diff options
Diffstat (limited to '.html/git/theory-and-practice/objects.html')
| -rw-r--r-- | .html/git/theory-and-practice/objects.html | 202 |
1 files changed, 0 insertions, 202 deletions
diff --git a/.html/git/theory-and-practice/objects.html b/.html/git/theory-and-practice/objects.html deleted file mode 100644 index ff6c53b..0000000 --- a/.html/git/theory-and-practice/objects.html +++ /dev/null @@ -1,202 +0,0 @@ -<!DOCTYPE html> -<html> -<head> - <title> - The Codex » - Objects - </title> - - <link - rel='stylesheet' - type='text/css' - href='http://fonts.googleapis.com/css?family=Buenard:400,700&subset=latin,latin-ext'> - <link - rel="stylesheet" - type="text/css" - href="../../media/css/reset.css"> - <link - rel="stylesheet" - type="text/css" - href="../../media/css/grimoire.css"> -</head> -<body> - -<div id="shell"> - - <ol id="breadcrumbs"> - - <li class="crumb-0 not-last"> - - <a href="../../">index</a> - - </li> - - <li class="crumb-1 not-last"> - - <a href="../">git</a> - - </li> - - <li class="crumb-2 not-last"> - - <a href="./">theory-and-practice</a> - - </li> - - <li class="crumb-3 last"> - - objects - - </li> - - </ol> - - - - <div id="article"> - <h1 id="objects">Objects</h1> -<p>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.</p> -<p>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 <code>0b43b9e3e64793f5a222a644ed5ab074d8fa1024</code> is present in your repository, -then Git commands will understand <code>0b43</code>, <code>0b43b9</code>, and other patterns to all -refer to the same object, so long as no other object has the same SHA-1 -prefix.</p> -<h2 id="blobs">Blobs</h2> -<p>The contents of every file that's ever been stored in a Git repository are -stored as <code>blob</code> objects. These objects are very simple: they contain the file -contents, byte for byte.</p> -<h2 id="trees">Trees</h2> -<p>File contents (and trees, and Other Things we'll get to later) are tied -together into a directory structure by <code>tree</code> 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.</p> -<p>A directory containing only files might be represented as the tree</p> -<pre><code>100644 blob 511542ad6c97b28d720c697f7535897195de3318 config.md -100644 blob 801ddd5ae10d6282bbf36ccefdd0b052972aa8e2 integrate.md -100644 blob 61d28155862607c3d5d049e18c5a6903dba1f85e scratch.md -100644 blob d7a79c144c22775239600b332bfa120775bab341 survival.md -</code></pre> -<p>while a directory with subdirectories would also have some <code>tree</code> children:</p> -<pre><code>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 -</code></pre> -<h2 id="commits">Commits</h2> -<p>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 <code>commit</code> objects, which contain:</p> -<pre><code>* 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. -</code></pre> -<p>Commit objects' parent references form a directed acyclic graph; the subgraph -reachable from a specific commit is that commit's <em>history</em>.</p> -<p>When working with Git's user interface, commit parents are given in a -predictable order determined by the <code>git checkout</code> and <code>git merge</code> commands.</p> -<h2 id="tags">Tags</h2> -<p>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 <code>tag</code> object type. These annotated tag -objects contain</p> -<pre><code>* 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. -</code></pre> -<h2 id="anonymity">Anonymity</h2> -<p>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 <a href="refs-and-names">Git's refs mechanism</a>, which I'll get to -shortly. This means that the same <code>blob</code> object can be reused for multiple -files (or, more probably, the same file in multiple commits), if they happen -to have the same contents.</p> -<p>This also applies to tag objects, even though their role is part of a system -for providing stable, meaningful names for commits.</p> -<h2 id="examining-objects">Examining objects</h2> -<ul> -<li> -<p><code>git cat-file <type> <sha1></code>: decodes the object <code><sha1></code> and prints its - contents to stdout. This prints the object's contents in their raw form, - which is less than useful for <code>tree</code> objects.</p> -</li> -<li> -<p><code>git cat-file -p <sha1></code>: decodes the object <code><sha1></code> and pretty-prints it. - This pretty-printing stays close to the underlying disk format; it's most - useful for decoding <code>tree</code> objects.</p> -</li> -<li> -<p><code>git show <sha1></code>: decodes the object <code><sha1></code> and formats its contents to - stdout. For blobs, this is identical to what <code>git cat-file blob</code> would do, - but for trees, commits, and tags, the output is reformated to be more - readable.</p> -</li> -</ul> -<h2 id="storage">Storage</h2> -<p>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.</p> -<p>Loose objects are stored directly on the filesystem, in the Git repository's -<code>objects</code> directory. Git takes a two-character prefix off of each object's -SHA-1 hash, and uses that to pick a subdirectory of <code>objects</code> 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.</p> -<p>Packed objects are stored together in packed files, which live in the -repository's <code>objects/pack</code> directory. These packed files are both compressed -and delta-encoded, allowing groups of similar objects to be stored very -compactly.</p> - </div> - - - -<div id="comments"> -<div id="disqus_thread"></div> -<script type="text/javascript"> - /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */ - var disqus_shortname = 'grimoire'; // required: replace example with your forum shortname - - /* * * DON'T EDIT BELOW THIS LINE * * */ - (function() { - var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true; - dsq.src = 'http://' + disqus_shortname + '.disqus.com/embed.js'; - (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq); - })(); -</script> -<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript> -<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a> -</div> - - - - <div id="footer"> - <p> - - The Codex — - - Powered by <a href="http://markdoc.org/">Markdoc</a>. - -<a href="https://bitbucket.org/ojacobson/grimoire.ca/src/master/wiki/git/theory-and-practice/objects.md">See this page on Bitbucket</a> (<a href="https://bitbucket.org/ojacobson/grimoire.ca/history-node/master/wiki/git/theory-and-practice/objects.md">history</a>). - - </p> - </div> - -</div> -</body> -</html>
\ No newline at end of file |