git bisect
Use binary search to find the commit that introduced a bug
Git bisect uses a binary search algorithm to find which commit in your project's history introduced a bug. You use it by first telling it a "bad"
commit that is known to contain the bug, and a "good" commit that is known to be before the bug was introduced. Then git bisect picks a commit between
those two endpoints and asks you whether the selected commit is "good" or "bad". It continues narrowing down the range until it finds the exact commit
that introduced the change.
In fact, git bisect can be used to find the commit that changed any property of your project; e.g., the commit that fixed a bug, or the commit that
caused a benchmark's performance to improve. To support this more general usage, the terms "old" and "new" can be used in place of "good" and "bad", or
you can choose your own terms.
Basic bisect commands: start, bad, good
As an example, suppose you are trying to find the commit that broke a feature that was known to work in version v2.6.13-rc2 of your project. You start a bisect session as follows:
git bisect start
git bisect bad # Current version is bad
git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good
Once you have specified at least one bad and one good commit, git bisect selects a commit in the middle of that range of history, checks it out, and outputs something similar to the following:
Bisecting: 675 revisions left to test after this (roughly 10 steps)
You should now compile the checked-out version and test it. If that version works correctly, type
git bisect good
If that version is broken, type
git bisect bad
Then git bisect will respond with something like
Bisecting: 337 revisions left to test after this (roughly 9 steps)
Keep repeating the process: compile the tree, test it, and depending on whether it is good or bad run git bisect good or git bisect bad to ask for the
next commit that needs testing.
Eventually there will be no more revisions left to inspect, and the command will print out a description of the first bad commit. The reference
refs/bisect/bad will be left pointing at that commit.