Git has a very powerful command for developers to systematically find the commit that introduce a bug.
You mark commits as good OR bad, and using divide-and-conquer strategy, you will find the first bad commit in a few pass (even with hundreds of commits).
Steps
# 1. Mark current branch that has the bug
git bisect bad
# 2. Mark a tag that is working, or use a commit SHA directly
git bisect good v1.2.3
# 3. Git will automatically checkout the commit between the good and bad
# You will have to identify if that commit is good or bad, and mark with
git bisect bad
# OR
git bisect good
Eventually, you will find the first bad commit.
You can return to your previous branch with git bisect reset.
For more features, refer to git-scm doc.
What if you are looking for the first change?
Sometimes, you are not finding the first commit that is bad.
Rather, you want to find the first commit that has new changes.
Since Git v2.7.0, they introduced new and bad as new terms. If you want to use your own terms, or if you are using older version of git, you can define your terms.
Example to have fixed and unfixed,
git bisect start --term-new=fixed --term-old=unfixed
Then mark with,
git bisect fixed
git bisect unfixed
What is the very first commit SHA?
The “Initial Commit” SHA can be retrieved with:
git rev-list --max-parents=0 HEAD