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diff --git a/wiki/git/theory-and-practice/objects.md b/wiki/git/theory-and-practice/objects.md deleted file mode 100644 index 6bf975a..0000000 --- a/wiki/git/theory-and-practice/objects.md +++ /dev/null @@ -1,125 +0,0 @@ -# Objects - -Git's basest level is a storage and naming system for things Git calls -“objects.” These objects hold the bulk of the data about files and projects -tracked by Git: file contents, directory trees, commits, and so on. Every -object is identified by a SHA-1 hash, which is derived from its contents. - -SHA-1 hashes are obnoxiously long, so Git allows you to substitue any unique -prefix of a SHA-1 hash, so long as it's at least four characters long. If the -hash `0b43b9e3e64793f5a222a644ed5ab074d8fa1024` is present in your repository, -then Git commands will understand `0b43`, `0b43b9`, and other patterns to all -refer to the same object, so long as no other object has the same SHA-1 -prefix. - -## Blobs - -The contents of every file that's ever been stored in a Git repository are -stored as `blob` objects. These objects are very simple: they contain the file -contents, byte for byte. - -## Trees - -File contents (and trees, and Other Things we'll get to later) are tied -together into a directory structure by `tree` objects. These objects contain a -list of records, with one child per record. Each record contains a permissions -field corresponding to the POSIX permissions mask of the object, a type, a -SHA-1 for another object, and a name. - -A directory containing only files might be represented as the tree - - 100644 blob 511542ad6c97b28d720c697f7535897195de3318 config.md - 100644 blob 801ddd5ae10d6282bbf36ccefdd0b052972aa8e2 integrate.md - 100644 blob 61d28155862607c3d5d049e18c5a6903dba1f85e scratch.md - 100644 blob d7a79c144c22775239600b332bfa120775bab341 survival.md - -while a directory with subdirectories would also have some `tree` children: - - 040000 tree f57ef2457a551b193779e21a50fb380880574f43 12factor - 040000 tree 844697ce99e1ef962657ce7132460ad7a38b7584 authnz - 100644 blob 54795f9b774547d554f5068985bbc6df7b128832 cool-urls-can-change.md - 040000 tree fc3f39eb5d1a655374385870b8be56b202be7dd8 dev - 040000 tree 22cbfb2c1d7b07432ea7706c36b0d6295563c69c devops - 040000 tree 0b3e63b4f32c0c3acfbcf6ba28d54af4c2f0d594 git - 040000 tree 5914fdcbd34e00e23e52ba8e8bdeba0902941d3f java - 040000 tree 346f71a637a4f8933dc754fef02515a8809369c4 mysql - 100644 blob b70520badbb8de6a74b84788a7fefe64a432c56d packaging-ideas.md - 040000 tree 73ed6572345a368d20271ec5a3ffc2464ac8d270 people - -## Commits - -Blobs and trees are sufficient to store arbitrary directory trees in Git, and -you could use them that way, but Git is mostly used as a revision-tracking -system. Revisions and their history are represented by `commit` objects, which contain: - -* The SHA-1 hash of the root `tree` object of the commit, -* Zero or more SHA-1 hashes for parent commits, -* The name and email address of the commit's “author,” -* The name and email address of the commit's “committer,” -* Timestamps representing when the commit was authored and committed, and -* A commit message. - -Commit objects' parent references form a directed acyclic graph; the subgraph -reachable from a specific commit is that commit's _history_. - -When working with Git's user interface, commit parents are given in a -predictable order determined by the `git checkout` and `git merge` commands. - -## Tags - -Git's revision-tracking system supports “tags,” which are stable names for -specific configurations. It also, uniquely, supports a concept called an -“annotated tag,” represented by the `tag` object type. These annotated tag -objects contain - -* The type and SHA-1 hash of another object, -* The name and email address of the person who created the tag, -* A timestamp representing the moment the tag was created, and -* A tag message. - -## Anonymity - -There's a general theme to Git's object types: no object knows its own name. -Every object only has a name in the context of some containing object, or in -the context of [Git's refs mechanism](refs-and-names), which I'll get to -shortly. This means that the same `blob` object can be reused for multiple -files (or, more probably, the same file in multiple commits), if they happen -to have the same contents. - -This also applies to tag objects, even though their role is part of a system -for providing stable, meaningful names for commits. - -## Examining objects - -* `git cat-file <type> <sha1>`: decodes the object `<sha1>` and prints its - contents to stdout. This prints the object's contents in their raw form, - which is less than useful for `tree` objects. - -* `git cat-file -p <sha1>`: decodes the object `<sha1>` and pretty-prints it. - This pretty-printing stays close to the underlying disk format; it's most - useful for decoding `tree` objects. - -* `git show <sha1>`: decodes the object `<sha1>` and formats its contents to - stdout. For blobs, this is identical to what `git cat-file blob` would do, - but for trees, commits, and tags, the output is reformated to be more - readable. - -## Storage - -Objects are stored in two places in Git: as “loose objects,” and in “pack -files.” Newly-created objects are initially loose objects, for ease of -manipulation; transferring objects to another repository or running certain -administrative commands can cause them to be placed in pack files for faster -transfer and for smaller storage. - -Loose objects are stored directly on the filesystem, in the Git repository's -`objects` directory. Git takes a two-character prefix off of each object's -SHA-1 hash, and uses that to pick a subdirectory of `objects` to store the -object in. The remainder of the hash forms the filename. Loose objects are -compressed with zlib, to conserve space, but the resulting directory tree can -still be quite large. - -Packed objects are stored together in packed files, which live in the -repository's `objects/pack` directory. These packed files are both compressed -and delta-encoded, allowing groups of similar objects to be stored very -compactly. |