Defame1297
7cb8e95379
Description 450 -> 266 chars, body 1044 -> 462 words. The bisect procedure moves to references/bisect.md. The boundary clause read 'not writing or rewriting commits', which disclaimed the cherry-pick and revert this skill's own Step 3 executes; it now excludes authoring and rebasing only. Adds 'backport' so cherry-pick is reachable by natural language at all, accepting 266 chars against a 250 soft target and a 400 ceiling.
69 lines
2.6 KiB
Markdown
69 lines
2.6 KiB
Markdown
---
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topic: bisect
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source_keys:
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- git-scm-bisect-docs
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---
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# Finding a commit with `git bisect`
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Read this when the question is *which commit changed the behaviour* and there is no string, file,
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or line range to search the log for. Binary search reduces the trials from O(N) to O(log N).
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## Manual flow
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```bash
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git bisect start
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git bisect bad [HEAD] # mark current (or specified) as broken
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git bisect good <commit> # mark known-good baseline
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# Git checks out the midpoint; test it
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git bisect good # test passes
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git bisect bad # test fails
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# Repeat until git reports "X is the first bad commit"
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git bisect reset # return to the original HEAD
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```
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## Automated
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With a test command available, use `git bisect run <cmd>`. Git reads the exit code: `0` good,
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`1`–`124` bad, `125` skip (build broken), `126`–`127` POSIX shell errors, treated as bad, and
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`128` or above aborts the session outright rather than marking the commit bad.
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## Untestable commits
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`git bisect skip` excludes a commit that cannot be built or tested without deciding good or bad
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for it. When the first bad commit is adjacent to a skipped range, bisect reports that it cannot
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pinpoint the culprit and lists the candidates — that is the precise answer the skip range allows,
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not a failure.
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## Undoing a wrong good/bad call
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`git bisect log` prints the session's decision history. Save it, edit out the mistaken entry, and
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resume from the corrected log rather than restarting the search:
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```bash
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git bisect log > bisect.log
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# edit bisect.log, removing the wrong decision
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git bisect reset && git bisect replay bisect.log
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```
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## Narrowing and speeding up
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- `git bisect start HEAD v1.2 -- src/` restricts bisection to a path, cutting the trial count.
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- `--no-checkout` updates the `BISECT_HEAD` ref instead of checking out a working tree — useful
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for tests that do not need one, and automatic in bare repos.
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- `--first-parent` follows only first parents at merges, finding the integration commit that
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introduced a regression while ignoring broken side branches.
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## Inspecting the remaining candidates
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`git bisect visualize` (alias `view`) opens the suspects in gitk, falling back to `git log` when
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no graphical display is detected. Add `--stat` or `-p` for a diffstat or full patches.
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## Hunting a non-bug property change
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`git bisect start --term-new <new> --term-old <old>` searches for any property change — a
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performance regression, say — instead of a bug. Use the custom terms in place of `good` and `bad`
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for the rest of the session.
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Once the first bad commit is identified, return to Step 3 to act on it and Step 4 to report it.
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