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What `git bisect run` does
Git bisect performs a binary search between a revision known to be good and one known to be bad. With git bisect run, Git checks out candidate revisions and runs your command at each one. The command’s exit status tells Git how to classify that revision, allowing it to choose the next candidate.
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The result is only as reliable as the two endpoint classifications and the test. The test must measure the same suspected behavior at each revision; a test that is inconsistent across versions or produces flaky results can lead to an unreliable classification. See the Git bisect documentation for the command’s rules and examples.
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Possibly, but four is an idealized estimate, not a fixed execution count. Each good-or-bad result divides the remaining candidate range. Four binary decisions can distinguish among up to sixteen equally selectable possibilities, so four runs can be enough when searching a range of about twelve candidate revisions.
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The phrase “twelve commits” needs a precise meaning. If the two known endpoints are included in the twelve, fewer than twelve commits remain as possible culprits. If twelve means twelve candidate revisions between known endpoints, that is a different search range. Git’s documentation describes approximate step counts rather than promising exactly four test executions for every twelve-commit history.
History shape and testability also matter. Uneven histories, merges, and revisions that must be skipped can change the number of useful decisions. A skipped commit near the boundary may leave Git unable to name one unique first bad commit.
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Set known-good and known-bad endpoints
Choose a revision where the test passes and one where it fails, then make that range explicit. The Git documentation demonstrates this command:
git bisect start HEAD HEAD~10 --In this example, HEAD is treated as bad and HEAD~10 as good. These are illustrative endpoints, not defaults to copy blindly; replace them with revisions that actually bracket the regression in your repository.
Write a test command that classifies each revision
For git bisect run, the exit status has a specific meaning:
- 0: the revision is good (the old behavior).
- 1 through 127, except 125: the revision is bad (the new behavior).
- 125: the revision cannot be tested, so Git should skip it.
- Any other status: the bisect run aborts.
A test script should return 0 when the suspected regression is absent and a bad status when it is present. If a revision cannot build for an unrelated reason, returning 125 prevents that build failure from being mistaken for the regression. The documented example uses:
#!/bin/sh
make || exit 125
~/check_test_case.shHere, a failed build makes the revision untestable, while the checker supplies the good-or-bad result. Use 125 only when the selected revision cannot be tested—not as a general way to report a failing test. When practical, keep the scripts outside the repository to avoid interactions among the bisect, build, and test processes.
Run the automated search
- Start the bisect with the known bad and good endpoints, as in
git bisect start HEAD HEAD~10 --, after adapting the revisions to your regression window. - Run your test script with
git bisect run ~/test.sh. Git will check out candidates and use the script’s exit status to narrow the range. - Read Git’s result in light of any skipped revisions. If a skipped commit is adjacent to the suspected change, the output may identify a boundary or narrowed region rather than a uniquely proven culprit.
To compare approaches: manual bisecting lets you inspect each revision and decide how to classify it, while git bisect run is suited to a repeatable, machine-checkable test. Automation is not a substitute for a usable test: revisions that cannot be tested need explicit handling, and an unstable test weakens the result.
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Handle skipped commits and verify the result
When Git skips an untestable revision, it may be unable to determine exactly which adjacent commit was first bad. Treat that result as a narrowed region, not as proof of one culprit. If possible, test neighboring commits manually or improve the script or environment so those revisions can be classified.
Even with no skipped revisions, confirm that the candidate reproduces the failure and that the preceding revision does not. The bisect classification relies on the endpoints and on the test applying consistently across the history; a flaky test or one that depends on changing external services can undermine that assumption.
Reset after the search
When you are finished, run:
git bisect resetBy default, this returns the checkout to the commit that was active before git bisect start. You can supply a different target commit to git bisect reset if you want to return elsewhere.
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