是“blob”一个文件内容的同义词我放在一个git仓库？ [英] Is "blob" a synonym for the contents of a file I put in a git repository?

问题描述

一位有见识的人回答了我的一个更复杂的git问题，似乎在这个问题的标题中有解释，我不同意这个问题，所以这是一种误解，甚至会让专家git用户感到困扰。

我将提供我自己的理解作为答案 - 我很乐意为您改进或批准（或两者兼得！） - 但随时可以添加得到良好支持的答案，这些答案与我的解释。

解决方案

不，虽然这个术语的用法很矛盾很容易找到，但在一些重要的方面，blob而不是放置在git仓库中的文件内容的同义词。

其中一个blob对象包含（顺序地）：

• 单词blob
  
• 一个空格
  
• 以空字符结尾的字符串表示您文件中的字节数


• 文件中的实际（逐字）数据
  

如果你的文件数据是blob，那么blob内容的定义将是递归的。其次，git存储blob的散列值，而不是文件的散列值。从 git（1） 手册页：

所有对象都由其内容的SHA-1散列命名，通常写为40个十六进制数字

如果blob对象和您的文件是同一个东西，它们将具有相同的散列。他们没有：

  $ printfa>文件
 $ openssl sha1文件
 SHA1（文件）= 86f7e437faa5a7fce15d1ddcb9eaeaea377667b8 
 $ git散列对象文件
 2e65efe2a145dda7ee51d1741299f848e5bf752e 
 $ printfblob％d\000 $（cat文件）$（wc -c file | awk'{print $ 1}'）> file-blob 
 $ openssl sha1 file-blob 
 SHA1（file-blob）= 2e65efe2a145dda7ee51d1741299f848e5bf752e 
  

正如你所看到的，根据blob内容的上述定义构造的 file-blob 的SHA1匹配git存储的代表 file ，我们从 <强> GIT-哈希对象（1） 。第三，也许更迂腐，原因是因为把blob当作同义词可能会误导那些从其他上下文中知道blob的人，比如数据库，其中一个BLOB（二进制大对象）可能是你存储内容的逐字表示。

总结：虽然很多人使用blob作为仓库中文件内容的替身，按照git的说法，它不是一样。虽然blob对象表示您的文件并包含其数据（在ASCII头字符串之后），但任何相似之处纯属巧合。

A knowledgeable person who answered one of my more complicated git questions seemed to have the interpretation in this question's title, which I disagree with, so this is a misunderstanding that can even afflict expert git users.

I'll be offering my own understanding as an answer - which I'm happy for you to improve or approve (or both!) - but feel free to add well-supported answers that differ significantly from my interpretation.

解决方案

No, though conflicting usages of the term are easy to find, in some important ways, "blob" is not a synonym for the contents of a file that has been placed in a git repository.

For one, a blob object contains (sequentially):

• the word "blob"
• one space
• a null-terminated string representing the number of bytes in your file
• the actual (verbatim) data from your file

If your file data was the blob, then this definition of a blob's contents would be recursive.

Secondly, git stores the hash of the blob, not the hash of your file. From the git(1) man page:

All objects are named by the SHA-1 hash of their contents, normally written as a string of 40 hex digits

If the blob object and your file were the same thing, they would have the same hash. They do not:

$ printf "a" > file
$ openssl sha1 file
SHA1(file)= 86f7e437faa5a7fce15d1ddcb9eaeaea377667b8
$ git hash-object file
2e65efe2a145dda7ee51d1741299f848e5bf752e
$ printf "blob %d\000$(cat file)" $(wc -c file | awk '{print $1}') > file-blob
$ openssl sha1 file-blob
SHA1(file-blob)= 2e65efe2a145dda7ee51d1741299f848e5bf752e

As you can see, the SHA1 of file-blob, constructed according to the above definition of a blob's contents, matches the hash that git stores to represent file, which we obtained from git-hash-object(1).

A third, maybe more pedantic, reason is because treating the blob as a synonym can be misleading to those who know "blob" from other contexts, such as databases, where a "BLOB" (Binary Large OBject) might be a verbatim representation of what you're storing.

To conclude: Although many people use "blob" as a sort of stand-in for the contents of a file in a repository, in git's parlance, it is not the same thing. Although the blob object represents your file and contains its data (after an ASCII header string), any resemblance is purely coincidental.

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