A workflow I wish I’d learned years earlier: git bisect run fully automates “which commit introduced this bug.”
Manual bisect is already good — git bisect start, mark one bad and one good commit, and git checks out the midpoint for you to test. But if you can express “is this commit broken?” as a script that exits 0 (good) or non-zero (bad), you hand the whole search to git:
git bisect start HEAD v1.4.0 # HEAD is bad, v1.4.0 was good
git bisect run ./check.sh
git then binary-searches the range on its own and prints the first bad commit. check.sh can be a single test, a curl | grep, a build command — anything with a meaningful exit code.
Two gotchas that bite people:
- Exit code 125 is special: it tells git “this commit is untestable, skip it” (e.g. it doesn’t compile for an unrelated reason). Use it instead of returning bad/good on commits your script can’t evaluate.
- Guard against the flaky case — if your test is nondeterministic, bisect will happily converge on the wrong commit. Make
check.shdeterministic first.
Over ~14 commits that’s 4 checkouts instead of 14. Over a 1000-commit range it’s ~10. What’s the most time this has ever saved you?
Does bisect not treat a merge commit as a singular commit in the history of the target branch?
From memory, I believe it does. But that means the bisection would give the result that the merge commit is responsible for the bug. Which isn’t wrong, but most workflows do not just blindly revert a whole merge, but would have to follow-up to surgically find and fix the bug in the merged-in branch.
So that would be yet another invocation of Git bisect. The hazard is believing that bisect is an fully automated bug finder, when it would just stop at the first merge commit if the bug came in from elsewhere.
The other hazard that I didn’t mention is a presumption that every commit – merge or not – is individually buildable and testable. For good workflows, this is the case. For some… more casual workflows where broken code is pushed and then fixes thereafter, bisect will not be able to hone down to a singular commit.