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316 lines
10 KiB
Markdown
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[#]: subject: "Inside .git"
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[#]: via: "https://jvns.ca/blog/2024/01/26/inside-git/"
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[#]: author: "Julia Evans https://jvns.ca/"
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[#]: collector: "lujun9972/lctt-scripts-1705972010"
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[#]: translator: " "
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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Inside .git
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======
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Hello! I posted a comic on Mastodon this week about what’s in the `.git` directory and someone requested a text version, so here it is. I added some extra notes too. First, here’s the image. It’s a ~15 word explanation of each part of your `.git` directory.
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[![][1]][2]
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You can `git clone https://github.com/jvns/inside-git` if you want to run all these examples yourself.
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Here’s a table of contents:
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* [HEAD: .git/head][3]
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* [branch: .git/refs/heads/main][4]
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* [commit: .git/objects/10/93da429…][5]
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* [tree: .git/objects/9f/83ee7550…][6]
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* [blobs: .git/objects/5a/475762c…][7]
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* [reflog: .git/logs/refs/heads/main][8]
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* [remote-tracking branches: .git/refs/remotes/origin/main][9]
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* [tags: .git/refs/tags/v1.0][10]
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* [the stash: .git/refs/stash][11]
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* [.git/config][12]
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* [hooks: .git/hooks/pre-commit][13]
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* [the staging area: .git/index][14]
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* [this isn’t exhaustive][15]
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* [this isn’t meant to completely explain git][16]
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The first 5 parts (`HEAD`, branch, commit, tree, blobs) are the core of git.
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### HEAD: `.git/head`
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**`HEAD`** is a tiny file that just contains the name of your current **branch**.
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Example contents:
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```
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$ cat .git/HEAD
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ref: refs/heads/main
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```
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`HEAD` can also be a commit ID, that’s called “detached HEAD state”.
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### branch: `.git/refs/heads/main`
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A **branch** is stored as a tiny file that just contains 1 **commit ID**. It’s stored in a folder called `refs/heads`.
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Example contents:
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```
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$ cat .git/refs/heads/main
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1093da429f08e0e54cdc2b31526159e745d98ce0
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```
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### commit: `.git/objects/10/93da429...`
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A **commit** is a small file containing its parent(s), message, **tree** , and author.
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Example contents:
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```
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$ git cat-file -p 1093da429f08e0e54cdc2b31526159e745d98ce0
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tree 9f83ee7550919867e9219a75c23624c92ab5bd83
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parent 33a0481b440426f0268c613d036b820bc064cdea
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author Julia Evans <julia@example.com> 1706120622 -0500
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committer Julia Evans <julia@example.com> 1706120622 -0500
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add hello.py
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```
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These files are compressed, the best way to see objects is with `git cat-file -p HASH`.
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### tree: `.git/objects/9f/83ee7550...`
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**Trees** are small files with directory listings. The files in it are called **blobs**.
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Example contents:
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```
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$ git cat-file -p 9f83ee7550919867e9219a75c23624c92ab5bd83
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100644 blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 .gitignore
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100644 blob 665c637a360874ce43bf74018768a96d2d4d219a hello.py
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040000 tree 24420a1530b1f4ec20ddb14c76df8c78c48f76a6 lib
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```
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The permissions here LOOK like unix permissions, but they’re actually super restricted, only 644 and 755 are allowed.
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### blobs: `.git/objects/5a/475762c...`
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**blobs** are the files that contain your actual code
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Example contents:
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```
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$ git cat-file -p 665c637a360874ce43bf74018768a96d2d4d219a
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print("hello world!")
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```
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Storing a new blob with every change can get big, so `git gc` periodically [packs them][17] for efficiency in `.git/objects/pack`.
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### reflog: `.git/logs/refs/heads/main`
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The reflog stores the history of every branch, tag, and HEAD. For (mostly) every file in `.git/refs`, there’s a corresponding log in `.git/logs/refs`.
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Example content for the `main` branch:
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```
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$ tail -n 1 .git/logs/refs/heads/main
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33a0481b440426f0268c613d036b820bc064cdea
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1093da429f08e0e54cdc2b31526159e745d98ce0
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Julia Evans <julia@example.com>
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1706119866 -0500
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commit: add hello.py
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```
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each line of the reflog has:
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* before/after commit IDs
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* user
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* timestamp
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* log message
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Normally it’s all one line, I just wrapped it for readability here.
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### remote-tracking branches: `.git/refs/remotes/origin/main`
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**Remote-tracking branches** store the most recently seen **commit ID** for a remote branch
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Example content:
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```
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$ cat .git/refs/remotes/origin/main
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fcdeb177797e8ad8ad4c5381b97fc26bc8ddd5a2
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```
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When git status says “you’re up to date with `origin/main`”, it’s just looking at this. It’s often out of date, you can update it with `git fetch origin main`.
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### tags: `.git/refs/tags/v1.0`
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A tag is a tiny file in `.git/refs/tags` containing a commit ID.
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Example content:
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```
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$ cat .git/refs/tags/v1.0
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1093da429f08e0e54cdc2b31526159e745d98ce0
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```
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Unlike branches, when you make new commits it doesn’t update the tag.
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### the stash: `.git/refs/stash`
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The stash is a tiny file called `.git/refs/stash`. It contains the commit ID of a commit that’s created when you run `git stash`.
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```
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cat .git/refs/stash
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62caf3d918112d54bcfa24f3c78a94c224283a78
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```
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The stash is a stack, and previous values are stored in `.git/logs/refs/stash` (the reflog for `stash`).
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```
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cat .git/logs/refs/stash
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62caf3d9 e85c950f Julia Evans <julia@example.com> 1706290652 -0500 WIP on main: 1093da4 add hello.py
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00000000 62caf3d9 Julia Evans <julia@example.com> 1706290668 -0500 WIP on main: 1093da4 add hello.py
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```
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Unlike branches and tags, if you `git stash pop` a commit from the stash, it’s **deleted** from the reflog so it’s almost impossible to find it again. The stash is the only reflog in git where things get deleted very soon after they’re added. (entries expire out of the branch reflogs too, but generally only after 90 days)
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**A note on refs:**
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At this point you’ve probably noticed that a lot of things (branches, remote-tracking branches, tags, and the stash) are commit IDs in `.git/refs`. They’re called “references” or “refs”. Every ref is a commit ID, but the different types of refs are treated VERY differently by git, so I find it useful to think about them separately even though they all use the same file format. For example, git deletes things from the stash reflog in a way that it won’t for branch or tag reflogs.
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### .git/config
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`.git/config` is a config file for the repository. It’s where you configure your remotes.
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Example content:
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```
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[remote "origin"]
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url = git@github.com: jvns/int-exposed
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fetch = +refs/heads/*: refs/remotes/origin/*
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[branch "main"]
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remote = origin
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merge refs/heads/main
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```
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git has local and global settings, the local settings are here and the global ones are in `~/.gitconfig` hooks
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### hooks: `.git/hooks/pre-commit`
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Hooks are optional scripts that you can set up to run (eg before a commit) to do anything you want.
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Example content:
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```
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#!/bin/bash
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any-commands-you-want
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```
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(this obviously isn’t a real pre-commit hook)
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### the staging area: `.git/index`
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The staging area stores files when you’re preparing to commit. This one is a binary file, unlike a lot of things in git which are essentially plain text files.
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As far as I can tell the best way to look at the contents of the index is with `git ls-files --stage`:
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```
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$ git ls-files --stage
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100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 .gitignore
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100644 665c637a360874ce43bf74018768a96d2d4d219a 0 hello.py
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100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 lib/empty.py
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```
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### this isn’t exhaustive
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There are some other things in `.git` like `FETCH_HEAD`, `worktrees`, and `info`. I only included the ones that I’ve found it useful to understand.
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### this isn’t meant to completely explain git
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One of the most common pieces of advice I hear about git is “just learn how the `.git` directory is structured and then you’ll understand everything!“.
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I love understanding the internals of things more than anyone, but there’s a LOT that “how the .git directory is structured” doesn’t explain, like:
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* how merges and rebases work and how they can go wrong (for instance this list of [what can go wrong with rebase][18])
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* how exactly your colleagues are using git, and what guidelines you should be following to work with them successfully
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* how pushing/pulling code from other repositories works
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* how to handle merge conflicts
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Hopefully this will be useful to some folks out there though.
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### some other references:
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* the book [building git][19] by James Coglan (side note: looks like there’s a [50% off discount for the rest of January][20])
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* [git from the inside out][21] by mary rose cook
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* the official [git repository layout docs][22]
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--------------------------------------------------------------------------------
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via: https://jvns.ca/blog/2024/01/26/inside-git/
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作者：[Julia Evans][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://jvns.ca/
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[b]: https://github.com/lujun9972
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[1]: https://wizardzines.com/images/uploads/inside-git.png
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[2]: https://wizardzines.com/comics/inside-git
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[3]: tmp.MwNheYBHl9#head-git-head
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[4]: tmp.MwNheYBHl9#branch-git-refs-heads-main
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[5]: tmp.MwNheYBHl9#commit-git-objects-10-93da429
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[6]: tmp.MwNheYBHl9#tree-git-objects-9f-83ee7550
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[7]: tmp.MwNheYBHl9#blobs-git-objects-5a-475762c
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[8]: tmp.MwNheYBHl9#reflog-git-logs-refs-heads-main
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[9]: tmp.MwNheYBHl9#remote-tracking-branches-git-refs-remotes-origin-main
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[10]: tmp.MwNheYBHl9#tags-git-refs-tags-v1-0
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[11]: tmp.MwNheYBHl9#the-stash-git-refs-stash
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[12]: tmp.MwNheYBHl9#git-config
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[13]: tmp.MwNheYBHl9#hooks-git-hooks-pre-commit
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[14]: tmp.MwNheYBHl9#the-staging-area-git-index
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[15]: tmp.MwNheYBHl9#this-isn-t-exhaustive
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[16]: tmp.MwNheYBHl9#this-isn-t-meant-to-completely-explain-git
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[17]: https://codewords.recurse.com/issues/three/unpacking-git-packfiles
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[18]: https://jvns.ca/blog/2023/11/06/rebasing-what-can-go-wrong-/
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[19]: https://shop.jcoglan.com/building-git/
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[20]: https://mastodon.social/@jcoglan/111807463940323655
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[21]: https://maryrosecook.com/blog/post/git-from-the-inside-out
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[22]: https://git-scm.com/docs/gitrepository-layout#Documentation/gitrepository-layout.txt-index
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