notebook/notes/git/objects.md

---
title: Objects
TARGET DECK: Obsidian::STEM
FILE TAGS: fs::cas git
tags:
  - cas
  - fs
  - git
---

## Overview

Git has a few fundamental types that serve as the foundation for its [[cas|CAS]] system, expanded upon in each subsequent section.

%%ANKI
Basic
What cryptographic hash function does git use internally?
Back: SHA-1
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255577-->
END%%

%%ANKI
Basic
How many bytes make up a SHA-1 digest?
Back: 20
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255589-->
END%%

%%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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709674569885-->
END%%

%%ANKI
Basic
What is "oid" an acronym for?
Back: **O**bject **id**.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
Tags: fs::cas
<!--ID: 1709177255603-->
END%%

%%ANKI
Cloze
When interpreting git as a key-value store, {oids} are to keys as {objects} are to values.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255612-->
END%%

%%ANKI
Basic
What is a "porcelain" command?
Back: A user-friendly command.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255618-->
END%%

%%ANKI
Basic
What is a "plumbing" command?
Back: A lower-level command.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255622-->
END%%

%%ANKI
Cloze
{Porcelain} commands are more user-friendly than {plumbing} commands.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255627-->
END%%

%%ANKI
Cloze
The term "{object database}" refers to the {`.git/objects`} directory.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255644-->
END%%

%%ANKI
Basic
Where does `d670460b4b4aece5915caf5c68d12f560a9fe3e4` live in the object database?
Back: At `.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4`.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255668-->
END%%

%%ANKI
Basic
What regular files exist in the object database after `git init`?
Back: None.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255672-->
END%%

%%ANKI
Basic
Files in the object database are compressed using what format?
Back: The `zlib` data format.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709345254695-->
END%%

%%ANKI
Basic
What compression library does git use internally?
Back: `zlib`
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709345254701-->
END%%

%%ANKI
Basic
What plumbing subcommand is used to inspect git objects?
Back: `cat-file`
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709345254716-->
END%%

%%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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709674569890-->
END%%

## Blobs

The **b**inary **l**arge **ob**ject (blob) is used to represent arbitrary files.

%%ANKI
Basic
What are git blobs an acronym for?
Back: **B**inary **l**arge **ob**ject.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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
```
Back: `blob 4<NUL>abcd`
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255637-->
END%%

%%ANKI
Basic
What does the `hash-object` subcommand return?
Back: The oid of the new blob object.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709177255664-->
END%%

%%ANKI
Basic
What is the result of the following command?
```bash
$ echo -n "abcd" | git hash-object --stdin
```
Back: An oid (usually a SHA-1 hash).
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709345254736-->
END%%

%%ANKI
Basic
What plumbing subcommand is used to create blobs?
Back: `hash-object`
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709345254740-->
END%%

%%ANKI
Basic
What kind of git objects can a tree contain?
Back: Blobs and trees.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709674569895-->
END%%

%%ANKI
Cloze
{`hash-object`} is to blobs as {`write-tree`} is to trees.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709349279660-->
END%%

## 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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709674569920-->
END%%

%%ANKI
Basic
Commits points to what kind of git object?
Back: Trees.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--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’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1709674569928-->
END%%

%%ANKI
Basic
How many parents does an initial commit have?
Back: Zero.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1716397645567-->
END%%

%%ANKI
Basic
How many parents does a "normal" commit have?
Back: One.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1716397645568-->
END%%

%%ANKI
Basic
How many parents does a "merge" commit have?
Back: Two or more.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1716397645570-->
END%%

## Tags

Tags are (possibly indirect) pointers to a git object. They *usually* point to a commit but aren't required to. There are two types of tags:

* **Lightweight** tags are [[refs|git refs]] that never move. These aren't really git objects in the same way the other objects are.
* **Annotated** tags are real objects that provide metadata about the object being pointed to.

%%ANKI
Cloze
{1:Lightweight} tags are to git {2:refs} whereas {2:annotated} tags are to git {1:objects}.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782248-->
END%%

%%ANKI
Basic
Creation of which of lightweight and/or annotated tags writes a new ref to disk?
Back: Both.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120977416-->
END%%

%%ANKI
Basic
Creation of which of lightweight and/or annotated tags writes a new object to disk?
Back: Just annotated tags.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120977419-->
END%%

%%ANKI
Basic
What two kinds of git tags are there?
Back: Lightweight and annotated.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782251-->
END%%

%%ANKI
Basic
What do git tags point to?
Back: Any git object.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782253-->
END%%

%%ANKI
Basic
What do git tags *usually* point to?
Back: Commits.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782256-->
END%%

%%ANKI
Basic
What primitive type does a lightweight tag correspond to?
Back: A git ref.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782258-->
END%%

%%ANKI
Basic
What primitive type does an annotated tag correspond to?
Back: A git object.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782261-->
END%%

%%ANKI
Basic
How many pointers does a lightweight tag introduce?
Back: One.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782264-->
END%%

%%ANKI
Basic
How many pointers does an annotated tag introduce?
Back: Two.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782267-->
END%%

%%ANKI
Basic
Why is the term "tag object" a slight misnomer?
Back: A lightweight tag is just a ref, not an object.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782269-->
END%%

%%ANKI
Cloze
Git objects are to {`.git/objects`} whereas git references are to {`.git/refs`}.
Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
<!--ID: 1711120782272-->
END%%

## Bibliography

* Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).