git reset hard,mixed,soft

首先，我们得了解git reset命令的形式之一：

git reset [] []

此命令的作用是恢复HEAD分支到位置，并根据决定是否恢复index file和working tree。恢复是指将staging area和working tree的状态还原到commit的状态（如果不指定commit，则默认为last commit）。如果没有pick哪个mode，即omitted mode，则mode默认为–mixed。可以为commit的hash id或引用值。

此外，mode还有两个值：–soft和–hard。

–soft不会改动(touch) index file和working tree，但是会撤销head到commit之间的all history，这是三个模式所共有的作用。

$ git status

On branch main

Changes not staged for commit:

 (use "git add ..." to update what will be committed)

 (use "git restore ..." to discard changes in working directory)

​    modified:  README.md

no changes added to commit (use "git add" and/or "git commit -a")

$ git add README.md

$ git status

On branch main

Changes to be committed:

 (use "git restore --staged ..." to unstage)

​    modified:  README.md

$ git log --graph --oneline

\* 0a071ac (HEAD -> main) A

\* 0c888f4 Initial Commit

$ git reset --soft 0c888f4

$ git status

On branch main

Changes to be committed:

 (use "git restore --staged ..." to unstage)

​    modified:  README.md

$ git log --graph --oneline

\* 0c888f4 (HEAD -> main) Initial Commit
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可以观察到commit A的committed changes都reset为staged changes to be committed。原本已经stage等待commit的changes不会被reset。HEAD指向，HEAD到的提交都会被undo。

–mixed在–soft的基础上，进一步reset，它会恢复index到时的状态。已经在index中的changes会reset回working tree，note that modify working tree≠reset working tree。

$ git reset 0c888f4

Unstaged changes after reset:

M    README.md
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git reset会报告影响到的changes。

–hard会在–mixed的基础上恢复working tree的状态。被跟踪的changes会被丢弃。

Any untracked files or directories in the way of writing any tracked files are simply deleted.

Git文档中的这句话我不是太理解，通过询问ChatGPT，翻译为：任何阻碍跟踪文件的写入的未跟踪文件或目录将被删除。

下面是一个Stack Overflow对该疑问的解释代码例子：

There is a case in which git reset --hard has effects on untracked files.

$ touch a.txt

$ git add .

$ git commit -m'A'

$ echo hello >> a.txt

$ git add .

$ git commit -m'B'

$ git rm a.txt

$ echo world > a.txt

$ git status -s

The status output is

D  a.txt

?? a.txt

The a.txt in the index is removed and the one in the work tree is untracked.

$ cat a.txt

world

Reset in the mode of --hard,

$ git reset --hard

$ cat a.txt

hello

$ git status

On branch master

nothing to commit, working tree clean
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The untracked a.txt in the work tree is deleted. But we could also say it’s been overwritten with the tracked a.txt in HEAD.