notebook/notes/git/objects.md

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

Overview

Git has a few fundamental types that serve as the foundation for its 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).

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%%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).

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%%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).

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%%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).

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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’s Voice in Software Development (New York, NY: Apress, 2014).

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%%ANKI Basic What is "oid" an acronym for? Back: Object id. Reference: Scott Chacon, Pro Git, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).

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%%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).

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%%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

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%%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

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%ANKI Basic What is the difference in output of manually decompressing <OID> and the following?

$ 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).

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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’s Voice in Software Development (New York, NY: Apress, 2014).

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Blobs

The binary large object (blob) is used to represent arbitrary files.

%%ANKI Basic What are git blobs an acronym for? Back: Binary large object. Reference: Scott Chacon, Pro Git, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).

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%%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).

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%%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).

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%%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).

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%%ANKI Basic What is the decompressed text of the blob generated by the following?

$ 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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%ANKI Basic What is the result of the following command?

$ 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).

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%%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).

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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).

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%%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).

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%%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).

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%%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).

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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’s Voice in Software Development (New York, NY: Apress, 2014).

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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’s Voice in Software Development (New York, NY: Apress, 2014).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%ANKI Basic How does the following command update the index?

$ 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).

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%%ANKI Basic How does the following command update the index?

$ 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).

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%%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).

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%%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).

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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’s Voice in Software Development (New York, NY: Apress, 2014).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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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 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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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%%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).

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Bibliography

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