https://www.cnblogs.com/sunshine-xin/articles/3521481.html
1. git rm --cached file will remove the file from the stage. That is, when you commit the file will be removed. git reset HEAD -- file will simply reset file in the staging area to the state where it was on the HEAD commit, i.e. will undo any changes you did to it since last commiting. If that change happens to be newly adding the file, then they will be equivalent.
意思是说：git rm -- cached 是从stage（index，暂存区） 里面删除文件，当你提交（commit）之后文件就会删除了。
git reset HEAD -- file : 回退暂存区里的文件（还原为HEAD commit里面该文件的状态），会撤销从上一次提交（commit）之后的一些操作。
如果是对于新增文件，这两个操作时等效的。
这两个命令都是对stage,index的操作。
2. git rm --cached is used to remove a file from the index. In the case where the file is already in the repo, git rm --cached will remove the file from the index, leaving it in the working
directory and a commit will now remove it from the repo as well. Basically, after the commit, you would have unversioned the file and kept a local copy.
git rm --cached 作用： 从索引里删除文件。 如果要删除的文件已经在仓库里了，git rm --cached 将会从索引里删除该文件，但本地工作目录还会保存源代码，提交之后将会同时从仓库里删除该文件。
git reset HEAD file ( which by default is using the --mixed flag) is different in that in the case where the file is already in the repo, it replaces the index version of the file with the one
from repo (HEAD), effectively unstaging the modifications to it.
git reset HEAD file (命令默认参数为 --mixed) 不同于文件已经在仓库中，该命令的作用是 用repo(HEAD)替换index中file的版本，使file的版本回退到HEAD版本，这个命令可以用于unstageing 对该文件的 改动。
3. There are three places where a file, say, can be - the tree, the index and the working copy. When you just add a file to a folder, you are adding it to the working copy.
When you do something like git add file you add it to the index. And when you commit it, you add it to the tree as well.
It will probably help you to know the three more common flags in git reset:
git reset [--<mode>] [<commit>]
This form resets the current branch head to <commit> and possibly updates the index (resetting it to the tree of <commit>) and the working tree depending on <mode>, which must be one of the following:
--soft
Does not touch the index file nor the working tree at all (but resets the head to <commit>, just like all modes do). This leaves all your changed files "Changes to be committed", as git status would put it.
--mixed
Resets the index but not the working tree (i.e., the changed files are preserved but not marked for commit) and reports what has not been updated. This is the default action.
--hard
Resets the index and working tree. Any changes to tracked files in the working tree since <commit> are discarded.
Now, when you do something like git reset HEAD - what you are actually doing is git reset HEAD --mixed and it will "reset" the index to the state it was before you started adding files / adding modifications to the index ( via git add ) In this case, the working copy and the index ( or staging ) were in sync, but you made the HEAD and the index to be in sync after the reset.
git rm on the other hand removes a file from the working directory and the index and when you commit, the file is removed from the tree as well. git rm --cached however removes the file from index alone and keeps it in your working copy. This is the exact opposite of git add file In this case, you made index to be different from the HEAD and the working, in it that the HEAD has the previously committed version of the file, working copy had the las modification if any or content from HEAD of the file and you removed the file from the index. A commit now will sync the index and tree and the file will be removed.
git管理项目，有三个地方存放文件：the tree, the index and the working copy, （树，索引，工作目录/工作副本）。
当你向目录里添加一个文件的时候，这个文件存放在工作目录中。命令git add file 将文件添加到索引，当提交（coommit）的时候, 该文件也同时添加到了树中。
git reset命令有三种模式：git reset [--<mode>] [<commit>]
该命令用来回退当前的分支head 到 <commit>, 根据<mode>不同 --soft --mixed --hard，会对working tree和index和HEAD进行重置, mode分为以下几种
1） --soft: 回退到某个版本，只回退了commit的信息，不会恢复到index file一级（对索引不做改动）。如果还要提交，直接commit即可，执行该模式之后，git status 对所有修改的文件会提示（Changes to be committed）
2） --mixed: 此为默认方式，不带任何参数的git reset，即是这种方式，它回退到某个版本，只保留源码，回退commit和index信息。
3) --hard: 彻底回退到某个版本，本地的源码也会变为上一个版本的内容（所有的修改都会被丢弃）。