git diff 命令比较文件的不同，即比较文件在暂存区和工作区的差异。
git diff 命令显示已写入暂存区和已经被修改但尚未写入暂存区文件对区别。
git diff 有两个主要的应用场景。
- 尚未缓存的改动：git diff
- 查看已缓存的改动： git diff --cached
- 查看已缓存的与未缓存的所有改动：git diff HEAD
- 显示摘要而非整个 diff：git diff --stat
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此仓库里面的文件是最新clone下来的，工作区与暂存区的一样：
git diff命令：显示工作区与暂存区没有区别
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此时我在工作区里面的5.txt里面增加内容，然后再操作：
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我把修改好的5.txt进行git add . 然后再执行git diff命令，就发现没有区别了：
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5.txt文件已经被提交了，现在我再更改工作区的5.txt文件，然后再执行git diff命令：
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显示暂存区和工作区的差异:
$ git diff [file]
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显示暂存区和上一次提交(commit)的差异:
$ git diff --cached [file] 或 $ git diff --staged [file]
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看已缓存的与未缓存的所有改动：git diff HEAD
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显示两次提交之间的差异:
$ git diff [first-branch]...[second-branch]
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- 显示摘要而非整个 diff：git diff --stat
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