在实际开发过程中，作为一名程序猿，可能会遇到这样一个问题，有时候提交的代码太多了，希望将代码回滚，有时候要将本地的修改废弃掉。这些操作都会在日常开发中遇到，今天希望把提交中一个文件回滚，但是不小心把所有文件全部会滚了，一脸懵逼。所以，这篇文章就来好好总结以下git的撤销修改操作。
撤销修改：
情形1:文件修改后还没有被放到暂存区
当对git工作区中的文件进行修改之后，通过git status命名会出现以下信息：
On branch master
Your branch is up to date with 'origin/master'.
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout -- <file>..." to discard changes in working directory)
modified: README.md
no changes added to commit (use "git add" and/or "git commit -a")
情形2：文件被添加到了暂存区
如果将文件README.md通过git add放入stage（暂存区），通过git status会出现以下信息：
On branch master
Your branch is up to date with 'origin/master'.
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
modified: README.md
根据提示，只需要通过git reset就可以将修改从暂存区中清除，此时实际上是回到了情形1中:
git reset HEAD README.md
Unstaged changes after reset:
M README.md
git status
On branch master
Your branch is up to date with 'origin/master'.
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout -- <file>..." to discard changes in working directory)
modified: README.md
no changes added to commit (use "git add" and/or "git commit -a")
情形3：文件被提交到了本地分支
如果通过git commit将修改提交到了本地分支，这次修改又该怎么撤销？
1.通过git log查看本地分支的历史提交记录：
commit d24b19e5cee389f1e1f2bb906fc5765d321da8cd (HEAD -> master)
Author: xxxxxxxxxx
Date: Wed Jul 25 22:44:48 2018 +0800
修改README.md
commit 2f4c6e119c38a8f0b71dd20244903f57079fbc3c (origin/master)
Author: xxxxxxxxxx
Date: Wed Jul 25 20:53:13 2018 +0800
udpate
2.通过git reset --hard {commitId}或者git reset --hard HEAD~1
HEAD is now at 2f4c6e1 udpate
可以看到HEAD现在指向的是2f4c6e1这次提交，最新的提交也已经没了。
3.但是如果在一次提交中包含了多个文件，撤销修改只希望撤销其中的一个文件，上述这个命令显然是不能够满足需求的。如果是这样的话，可以通过git reset --soft HEAD~1来实现：
On branch master
Your branch is up to date with 'origin/master'.
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
modified: README.md
命令 git reset --soft {commitId} 的作用就是将修改从本地分支回滚，但是会保留在暂存区中。
所以这里就有必要讲讲git reset这个命令：
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>. If <mode> is omitted, defaults to
"--mixed". The <mode> must be one of the following:
--soft
Does not touch the index file or 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.
If -N is specified, removed paths are marked as intent-to-add
(see git-add(1)).
--hard
Resets the index and working tree. Any changes to tracked files
in the working tree since <commit> are discarded.
--merge
Resets the index and updates the files in the working tree that
are different between <commit> and HEAD, but keeps those which
are different between the index and working tree (i.e. which
have changes which have not been added). If a file that is
different between <commit> and the index has unstaged changes,
reset is aborted.
In other words, --merge does something like a git read-tree -u
-m <commit>, but carries forward unmerged index entries.
--keep
Resets index entries and updates files in the working tree that
are different between <commit> and HEAD. If a file that is
different between <commit> and HEAD has local changes, reset is
aborted.
这里面值得注意的就是：
1.如果使用--hard模式的话，commit之后的所有修改都会被废弃，无法找回。之前不慎用了--hard,欲哭无泪。
2.默认使用的是--mixed，重置了暂存区（index），但不会重置工作区（working tree）。所有的修改都还在。
其实上述三种模式总结以下就是：
| reset模式 | 工作区 | 暂存区 | 本地分支 |
|---|---|---|---|
| --hard | 无 | 无 | 无 |
| --mixed | 保留 | 无 | 无 |
| --soft | 保留 | 保留 | 无 |
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