这里主要讲分支branch , 标签tag的真实含义

1， 再看 .git的结构

chuangchuang@chuang:~/work/temp/gittest[dev*]$ tree .git/
.git/
├── branches
├── COMMIT_EDITMSG
├── config
├── description
├── HEAD
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── prepare-commit-msg.sample
│   ├── pre-push.sample
│   ├── pre-rebase.sample
│   └── update.sample
├── index
├── info
│   └── exclude
├── logs
│   ├── HEAD
│   └── refs
│       ├── heads
│       │   ├── dev
│       │   └── master
│       └── remotes
│           └── origin
│               └── master
├── objects
│   ├── 45
│   │   └── 776d6cb67571cfe31656fb1d114e520c9e0ab2
│   ├── 68
│   │   └── d98b1781c29d1df8b4ac7b6d457bccecf9756e
│   ├── a3
│   │   └── c8effc24eeb66de97a66b073c6d62c48fbc9ed
│   ├── e4
│   │   └── 626567ec9ae5a6f3abd77af37c25a85ec4e7f1
│   ├── e6
│   │   └── 9de29bb2d1d6434b8b29ae775ad8c2e48c5391
│   ├── f1
│   │   └── 3f51556efabe074d5b255eabcdd3ec33520c55
│   ├── info
│   └── pack
└── refs
    ├── heads
    │   ├── dev
    │   └── master
    ├── remotes
    │   └── origin
    │       └── master
    └── tags

2.讲讲 HEAD

HEAD代表了当前仓库的最新版本
看看里面有啥

chuangchuang@chuang:~/work/temp/gittest[dev*]$ cat .git/HEAD 
ref: refs/heads/dev

可以看到，这个head指向的是一个分支dev
在看看这个refs/heads/dev中存放的是什么

chuangchuang@chuang:~/work/temp/gittest[dev*]$ cat .git/refs/heads/dev
45776d6cb67571cfe31656fb1d114e520c9e0ab2

可以看到是指向了一个commit而已

3, 讲讲tag

上面的.git目录中没有对应tag信息
3.1 新建一个tag

chuangchuang@chuang:~/work/temp/gittest[dev*]$ git tag -a v1.0.0 -m "添加一个tag1.0.0"
chuangchuang@chuang:~/work/temp/gittest[dev*]$ git tag
v1.0.0

3.2 查看该tag的信息

chuangchuang@chuang:~/work/temp/gittest[dev*]$ git show v1.0.0
tag v1.0.0
Tagger: chuang <chuangchuang.chen@cafintech.com>
Date:   Sun Nov 12 19:52:26 2017 +0800

添加一个tag1.0.0

commit 45776d6cb67571cfe31656fb1d114e520c9e0ab2
Author: chuang <chuangchuang.chen@cafintech.com>
Date:   Sun Nov 12 10:12:52 2017 +0800

    di er ci ti jiao test

diff --git a/aa.txt b/aa.txt
index e69de29..e462656 100644
--- a/aa.txt
+++ b/aa.txt
@@ -0,0 +1 @@
+1 di yi ci wang li tian jia nei rong

3.3 查看当前最后一个commit

chuangchuang@chuang:~/work/temp/gittest[dev*]$ git log -1 --pretty=raw
commit 45776d6cb67571cfe31656fb1d114e520c9e0ab2
tree 68d98b1781c29d1df8b4ac7b6d457bccecf9756e
parent a3c8effc24eeb66de97a66b073c6d62c48fbc9ed
author chuang <chuangchuang.chen@cafintech.com> 1510452772 +0800
committer chuang <chuangchuang.chen@cafintech.com> 1510452772 +0800

    di er ci ti jiao test

可以看到tag中显示的信息，追踪的是 id为 45776d6cb67571cfe31656fb1d114e520c9e0ab2
的最后一次commit

3.4 查看物理结构中的tag
再次查看.git中的tag(删减无用的)

.git/
├── branches
├── objects
│   ├── 45
│   │   └── 776d6cb67571cfe31656fb1d114e520c9e0ab2
│   ├── 68
│   ├── info
│   └── pack
└── refs
    ├── heads
    │   ├── dev
    │   └── master
    ├── remotes
    │   └── origin
    │       └── master
    └── tags
        └── v1.0.0

看看tags目录下的v1.0.0

chuangchuang@chuang:~/work/temp/gittest[dev*]$ cat .git/refs/tags/v1.0.0 
c8ec46b8063b47a12de692106db06822945d4b09

里面存储了id为 c8ec46b8063b47a12de692106db06822945d4b09的文件名
再查看该文件的存储

chuangchuang@chuang:~/work/temp/gittest[dev*]$ git cat-file -p c8ec4
object 45776d6cb67571cfe31656fb1d114e520c9e0ab2
type commit
tag v1.0.0
tagger chuang <chuangchuang.chen@cafintech.com> 1510487546 +0800

添加一个tag1.0.0

可以看到这个里面已经关联到了commit
id为45776d6cb67571cfe31656fb1d114e520c9e0ab2

4 总结

branch 和 tag实际上都是关联到了 commit id