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---
title: Objects
TARGET DECK: Obsidian::STEM
FILE TAGS: fs::cas git
tags:
- cas
- fs
- git
---
## Overview
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Git has a few fundamental types that serve as the foundation for its [[cas|CAS]] system, expanded upon in each subsequent section.
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%%ANKI
Basic
What cryptographic hash function does git use internally?
Back: SHA-1
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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END%%
%%ANKI
Basic
How many bytes make up a SHA-1 digest?
Back: 20
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255580 -->
END%%
%%ANKI
Basic
How many hexadecimal digits make up a SHA-1 digest?
Back: 40
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255584 -->
END%%
%%ANKI
Basic
How is a SHA-1 digest typically represented?
Back: As a string of 40 hexadecimal digits.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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END%%
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%%ANKI
Basic
How is the SHA-1 of a git object generated?
Back: By calculating the checksum of the object header + contents.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569885 -->
END%%
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%%ANKI
Basic
What is "oid" an acronym for?
Back: **O**bject **id** .
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255594 -->
END%%
%%ANKI
Basic
What *is* an object id?
Back: The digest produced when hashing/storing content.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255599 -->
END%%
%%ANKI
Basic
What does it mean for git to be a content-addressed storage system?
Back: Keys generated from hashing content are used to later retrieve said content.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
Tags: fs::cas
<!-- ID: 1709177255603 -->
END%%
%%ANKI
Cloze
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When interpreting git as a key-value store, {oids} are to keys as {objects} are to values.
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
Tags: fs::cas
<!-- ID: 1709177255607 -->
END%%
%%ANKI
Basic
Why prefer term "oid" over e.g. "SHA-1 digest" or similar?
Back: Git is transitioning to the use of other cryptographic hash functions.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255612 -->
END%%
%%ANKI
Basic
What is a "porcelain" command?
Back: A user-friendly command.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255618 -->
END%%
%%ANKI
Basic
What is a "plumbing" command?
Back: A lower-level command.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255622 -->
END%%
%%ANKI
Cloze
{Porcelain} commands are more user-friendly than {plumbing} commands.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255627 -->
END%%
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%%ANKI
Cloze
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The term "{object database}" refers to the {`.git/objects`} directory.
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255644 -->
END%%
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%%ANKI
Basic
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Where does `d670460b4b4aece5915caf5c68d12f560a9fe3e4` live in the object database?
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Back: At `.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4` .
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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<!-- ID: 1709177255668 -->
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END%%
%%ANKI
Basic
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What regular files exist in the object database after `git init` ?
Back: None.
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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<!-- ID: 1709177255672 -->
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END%%
%%ANKI
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Basic
Files in the object database are compressed using what format?
Back: The `zlib` data format.
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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<!-- ID: 1709345254695 -->
END%%
%%ANKI
Basic
What compression library does git use internally?
Back: `zlib`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254701 -->
END%%
%%ANKI
Basic
How do you decode e.g. `.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4` ?
Back: By decompressing the `zlib` data formatted file.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254705 -->
END%%
%%ANKI
Basic
What plumbing subcommand is used to inspect git objects?
Back: `cat-file`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254709 -->
END%%
%%ANKI
Basic
What plumbing subcommand is typically used over manually decompressing with `zlib` ?
Back: `cat-file`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254713 -->
END%%
%%ANKI
Basic
What is the difference in output of manually decompressing `<OID>` and the following?
```bash
$ git cat-file -p < OID >
```
Back: Manual decompression includes the object header.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254716 -->
END%%
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%%ANKI
Basic
What plumbing command can be used to find the git object type of an oid?
Back: `cat-file`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569890 -->
END%%
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## Blobs
The **b**inary **l**arge **ob**ject (blob) is used to represent arbitrary files.
%%ANKI
Basic
Why are git blobs named the way they are?
Back: It is an acronym for **b**inary **l**arge **ob**ject.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254720 -->
END%%
%%ANKI
Basic
A regular file is stored as what kind of git object?
Back: A blob.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254723 -->
END%%
%%ANKI
Basic
What does the decompressed header of a blob object look like?
Back: `blob <size><NUL>`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254726 -->
END%%
%%ANKI
Basic
What is the value of `<size>` found in the header of a blob file?
Back: The number of bytes of the decompressed content.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254730 -->
END%%
%%ANKI
Basic
What is the decompressed text of the blob generated by the following?
```bash
$ echo -n "abcd" | git hash-object -w --stdin
```
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Back: `blob 4<NUL>abcd`
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254733 -->
END%%
%%ANKI
Basic
The `hash-object` subcommand optionally stores a created object in what directory?
Back: `.git/objects`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255637 -->
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END%%
%%ANKI
Basic
What does the `hash-object` subcommand return?
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Back: The oid of the new blob object.
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255651 -->
END%%
%%ANKI
Basic
What optional functionality does the `hash-object` subcommand support?
Back: It can write an object into the object database.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255659 -->
END%%
%%ANKI
Basic
What flag lets the `hash-object` subcommand write to the object database?
Back: `-w`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709177255664 -->
END%%
%%ANKI
Basic
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What is the result of the following command?
```bash
$ echo -n "abcd" | git hash-object --stdin
```
Back: An oid (usually a SHA-1 hash).
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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<!-- ID: 1709345254736 -->
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END%%
%%ANKI
Basic
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What plumbing subcommand is used to create blobs?
Back: `hash-object`
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Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
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<!-- ID: 1709348481684 -->
END%%
## Trees
The tree is used to represent directories. It maintains filenames of any contained blobs.
%%ANKI
Basic
What git object maintains filenames?
Back: Trees.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481689 -->
END%%
%%ANKI
Basic
A directory is stored as what kind of git object?
Back: Trees.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481693 -->
END%%
%%ANKI
Basic
All the *content* of a git repository is stored as what kind of git objects?
Back: Trees and blobs.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481696 -->
END%%
%%ANKI
Cloze
A regular file is to {blob} objects as directories are to {tree} objects.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709345254740 -->
END%%
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%%ANKI
Basic
What kind of git objects can a tree contain?
Back: Blobs and trees.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569895 -->
END%%
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%%ANKI
Cloze
{`hash-object`} is to blobs as {`write-tree`} is to trees.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481699 -->
END%%
%%ANKI
Basic
What does the decompressed header of a tree object look like?
Back: `tree <size><NUL>`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481704 -->
END%%
%%ANKI
Basic
What does the decompressed header of an empty tree object look like?
Back: `tree 0<NUL>`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481711 -->
END%%
%%ANKI
Basic
What plumbing subcommand is used to create trees?
Back: `write-tree`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481715 -->
END%%
%%ANKI
Basic
The `write-tree` subcommand stores the new tree object in what directory?
Back: `.git/objects`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481718 -->
END%%
%%ANKI
Basic
The `write-tree` subcommand creates a tree object from what?
Back: The current index.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481721 -->
END%%
%%ANKI
Basic
What does the `write-tree` subcommand return?
Back: The oid of the new tree object.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481725 -->
END%%
%%ANKI
Basic
Why doesn't `write-tree` take any arguments?
Back: It creates a new tree from the current index.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709348481730 -->
END%%
%%ANKI
Basic
What plumbing subcommand is used to put tree objects into the staging area?
Back: `read-tree`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709349279634 -->
END%%
%%ANKI
Basic
How does the following command update the index?
```bash
$ git read-tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579
```
Back: It adds the contents of tree `d8329fc1cc938780ffdd9f94e0d364e0ea74f579` .
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709349279641 -->
END%%
%%ANKI
Basic
How does the following command update the index?
```bash
$ git read-tree --prefix=foo d8329fc1cc938780ffdd9f94e0d364e0ea74f579
```
Back: It adds tree `d8329fc1cc938780ffdd9f94e0d364e0ea74f579` as subtree `foo` .
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709349279646 -->
END%%
%%ANKI
Basic
Where does the `read-tree` subcommand read the tree into?
Back: The current index.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709349279652 -->
END%%
%%ANKI
Basic
The `read-tree` and `write-tree` interact indirectly through what?
Back: The staging area.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709349279660 -->
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END%%
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## Commits
Commit objects provide metadata information about particular trees.
> While a tree represents a particular directory state of a working directory, a commit represents that state in "time", and explains how to get there.
%%ANKI
Basic
What distinguishes a tree from a commit?
Back: A commit contains data about a tree (e.g. author, parent commits, etc.).
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569898 -->
END%%
%%ANKI
Basic
How do git commits maintain a traversable history?
Back: Each commit can have parent commits associated with it.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569902 -->
END%%
%%ANKI
Basic
What kind of git object is responsible for maintaining history?
Back: Commits.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569906 -->
END%%
%%ANKI
Basic
What plumbing subcommand is used to create commits?
Back: `commit-tree`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569911 -->
END%%
%%ANKI
Cloze
{`write-tree`} is to trees as {`commit-tree`} is to commits.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569916 -->
END%%
%%ANKI
Basic
What does the decompressed header of a commit object look like?
Back: `commit <size><NUL>`
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569920 -->
END%%
%%ANKI
Basic
Commits points to what kind of git object?
Back: Trees.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569924 -->
END%%
%%ANKI
Basic
What does the `commit-tree` subcommand return?
Back: The oid of the new commit object.
Reference: Scott Chacon, *Pro Git* , Second edition, The Expert’
<!-- ID: 1709674569928 -->
END%%
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## References
* Scott Chacon, *Pro Git* , Second edition, The Expert’
