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diff --git a/.html/git/theory-and-practice/objects.html b/.html/git/theory-and-practice/objects.html new file mode 100644 index 0000000..ff6c53b --- /dev/null +++ b/.html/git/theory-and-practice/objects.html @@ -0,0 +1,202 @@ +<!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>
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