Find the bug with bisect

Goal#

Binary-search history with git bisect, by hand and with a script, and name the commit that broke things.

Halving the haystack#

Tell git one commit that is good and one that is bad. It checks out the commit halfway between and asks you which it is. Each answer halves what is left: 40 commits take about six answers, a thousand take about ten.

git bisect start
git bisect bad              # the commit you are on is broken
git bisect good <hash>      # this old one worked
git bisect good             # or: git bisect bad
git bisect reset            # done: back to where you started

Let a script answer#

If a command can tell good from bad by its exit code (0 for good, anything from 1 to 127 except 125 for bad), git can run the whole search alone:

git bisect run sh check.sh

Any test command works: npm test, pytest tests/test_naps.py, or a three-line script.

Quest 10#

quest start 10

Forty commits of purring, and somewhere HEAD started hissing. check.sh passes while purr.txt has no hiss in it. Every commit has the same message, so reading the log will not help. Find the first commit where check.sh fails, put a branch called culprit on it, and finish the bisect.

Start with the newest commit as bad and the first commit as good:

git bisect start HEAD $(git rev-list --max-parents=0 HEAD)
git bisect run sh check.sh

This works the same in PowerShell. Git answers:

Bisecting: 19 revisions left to test after this (roughly 4 steps)
...
1d035425aadde61fcca1784ebfce810594897d8d is the first bad commit

The hash is the same for everyone. Then git branch culprit <hash>, git bisect reset, and quest check.

Check it works#

• culprit points at 1d03542, and git show culprit adds the hiss.
• quest check passes.

Exercise#

Start the quest again and bisect by hand: answer git bisect good or git bisect bad yourself after running sh check.sh (or reading purr.txt). Count your steps. Did you beat six?

More to try

• git log -S hiss --oneline finds the commit that added the word, without bisect. When does bisect win anyway? (Think: a bug, not a word.)
• If a commit cannot be tested, git bisect skip moves on.