522 lines
16 KiB
Markdown
522 lines
16 KiB
Markdown
---
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title: Objects
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TARGET DECK: Obsidian::STEM
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FILE TAGS: fs::cas git
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tags:
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- cas
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- fs
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- git
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---
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## 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
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Basic
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What cryptographic hash function does git use internally?
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Back: SHA-1
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255577-->
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END%%
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%%ANKI
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Basic
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How many bytes make up a SHA-1 digest?
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Back: 20
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255580-->
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END%%
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%%ANKI
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Basic
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How many hexadecimal digits make up a SHA-1 digest?
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Back: 40
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255584-->
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END%%
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%%ANKI
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Basic
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How is a SHA-1 digest typically represented?
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Back: As a string of 40 hexadecimal digits.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255589-->
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END%%
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%%ANKI
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Basic
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How is the SHA-1 of a git object generated?
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Back: By calculating the checksum of the object header + contents.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569885-->
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END%%
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%%ANKI
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Basic
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What is "oid" an acronym for?
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Back: **O**bject **id**.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255594-->
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END%%
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%%ANKI
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Basic
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What *is* an object id?
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Back: The digest produced when hashing/storing content.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255599-->
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END%%
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%%ANKI
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Basic
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What does it mean for git to be a content-addressed storage system?
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Back: Keys generated from hashing content are used to later retrieve said content.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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Tags: fs::cas
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<!--ID: 1709177255603-->
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END%%
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%%ANKI
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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’s Voice in Software Development (New York, NY: Apress, 2014).
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Tags: fs::cas
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<!--ID: 1709177255607-->
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END%%
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%%ANKI
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Basic
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Why prefer term "oid" over e.g. "SHA-1 digest" or similar?
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Back: Git is transitioning to the use of other cryptographic hash functions.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255612-->
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END%%
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%%ANKI
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Basic
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What is a "porcelain" command?
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Back: A user-friendly command.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255618-->
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END%%
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%%ANKI
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Basic
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What is a "plumbing" command?
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Back: A lower-level command.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255622-->
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END%%
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%%ANKI
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Cloze
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{Porcelain} commands are more user-friendly than {plumbing} commands.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255627-->
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END%%
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%%ANKI
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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’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255644-->
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END%%
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%%ANKI
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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’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255668-->
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END%%
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%%ANKI
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Basic
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What regular files exist in the object database after `git init`?
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Back: None.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255672-->
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END%%
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%%ANKI
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Basic
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Files in the object database are compressed using what format?
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Back: The `zlib` data format.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254695-->
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END%%
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%%ANKI
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Basic
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What compression library does git use internally?
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Back: `zlib`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254701-->
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END%%
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%%ANKI
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Basic
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How do you decode e.g. `.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4`?
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Back: By decompressing the `zlib` data formatted file.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254705-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to inspect git objects?
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Back: `cat-file`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254709-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is typically used over manually decompressing with `zlib`?
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Back: `cat-file`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254713-->
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END%%
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%%ANKI
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Basic
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What is the difference in output of manually decompressing `<OID>` and the following?
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```bash
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$ git cat-file -p <OID>
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```
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Back: Manual decompression includes the object header.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254716-->
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END%%
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%%ANKI
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Basic
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What plumbing command can be used to find the git object type of an oid?
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Back: `cat-file`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569890-->
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END%%
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## Blobs
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The **b**inary **l**arge **ob**ject (blob) is used to represent arbitrary files.
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%%ANKI
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Basic
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Why are git blobs named the way they are?
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Back: It is an acronym for **b**inary **l**arge **ob**ject.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254720-->
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END%%
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%%ANKI
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Basic
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A regular file is stored as what kind of git object?
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Back: A blob.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254723-->
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END%%
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%%ANKI
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Basic
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What does the decompressed header of a blob object look like?
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Back: `blob <size><NUL>`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254726-->
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END%%
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%%ANKI
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Basic
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What is the value of `<size>` found in the header of a blob file?
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Back: The number of bytes of the decompressed content.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254730-->
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END%%
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%%ANKI
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Basic
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What is the decompressed text of the blob generated by the following?
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```bash
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$ echo -n "abcd" | git hash-object -w --stdin
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```
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Back: `blob 4<NUL>abcd`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254733-->
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END%%
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%%ANKI
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Basic
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The `hash-object` subcommand optionally stores a created object in what directory?
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Back: `.git/objects`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255637-->
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END%%
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%%ANKI
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Basic
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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’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255651-->
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END%%
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%%ANKI
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Basic
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What optional functionality does the `hash-object` subcommand support?
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Back: It can write an object into the object database.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255659-->
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END%%
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%%ANKI
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Basic
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What flag lets the `hash-object` subcommand write to the object database?
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Back: `-w`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255664-->
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END%%
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%%ANKI
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Basic
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What is the result of the following command?
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```bash
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$ echo -n "abcd" | git hash-object --stdin
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```
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Back: An oid (usually a SHA-1 hash).
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254736-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to create blobs?
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Back: `hash-object`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481684-->
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END%%
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## Trees
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The tree is used to represent directories. It maintains filenames of any contained blobs.
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%%ANKI
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Basic
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What git object maintains filenames?
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Back: Trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481689-->
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END%%
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%%ANKI
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Basic
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A directory is stored as what kind of git object?
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Back: Trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481693-->
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END%%
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%%ANKI
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Basic
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All the *content* of a git repository is stored as what kind of git objects?
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Back: Trees and blobs.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481696-->
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END%%
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%%ANKI
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Cloze
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A regular file is to {blob} objects as directories are to {tree} objects.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709345254740-->
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END%%
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%%ANKI
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Basic
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What kind of git objects can a tree contain?
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Back: Blobs and trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569895-->
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END%%
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%%ANKI
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Cloze
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{`hash-object`} is to blobs as {`write-tree`} is to trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481699-->
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END%%
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%%ANKI
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Basic
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What does the decompressed header of a tree object look like?
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Back: `tree <size><NUL>`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481704-->
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END%%
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%%ANKI
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Basic
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What does the decompressed header of an empty tree object look like?
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Back: `tree 0<NUL>`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481711-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to create trees?
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Back: `write-tree`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481715-->
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END%%
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%%ANKI
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Basic
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The `write-tree` subcommand stores the new tree object in what directory?
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Back: `.git/objects`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481718-->
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END%%
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%%ANKI
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Basic
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The `write-tree` subcommand creates a tree object from what?
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Back: The current index.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481721-->
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END%%
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%%ANKI
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Basic
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What does the `write-tree` subcommand return?
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Back: The oid of the new tree object.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481725-->
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END%%
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%%ANKI
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Basic
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Why doesn't `write-tree` take any arguments?
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Back: It creates a new tree from the current index.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709348481730-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to put tree objects into the staging area?
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Back: `read-tree`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709349279634-->
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END%%
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%%ANKI
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Basic
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How does the following command update the index?
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```bash
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$ git read-tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579
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```
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Back: It adds the contents of tree `d8329fc1cc938780ffdd9f94e0d364e0ea74f579`.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709349279641-->
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END%%
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%%ANKI
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Basic
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How does the following command update the index?
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```bash
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$ git read-tree --prefix=foo d8329fc1cc938780ffdd9f94e0d364e0ea74f579
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```
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Back: It adds tree `d8329fc1cc938780ffdd9f94e0d364e0ea74f579` as subtree `foo`.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709349279646-->
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END%%
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%%ANKI
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Basic
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Where does the `read-tree` subcommand read the tree into?
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Back: The current index.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709349279652-->
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END%%
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%%ANKI
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Basic
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The `read-tree` and `write-tree` interact indirectly through what?
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Back: The staging area.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709349279660-->
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END%%
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## Commits
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Commit objects provide metadata information about particular trees.
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> 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.
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%%ANKI
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Basic
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What distinguishes a tree from a commit?
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Back: A commit contains data about a tree (e.g. author, parent commits, etc.).
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569898-->
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END%%
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%%ANKI
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Basic
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How do git commits maintain a traversable history?
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Back: Each commit can have parent commits associated with it.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569902-->
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END%%
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%%ANKI
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Basic
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What kind of git object is responsible for maintaining history?
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Back: Commits.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569906-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to create commits?
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Back: `commit-tree`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569911-->
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END%%
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%%ANKI
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Cloze
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{`write-tree`} is to trees as {`commit-tree`} is to commits.
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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%%
|
||
|
||
## References
|
||
|
||
* Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014). |