:

...-(Y)-( )-( )-...-(X)-...-(main)
       ( )-( )-(feature) 

To attach the branch back to X or to the tip of the updated main, the short version of git rebase is sufficient:

$ git rebase X # or "git rebase main"

If it feels scary to experiment with rewriting Git history, a safe approach is to create a copy of the current branch:

# (when on feature)
$ git branch feature_backup

Then, no matter what you do with the feature branch after that, its exact original copy will “live” safely, marked as feature_backup. Later when you don’t need the backup branch any more, you can force-delete it with the -D option:

$ git branch -D feature_backup

Further reading:

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