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What is Git bisect? How can you use it to determine the source of a (regression) bug?

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Git bisect is used to find the commit that introduced a bug by using binary search. The command for Git bisect is

git bisect <subcommand> <options>

Now since you have mentioned the command above explain to them what this command will do.

This command 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.
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Git provides a rather efficient mechanism to find bad commits. Instead of making the user try out every single commit to find out the first one that introduced some particular issue into the code, git bisect allows the user to perform a sort of binary search on the entire history of a repository.

By issuing the command git bisect start, the repository enters bisect mode. After this, all you have to do is identify a bad and a good commit:

git bisect bad # marks the current version as bad

git bisect good {hash or tag} # marks the given hash or tag as good, ideally of some earlier commit

Once this is done, Git will then have a range of commits that it needs to explore. At every step, it will checkout a certain commit from this range, and require you to identify it as good or bad. After which the range will be effectively halved, and the whole search will require a lot less number of steps than the actual number of commits involved in the range. Once the first bad commit has been found, or the bisect mode needs to be ended, the following command can be used to exit the mode and reset the bisection state:

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