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git reflog

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2013-04-26 23:28:01 +08:00
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---
layout: post
title: Git reflog 机制
categories:
- Programming
tags:
- git
---
git reflog是对reflog进行管理的命令，那么什么是reflog呢？
reflog是git用来记录引用变化的一种机制，比如记录分支的变化或者是HEAD引用的变化。
比如在某git库中运行git reflog,当git reflog命令不指定引用的时候默认列出HEAD的reflog。
2ab4043 (HEAD, refs/heads/master, refs/heads/a) HEAD@{0}: checkout: moving from master to a
2ab4043 (HEAD, refs/heads/master, refs/heads/a) HEAD@{1}: commit (merge): Merge branch 'mybranch'
bf98582 HEAD@{2}: rebase: aborting
bf98582 HEAD@{3}: checkout: moving from 7e9938d9f5b9f7835359ca87da8a329781ed74b6 to master
7e9938d (refs/remotes/origin/mybranch, refs/heads/mybranch) HEAD@{4}: checkout: moving from master to 7e9938d9f5b9f7835359ca87da8a329781ed74b6^0
bf98582 HEAD@{5}: reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
be93372 (refs/remotes/origin/master) HEAD@{6}: commit: add merge detail to read me
3eeedca HEAD@{7}: commit (merge): Merge branch 'mybranch'
bf98582 HEAD@{8}: reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
00fa1f0 HEAD@{9}: commit (merge): Merge branch 'mybranch'
bf98582 HEAD@{10}: reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
663feb9 HEAD@{11}: commit: merge mybranch
bf98582 HEAD@{12}: reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
aa73004 HEAD@{13}: commit: merge issue
bf98582 HEAD@{14}: commit: modify readme file
cd2eddb HEAD@{15}: checkout: moving from mybranch to master
7e9938d (refs/remotes/origin/mybranch, refs/heads/mybranch) HEAD@{16}: commit: modiy readme add issueFile
cd2eddb HEAD@{17}: checkout: moving from master to mybranch
cd2eddb HEAD@{18}: commit: prepare files
b39fed8 HEAD@{19}: commit (initial): first commit
这里涉及到一个修订版本引用的语法,比如 HEAD@{0}代表HEAD当前的值，HEAD@{2}代表HEAD两次变化之前的值。详情的语法可以参看这里 [https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html](https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html)。
从上面的输出结果可以看到HEAD所有的变化历史，从每一条记录中可以看出每次变化所对应的git操作，比如commit，checkout，rebase，merge等，以及变化的详情内容。
git reflog有时候可以帮助你找到丢失掉的commit，比如你在某个detached HEAD（即不在任何分支只是在某个历史的commit的节点上）的时候进行了一次commit，然后你切换到另一个分支先把刚才的东西合并进来，这个时候突然意识到刚才的提交的东西找不到了，这个时间你就可以通过HEAD@{1}引用到刚才的提交了，或者通过git reflog找到对应commit的sha1值，然后进行merge。
<br>
那么git系统是如何存储reflog的呢？这里继续拿HEAD来举例，git会将变化记录到HEADH对应的reflog文件中，其路径为.git/logs/HEAD，文件是一个纯文本文件。分支的reflog文件都放在.git/logs/refs目录下的子目录中。
下面是HEAD的reflog文件的内容：
0000000000000000000000000000000000000000 b39fed82cd3225eb524f6f0184c0ba49a4f6952c 卢克 <kejinlu@gmail.com> 1366871718 +0800 commit (initial): first commit
b39fed82cd3225eb524f6f0184c0ba49a4f6952c cd2eddb41a632f68b0655366d5ca99f4701bb9b4 卢克 <kejinlu@gmail.com> 1366871885 +0800 commit: prepare files
cd2eddb41a632f68b0655366d5ca99f4701bb9b4 cd2eddb41a632f68b0655366d5ca99f4701bb9b4 卢克 <kejinlu@gmail.com> 1366871950 +0800 checkout: moving from master to mybranch
cd2eddb41a632f68b0655366d5ca99f4701bb9b4 7e9938d9f5b9f7835359ca87da8a329781ed74b6 卢克 <kejinlu@gmail.com> 1366872039 +0800 commit: modiy readme add issueFile
7e9938d9f5b9f7835359ca87da8a329781ed74b6 cd2eddb41a632f68b0655366d5ca99f4701bb9b4 卢克 <kejinlu@gmail.com> 1366872046 +0800 checkout: moving from mybranch to master
cd2eddb41a632f68b0655366d5ca99f4701bb9b4 bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366872107 +0800 commit: modify readme file
bf985821e12129ea3dc9d4150792b8dae798773c aa7300408b6865b105d196c1acf60dc83ffccef1 卢克 <kejinlu@gmail.com> 1366872272 +0800 commit: merge issue
aa7300408b6865b105d196c1acf60dc83ffccef1 bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366872623 +0800 reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
bf985821e12129ea3dc9d4150792b8dae798773c 663feb94a406a8d3600f2d24bf4dafc565a5f9da 卢克 <kejinlu@gmail.com> 1366872760 +0800 commit: merge mybranch
663feb94a406a8d3600f2d24bf4dafc565a5f9da bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366872810 +0800 reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
bf985821e12129ea3dc9d4150792b8dae798773c 00fa1f0727bca42305b2a7c5dc53f44c33a17a96 卢克 <kejinlu@gmail.com> 1366872883 +0800 commit (merge): Merge branch 'mybranch'
00fa1f0727bca42305b2a7c5dc53f44c33a17a96 bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366873467 +0800 reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
bf985821e12129ea3dc9d4150792b8dae798773c 3eeedca7ea0b821088faba84b3157493eae4e13d 卢克 <kejinlu@gmail.com> 1366873579 +0800 commit (merge): Merge branch 'mybranch'
3eeedca7ea0b821088faba84b3157493eae4e13d be933721a15b18605aaf9d4d9f5c5eff3281b9b4 卢克 <kejinlu@gmail.com> 1366874038 +0800 commit: add merge detail to read me
be933721a15b18605aaf9d4d9f5c5eff3281b9b4 bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366882601 +0800 reset: moving to bf985821e12129ea3dc9d4150792b8dae798773c
bf985821e12129ea3dc9d4150792b8dae798773c 7e9938d9f5b9f7835359ca87da8a329781ed74b6 卢克 <kejinlu@gmail.com> 1366882618 +0800 checkout: moving from master to 7e9938d9f5b9f7835359ca87da8a329781ed74b6^0
7e9938d9f5b9f7835359ca87da8a329781ed74b6 bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366882688 +0800 checkout: moving from 7e9938d9f5b9f7835359ca87da8a329781ed74b6 to master
bf985821e12129ea3dc9d4150792b8dae798773c bf985821e12129ea3dc9d4150792b8dae798773c 卢克 <kejinlu@gmail.com> 1366882792 +0800 rebase: aborting
bf985821e12129ea3dc9d4150792b8dae798773c 2ab4043c6c4c9f59e7756cb7fb5df4fdf467fe4b 卢克 <kejinlu@gmail.com> 1366882860 +0800 commit (merge): Merge branch 'mybranch'
2ab4043c6c4c9f59e7756cb7fb5df4fdf467fe4b 2ab4043c6c4c9f59e7756cb7fb5df4fdf467fe4b 卢克 <kejinlu@gmail.com> 1366958770 +0800 checkout: moving from master to a
从上面HEAD的reflog的log文件的内容可以看到，每一个reflog的entry都包含了变化前commit节点的sha1值以及变化后的commit节点的sha1值，如果你阅读了git的源码你会看到这两个值对应的变量名为osha1（old sha1）和nsha1（new sha1），第一次提交产生的变化的老的sha1值为全0的特殊值；还包含了用户名，email ，变化的时间戳以及变化的具体内容。