如何计算git哈希值？ [英] How is the git hash calculated?

问题描述

我试图理解git如何计算refs的散列值。

  $ git ls-remote https://github.com/git/git 
 
 .. .. 
 29932f3915935d773dc8d52c292cadd81c81071d refs / tags / v2.4.2 
 9eabf5b536662000f79978c4d1b6e4eff5c8d785 refs / tags / v2.4.2 ^ {} 
 .... 
  

在本地克隆回购。检查 refs / tags / v2.4.2 ^ {} ref by sha

  $ git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785 
 
 tree 655a20f99af32926cbf6d8fab092506ddd70e49c 
 parent df08eb357dd7f432c3dcbe0ef4b3212a38b4aeff 
作者Junio C Hamano< [e-mail]> 1432673399 -0700 
提交者Junio C Hamano< [e-mail]> 1432673399 -0700 
 
 Git 2.4.2 
 
签名：Junio C Hamano< [e-mail]> 
  

复制解压后的内容以便散列它（AFAIK git在散列时使用未压缩的版本）

  git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785> 
 
 
 $ b 
让我们使用git自己的散列命令SHA-1内容
 
 
  git hash-object fi 
 3cf741bbdbcdeed65e5371912742e854a035e665 
  
为什么输出不是 [9e] abf5b536662000f79978c4d1b6e4eff5c8d785 ？我知道前两个字符（ 9e ）是十六进制的长度。我应该如何散列 fi 的内容，以便我可以获得git ref  abf5b536662000f79978c4d1b6e4eff5c8d785 ？
解决方案
如 git commit sha1 formed ，公式为： 
 
 
 （printf< type>％s\0 $（git cat-file< type>< ref> | wc -c）; git cat-file< type>< ref> | sha1sum 
  对于提交 9eabf5b536662000f79978c4d1b6e4eff5c8d785（即 v2.4.2 ^ {} ，并引用了一个树）： 
 
 
 （printfcommit％s\$（git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 | wc -c）; git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785）| sha1sum 
  
这会给9eabf5b536662000f79978c4d1b6e4eff5c8d785。 
 
 
 
如同：
 
 
 （printfcommit％s\$（git cat-file commit v2.4.2 {} | wc -c）; git cat-file commit v2.4.2 {}）| sha1sum 
  
（仍然是9eabf5b536662000f79978c4d1b6e4eff5c8d785）
 
 
 
同样，计算标签v2.4.2的SHA1应该是： 
 
 
 （ printftag％s\$（git cat-file tag v2.4.2 | wc -c）; git cat-file tag v2.4.2）| sha1sum 
  
这会给29932f3915935d773dc8d52c292cadd81c81071d。
 
I'm trying to understand how git calculates the hash of refs. 
$ git ls-remote https://github.com/git/git  

....
29932f3915935d773dc8d52c292cadd81c81071d    refs/tags/v2.4.2
9eabf5b536662000f79978c4d1b6e4eff5c8d785    refs/tags/v2.4.2^{}
....
Clone the repo locally. Check the refs/tags/v2.4.2^{} ref by sha
$ git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785 

tree 655a20f99af32926cbf6d8fab092506ddd70e49c
parent df08eb357dd7f432c3dcbe0ef4b3212a38b4aeff
author Junio C Hamano <[e-mail]> 1432673399 -0700
committer Junio C Hamano <[e-mail]> 1432673399 -0700

Git 2.4.2

Signed-off-by: Junio C Hamano <[e-mail]>
Copy the decompressed content so that we can hash it.(AFAIK git uses the uncompressed version when it's hashing)
git cat-file -p 9eabf5b536662000f79978c4d1b6e4eff5c8d785 > fi
Let's SHA-1 the content using git's own hash command
git hash-object fi
3cf741bbdbcdeed65e5371912742e854a035e665
Why the output is not [9e]abf5b536662000f79978c4d1b6e4eff5c8d785? I understand the first two characters (9e) is the length in hex. How should I hash the content of fi so that I can get the git ref abf5b536662000f79978c4d1b6e4eff5c8d785 ?
解决方案
As described in "How is git commit sha1 formed ", the formula is:
(printf "<type> %s\0" $(git cat-file <type> <ref> | wc -c); git cat-file <type> <ref>)|sha1sum
In the case of the commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 (which is v2.4.2^{}, and which referenced a tree) :
(printf "commit %s\0" $(git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 | wc -c); git cat-file commit 9eabf5b536662000f79978c4d1b6e4eff5c8d785 )|sha1sum
That will give 9eabf5b536662000f79978c4d1b6e4eff5c8d785.

As would:
(printf "commit %s\0" $(git cat-file commit v2.4.2{} | wc -c); git cat-file commit v2.4.2{})|sha1sum
(still 9eabf5b536662000f79978c4d1b6e4eff5c8d785)

Similarly, computing the SHA1 of the tag v2.4.2 would be:
(printf "tag %s\0" $(git cat-file tag v2.4.2 | wc -c); git cat-file tag v2.4.2)|sha1sum
That would give 29932f3915935d773dc8d52c292cadd81c81071d.

                        
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