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https://github.com/allouis/git-internals


https://github.com/allouis/git-internals

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README 

        
# Git Internals

 

 ## What makes a git repository?

 

 To create a git repository you just need the `.git` directory in your project, with a few essential subdirectories and files.

 

 Those subdirectories and files are:

 

 - HEAD - This file tells git where the HEAD of your repo currently is, it points to a ref

 - /refs - This directory conatains all the refs, or pointers, to commits, e.g. branches, tags etc...

 - /objects - This directory is the database of git, its the storage part of the key-value store on which git sits 



## The storage mechanism

 

 You can think of git as a wrapper over a content addressable key-value store, there are three contents types: blobs, trees and commits.

 

 Essentially, a blob is a file, a tree is a directory, which contains blobs or other trees and a commit is metadata for a tree.

 

 Git stores all of this in the objects directory. The process for doing so is like:

 

 - Add a header to the content consisting of the content type (blob/tree/commit) the content length and a null byte.

 - Use a SHA1 hashing algorithm on the header + content

 - Use zlib defalte compression on the hash

 

 The resulting compressed string is then stored in the objects directory, using the first 2 chars of the hash as the sub dir and the remaining as the file name.

 

 Git internals give use a command that wraps this process up for us: `git hash-object`. We also get the reverse of this command, which takes a hash and gives us the content which is `git cat-file`



## Creating trees

 

 A tree is simply a pointer to blobs and other trees, we can use the `git write-tree` command to take the current index and create a tree out of it. Our index can be thought of as staged changes. We can update the current index using the `git update-index` command, and using the `--cacheinfo` flag we pass it a filetype, a blob hash and a file name. This will create a `.git/index` file which contains the current state.

 

 Once we have the index in a state we would like to store as a tree, we can use `git write-tree`, this creates and stores a tree object and gives us back the hash for it.



## Creating commits

 

 A commit is metadata about a specific tree, including the tree parent, the author of the commit and a description. We can create a commit at any point by use the `git commit-tree` command, this takes a tree hash and a description, we can also use the `-p` flag to pass the parent tree