World 10: Deep Git

Find the bug with bisect

You will learn: how to find the commit that broke something, in a few quick tests.

The problem

The shop page shows the price as RM0! Last week it said RM10. Ten commits since then. Which one broke it?

Watch

~/shop $ git bisect start HEAD HEAD~9Bisecting: 4 revisions left to test after this (roughly 2 steps)[82a74e38e18f28bb624a2849ee62343753d9790d] Add a footer

plays “higher or lower” with your history. Give it one bad and one good commit. Git checks out the middle one for you to test.

Try it

Be the detective. Run the test, then mark the commit good or bad.

The oldest commit works. The newest one is broken. Find the first bad commit.

The test: the price in index.html must not be RM0.

87509ecAdd shop pagegood
a2fb616Add a photomaybe
70587efAdd opening hoursmaybe
0844232Add a menu linkmaybe
82a74e3Add a footermaybe
c35bad4Tidy up spacingmaybe
30a497fAdd phone numbermaybe
037d9a6Add map linkmaybe
823e211Add FAQ linkmaybe
accccefUpdate footer yearbad

Tests used: 0. For 9 commits, bisect needs at most 4.

Predict

Now there are 1,000 commits between good and bad. About how many tests does bisect need?

Watch

~/shop $ git bisect start HEAD HEAD~9Bisecting: 4 revisions left to test after this (roughly 2 steps)[82a74e38e18f28bb624a2849ee62343753d9790d] Add a footer~/shop $ git bisect run sh test.shrunning 'sh' 'test.sh'Bisecting: 2 revisions left to test after this (roughly 1 step)[30a497f8e9e2c392b1a4a259fd5d43e8cde21bfb] Add phone numberrunning 'sh' 'test.sh'Bisecting: 0 revisions left to test after this (roughly 0 steps)[c35bad4140074d2904a4cff307aafecb7d75173e] Tidy up spacingrunning 'sh' 'test.sh'c35bad4140074d2904a4cff307aafecb7d75173e is the first 'bad' commitcommit c35bad4140074d2904a4cff307aafecb7d75173e (HEAD)Author: You <you@example.com>Date:   Fri Oct 30 09:06:00 2026 +0800    Tidy up spacing index.html | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)bisect found first 'bad' commit

Got a test script? git bisect run sh test.sh does every step for you. Exit code 0 means good. 1 means bad.

Try it

Your turn. Start bisect, let test.sh judge, then go back home.

Hint

git bisect start HEAD HEAD~9, then git bisect run sh test.sh, then git bisect reset.

Check

Put a bisect hunt in order.

First step at the top. Put these in order. Use the arrows, or drag.

Test the commit Git checks out
Run git bisect reset
Run git bisect start
Mark it good or bad, until Git names the culprit
Mark one bad and one good commit

You did it!

You can now find the commit that broke something, fast.

Go deeper (optional)

Can’t test one commit, say it doesn’t build? git bisect skip picks a neighbour. git bisect log shows your steps. Hunting a fix, not a bug? Use git bisect old and git bisect new instead of good and bad. In bisect run, exit code 125 means skip.

~/shop $ git bisect start HEAD HEAD~9Bisecting: 4 revisions left to test after this (roughly 2 steps)[82a74e38e18f28bb624a2849ee62343753d9790d] Add a footer~/shop $ git bisect skipBisecting: 4 revisions left to test after this (roughly 2 steps)[c35bad4140074d2904a4cff307aafecb7d75173e] Tidy up spacing