git merge-base

How merge and rebase decide what's new on each side.

🎯 What & why

Find the best common ancestor between commits. This is the commit merge uses as its base and rebase uses to decide which patches are 'yours'.

🧠 Mental model

History is a DAG; merge-base walks back from each input until it finds the youngest commit reachable from all of them. Get the base wrong and merges/rebases produce nonsense.

🛠️ Synopsis

git merge-base [--all] <commit> <commit>...
            git merge-base --octopus <commit>...
            git merge-base --independent <commit>...
            git merge-base --is-ancestor <commit> <commit>
            git merge-base --fork-point <ref> [<commit>]

🎚️ Switches & options

Flag	What it does
`<commit>...`	Two or more commits to find an ancestor for.
`--all`	Print every best common ancestor, not just one (criss-cross history).
`--octopus`	Compute a single base suitable for an N-way octopus merge.
`--independent`	Reduce a list to those commits not reachable from the others.
`--is-ancestor <a> <b>`	Exit 0 if <a> is an ancestor of <b>; for scripts, no output.
`--fork-point <ref> [<commit>]`	Use the reflog of <ref> to find the point you originally branched from, even if <ref> was rewritten.

💡 Use cases

Show the diff your branch introduced: git diff $(git merge-base main HEAD) HEAD.
Sanity-check whether a fix has landed on a release branch (--is-ancestor).
Rebase cleanly onto a force-pushed upstream using --fork-point.
Resolve which commits are 'extra' across feature branches with --independent.

🧪 Examples

Base of feature vs main

git merge-base main feature

Is bugfix already in release?

git merge-base --is-ancestor abc1234 release/2.0 && echo yes

Fork point against rewritten upstream

git merge-base --fork-point origin/main

All bases (criss-cross)

git merge-base --all topic1 topic2

🎓 Recommendations

When scripting --is-ancestor, check $? directly; do not parse stdout, there is none.
Prefer --fork-point for git rebase after upstream force-pushes; otherwise you'll replay commits that no longer belong to you.

🪤 Common pitfalls

Multiple best ancestors exist with criss-cross merges; without --all you get just one and may misread history.
--fork-point depends on the local reflog of the upstream ref; on a fresh clone it may have nothing useful to work with.
An octopus base is not the same as pairwise bases; do not feed it to a regular two-way merge.

🔗 Related modules

📝 Quiz

Hit each option, then Check answers. Score is recorded; Next is always open.