git merge-base Command: Find Common Ancestor in CI

git merge-base finds the commit where two branches diverged, their best common ancestor.

Change-aware CI needs to diff a PR against the point it branched from, not against the tip of the target branch. merge-base computes exactly that fork point.

Common flags

• <a> <b> - print the best common ancestor of two commits
• --fork-point <ref> - find where the current branch forked, using reflog data
• --is-ancestor <a> <b> - exit zero if A is an ancestor of B (a test, no output)
• --all - print all best common ancestors when several exist
• --octopus - common ancestor for more than two commits

Example

# Diff a PR against the point it diverged from main
BASE="$(git merge-base origin/main HEAD)"
git diff --name-only "${BASE}...HEAD"

In CI

Computing the merge-base is the correct way to scope a PR diff; using origin/main directly includes unrelated commits merged after the branch was cut. This requires enough history to reach the fork point, so use fetch-depth: 0 (or a deepened fetch) on PR builds.

Key takeaways

• git merge-base finds the true PR fork point for accurate changed-files diffs.
• --is-ancestor is a clean boolean test for "is this commit reachable".
• It needs history back to the divergence, so PR builds want fetch-depth 0.

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