Git bisect - debugging with git

git bisect

快捷记忆：如何记住这个命令：binary search commit --》 bisect
bisect vt. 把…一分为二；把…二等分

 The command takes various subcommands, 
and different options depending on the subcommand:

           git bisect start [--term-{new,bad}= --term-{old,good}=]
               [--no-checkout] [--first-parent] [ [...]] [--] [...]
           git bisect (bad|new|) []
           git bisect (good|old|) [...]
           git bisect terms [--term-good | --term-bad]
           git bisect skip [(|)...]
           git bisect reset []
           git bisect (visualize|view)
           git bisect replay 
           git bisect log
           git bisect run ...
           git bisect help

在公司的博客大赛上，有人写了一篇文章，介绍使用该命令去debug一次bug的提交。

git-bisect - Use binary search to find the commit that introduced a bug
git bisect, 他可以让你标识某次 commit 工作正常还是不正常, 然后通过二分查找,快速找到第一次引起功能不正常的commit. 从而快速定位引进问题的代码。

用法: git bisect

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                 # 开始 bisect
 $ git bisect bad                 # Current version is bad，标记当前版本为 bad
 $ git bisect good v2.6.13-rc2    # v2.6.13-rc2 is known to be good， 标记之前一个版本为 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)

然后验证该commit是good 还是 bad，继续标记：
git bisect good or git bisect bad

Git 会继续使用二分查找选取中间一个commit，并且checkout。然后重复上述过程。

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.

最终会打印出第一次引入bug的commit信息。

git bisect run

one of git bisect subcommand is the git bisect run which accepts a command as an argument and allows to fully automate git bisect!

git bisect run

After running this command git automatically runs cmd on each checked-out commit and uses the return value of that call to decide if the commit is good or bad until it will find the first broken commit.

如何 automate git bisect?

$git bisect start [bad_commit] [good_commit]
$git bisect run npm run text
# exit 0 for 'good commit', non-0 for 'bad-commit'

一些人的经验tips

1. 对于存在merge提交的场景可能会有坑。比如我曾经就遇到客户bisect的过程跑到了merge分支里面的提交去，从而使得good/bad的annotate出现了莫名其妙的错误，最后找出来一个莫名其妙的commit。在线性提交的分支上使用这项技术更加保险

2. 另外git还有个很有用的命令，手残党的福利，git reflog，大家有兴趣可以去查下 :)

3. 再给大家分享一个 git rerere, 送给对 rebase 有执念的同学