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Git Bisect ​

If something broke in the past for unknown reasons, then a bisect is a great way of narrowing down causes.

A bisect is a semi-manual binary tree search for the first broken commit. For example, given 5 commits:

  1. A <- You are here
  2. B
  3. C
  4. D <- This is the broken commit
  5. E <- This is the latest known working version

The bisect would start by marking A as a "bad" commit, and marking E as a "good" commit. Bisect would then drop you in between them:

  1. A <- Marked as bad
  2. B
  3. C <- You are here (dropped here by bisect)
  4. D <- This is the broken commit
  5. E <- Marked as good

At this point you would then mark C as "bad", and the bisect would continue to D.

  1. A <- Marked as bad
  2. B <- Skipped over by bisect
  3. C <- Marked as bad
  4. D <- You are here
  5. E <- Marked as good

You would then mark D as "bad" and would be found as the final bisect. Performing this in the command line would look like:

shell
# Start the bisecting process, on your HEAD commit
git bisect start

# Mark A (your commit) as bad
git bisect bad

# Mark E as good
git bisect good <commit hash>

# From here, the steps are to reproduce your failure and mark as good or bad as fit
npm test
git bisect good|bad

# Once the bisect is done, reset back. Or run this whenever things break
git bisect reset

Different names for 'good' and 'bad' ​

Bisect semantically looks for the first commit with a problem, if instead you want the first commit with a fix, then bisect won't look quite right out of the box.

shell
# Start the bisect as usual
git bisect start

# Set the most recent version to be good
git bisect good HEAD

# Set the bad commit to whenever
git bisect bad PAST

> Some good revs are not ancestors of the bad rev.
> git bisect cannot work properly in this case.
> Maybe you mistook good and bad revs?

To help this out, you can use the --term-new and --term-old flags. Normally 'new' = 'bad' and 'old' = 'good'.

shell
# Now we have messaging here that makes more sense
git bisect start --term-new=fixed --term-old=unfixed

git bisect fixed HEAD
git bisect unfixed PAST