为什么说Git管理的是修改，而不是文件呢？我们还是做实验。第一步，对readme.txt做一个修改，比如加一行内容：

$ cat readme.txt

Git is a distributed version control system.

Git is free software distributed under the GPL.

Git has a mutable index called stage.

Git tracks changes.

然后，添加：

$ git add readme.txt

$ git status

# On branch master

# Changes to be committed:

#   (use "git reset HEAD <file>..." to unstage)

#

#       modified:   readme.txt

#

然后，再修改readme.txt：

$ cat readme.txt

Git is a distributed version control system.

Git is free software distributed under the GPL.

Git has a mutable index called stage.

Git tracks changes of files.

提交：

$ git commit -m "git tracks changes"

[master d4f25b6] git tracks changes

  file changed,  insertion(+)

提交后，再看看状态：

$ git status

# On branch 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.txt

#

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

咦，怎么第二次的修改没有被提交？

别激动，我们回顾一下操作过程：

第一次修改 -> git add -> 第二次修改 -> git commit

你看，我们前面讲了，Git管理的是修改，当你用git add命令后，在工作区的第一次修改被放入暂存区，准备提交，但是，在工作区的第二次修改并没有放入暂存区，所以，git commit只负责把暂存区的修改提交了，也就是第一次的修改被提交了，第二次的修改不会被提交。

提交后，用git diff HEAD -- readme.txt命令可以查看工作区和版本库里面最新版本的区别：

$ git diff HEAD -- readme.txt

diff --git a/readme.txt b/readme.txt

index 76d770f..a9c5755

--- a/readme.txt

+++ b/readme.txt

@@ -, +, @@

 Git is a distributed version control system.

 Git is free software distributed under the GPL.

 Git has a mutable index called stage.

-Git tracks changes.

+Git tracks changes of files.

可见，第二次修改确实没有被提交。