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| </head> | |
| <body class="manpage"> | |
| <div id="header"> | |
| <h1>gitdatamodel(7) Manual Page</h1> | |
| <h2 id="_name">NAME</h2> | |
| <div class="sectionbody"> | |
| <p>gitdatamodel - Git's core data model</p> | |
| </div> | |
| </div> | |
| <div id="content"> | |
| <div class="sect1"> | |
| <h2 id="_synopsis">SYNOPSIS</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>gitdatamodel</p> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_description">DESCRIPTION</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>It’s not necessary to understand Git’s data model to use Git, but it’s | |
| very helpful when reading Git’s documentation so that you know what it | |
| means when the documentation says "object", "reference" or "index".</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Git’s core operations use 4 kinds of data:</p> | |
| </div> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p><a href="#objects">Objects</a>: commits, trees, blobs, and tag objects</p> | |
| </li> | |
| <li> | |
| <p><a href="#references">References</a>: branches, tags, | |
| remote-tracking branches, etc</p> | |
| </li> | |
| <li> | |
| <p><a href="#index">The index</a>, also known as the staging area</p> | |
| </li> | |
| <li> | |
| <p><a href="#reflogs">Reflogs</a>: logs of changes to references ("ref log")</p> | |
| </li> | |
| </ol> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="objects">OBJECTS</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>All of the commits and files in a Git repository are stored as "Git objects". | |
| Git objects never change after they’re created, and every object has an ID, | |
| like <code>1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a</code>.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>This means that if you have an object’s ID, you can always recover its | |
| exact contents as long as the object hasn’t been deleted.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Every object has:</p> | |
| </div> | |
| <div id="object-id" class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>an <strong>ID</strong> (aka "object name"), which is a cryptographic hash of its | |
| type and contents. | |
| It’s fast to look up a Git object using its ID. | |
| This is usually represented in hexadecimal, like | |
| <code>1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a</code>.</p> | |
| </li> | |
| <li> | |
| <p>a <strong>type</strong>. There are 4 types of objects: | |
| <a href="#commit">commits</a>, <a href="#tree">trees</a>, <a href="#blob">blobs</a>, | |
| and <a href="#tag-object">tag objects</a>.</p> | |
| </li> | |
| <li> | |
| <p><strong>contents</strong>. The structure of the contents depends on the type.</p> | |
| </li> | |
| </ol> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Here’s how each type of object is structured:</p> | |
| </div> | |
| <div id="commit" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">commit</dt> | |
| <dd> | |
| <p>A commit contains these required fields | |
| (though there are other optional fields):</p> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>The full directory structure of all the files in that version of the | |
| repository and each file’s contents, stored as the <strong><a href="#tree">tree</a></strong> ID | |
| of the commit’s top-level directory</p> | |
| </li> | |
| <li> | |
| <p>Its <strong>parent commit ID(s)</strong>. The first commit in a repository has 0 parents, | |
| regular commits have 1 parent, merge commits have 2 or more parents</p> | |
| </li> | |
| <li> | |
| <p>An <strong>author</strong> and the time the commit was authored</p> | |
| </li> | |
| <li> | |
| <p>A <strong>committer</strong> and the time the commit was committed</p> | |
| </li> | |
| <li> | |
| <p>A <strong>commit message</strong></p> | |
| <div class="paragraph"> | |
| <p>Here’s how an example commit is stored:</p> | |
| </div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre>tree 1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a | |
| parent 4ccb6d7b8869a86aae2e84c56523f8705b50c647 | |
| author Maya <maya@example.com> 1759173425 -0400 | |
| committer Maya <maya@example.com> 1759173425 -0400 | |
| Add README</pre> | |
| </div> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Like all other objects, commits can never be changed after they’re created. | |
| For example, "amending" a commit with <code>git</code> <code>commit</code> <code>--amend</code> creates a new | |
| commit with the same parent.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Git does not store the diff for a commit: when you ask Git to show | |
| the commit with <a href="git-show.html">git-show(1)</a>, it calculates the diff from its | |
| parent on the fly.</p> | |
| </div> | |
| </li> | |
| </ol> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="tree" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">tree</dt> | |
| <dd> | |
| <p>A tree is how Git represents a directory. | |
| It can contain files or other trees (which are subdirectories). | |
| It lists, for each item in the tree:</p> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>The <strong>filename</strong>, for example <code>hello.py</code></p> | |
| </li> | |
| <li> | |
| <p>The <strong>file type</strong>, which must be one of these five types:</p> | |
| <div class="ulist"> | |
| <ul> | |
| <li> | |
| <p><strong>regular file</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>executable file</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>symbolic link</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>directory</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>gitlink</strong> (for use with submodules)</p> | |
| </li> | |
| </ul> | |
| </div> | |
| </li> | |
| <li> | |
| <p>The <a href="#object-id"><strong>object ID</strong></a> with the contents of the file, directory, | |
| or gitlink.</p> | |
| <div class="paragraph"> | |
| <p>For example, this is how a tree containing one directory (<code>src</code>) and one file | |
| (<code>README.md</code>) is stored:</p> | |
| </div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre>100644 blob 8728a858d9d21a8c78488c8b4e70e531b659141f README.md | |
| 040000 tree 89b1d2e0495f66d6929f4ff76ff1bb07fc41947d src</pre> | |
| </div> | |
| </div> | |
| </li> | |
| </ol> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div class="admonitionblock note"> | |
| <table> | |
| <tr> | |
| <td class="icon"> | |
| <div class="title">Note</div> | |
| </td> | |
| <td class="content"> | |
| In the output above, Git displays the file type of each tree entry | |
| using a format that’s loosely modelled on Unix file modes (<code>100644</code> is | |
| "regular file", <code>100755</code> is "executable file", <code>120000</code> is "symbolic | |
| link", <code>040000</code> is "directory", and <code>160000</code> is "gitlink"). It also | |
| displays the object’s type: <code>blob</code> for files and symlinks, <code>tree</code> for | |
| directories, and <code>commit</code> for gitlinks. | |
| </td> | |
| </tr> | |
| </table> | |
| </div> | |
| <div id="blob" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">blob</dt> | |
| <dd> | |
| <p>A blob object contains a file’s contents.</p> | |
| <div class="paragraph"> | |
| <p>When you make a commit, Git stores the full contents of each file that | |
| you changed as a blob. | |
| For example, if you have a commit that changes 2 files in a repository | |
| with 1000 files, that commit will create 2 new blobs, and use the | |
| previous blob ID for the other 998 files. | |
| This means that commits can use relatively little disk space even in a | |
| very large repository.</p> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="tag-object" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">tag object</dt> | |
| <dd> | |
| <p>Tag objects contain these required fields | |
| (though there are other optional fields):</p> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>The <strong>ID</strong> of the object it references</p> | |
| </li> | |
| <li> | |
| <p>The <strong>type</strong> of the object it references</p> | |
| </li> | |
| <li> | |
| <p>The <strong>tagger</strong> and tag date</p> | |
| </li> | |
| <li> | |
| <p>A <strong>tag message</strong>, similar to a commit message</p> | |
| </li> | |
| </ol> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Here’s how an example tag object is stored:</p> | |
| </div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre>object 750b4ead9c87ceb3ddb7a390e6c7074521797fb3 | |
| type commit | |
| tag v1.0.0 | |
| tagger Maya <maya@example.com> 1759927359 -0400 | |
| Release version 1.0.0</pre> | |
| </div> | |
| </div> | |
| <div class="admonitionblock note"> | |
| <table> | |
| <tr> | |
| <td class="icon"> | |
| <div class="title">Note</div> | |
| </td> | |
| <td class="content"> | |
| All of the examples in this section were generated with | |
| <code>git</code> <code>cat-file</code> <code>-p</code> <em><object-id></em>. | |
| </td> | |
| </tr> | |
| </table> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="references">REFERENCES</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>References are a way to give a name to a commit. | |
| It’s easier to remember "the changes I’m working on are on the <code>turtle</code> | |
| branch" than "the changes are in commit bb69721404348e". | |
| Git often uses "ref" as shorthand for "reference".</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>References can either refer to:</p> | |
| </div> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>An object ID, usually a <a href="#commit">commit</a> ID</p> | |
| </li> | |
| <li> | |
| <p>Another reference. This is called a "symbolic reference"</p> | |
| </li> | |
| </ol> | |
| </div> | |
| <div class="paragraph"> | |
| <p>References are stored in a hierarchy, and Git handles references | |
| differently based on where they are in the hierarchy. | |
| Most references are under <code>refs/</code>. Here are the main types:</p> | |
| </div> | |
| <div id="branch" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">branches: <code>refs/heads/</code><em><name></em></dt> | |
| <dd> | |
| <p>A branch refers to a commit ID. | |
| That commit is the latest commit on the branch.</p> | |
| <div class="paragraph"> | |
| <p>To get the history of commits on a branch, Git will start at the commit | |
| ID the branch references, and then look at the commit’s parent(s), | |
| the parent’s parent, etc.</p> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="tag" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">tags: <code>refs/tags/</code><em><name></em></dt> | |
| <dd> | |
| <p>A tag refers to a commit ID, tag object ID, or other object ID. | |
| There are two types of tags:</p> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>"Annotated tags", which reference a <a href="#tag-object">tag object</a> ID | |
| which contains a tag message</p> | |
| </li> | |
| <li> | |
| <p>"Lightweight tags", which reference a commit, blob, or tree ID | |
| directly</p> | |
| <div class="paragraph"> | |
| <p>Even though branches and tags both refer to a commit ID, Git | |
| treats them very differently. | |
| Branches are expected to change over time: when you make a commit, Git | |
| will update your <a href="#HEAD">current branch</a> to point to the new commit. | |
| Tags are usually not changed after they’re created.</p> | |
| </div> | |
| </li> | |
| </ol> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="HEAD" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">HEAD: <code>HEAD</code></dt> | |
| <dd> | |
| <p><code>HEAD</code> is where Git stores your current <a href="#branch">branch</a>, | |
| if there is a current branch. <code>HEAD</code> can either be:</p> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>A symbolic reference to your current branch, for example <code>ref:</code> | |
| <code>refs/heads/main</code> if your current branch is <code>main</code>.</p> | |
| </li> | |
| <li> | |
| <p>A direct reference to a commit ID. In this case there is no current branch. | |
| This is called "detached HEAD state", see the DETACHED HEAD section | |
| of <a href="git-checkout.html">git-checkout(1)</a> for more.</p> | |
| </li> | |
| </ol> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="remote-tracking-branch" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">remote-tracking branches: <code>refs/remotes/</code><em><remote></em><code>/</code><em><branch></em></dt> | |
| <dd> | |
| <p>A remote-tracking branch refers to a commit ID. | |
| It’s how Git stores the last-known state of a branch in a remote | |
| repository. <code>git</code> <code>fetch</code> updates remote-tracking branches. When | |
| <code>git</code> <code>status</code> says "you’re up to date with origin/main", it’s looking at | |
| this.</p> | |
| <div class="paragraph"> | |
| <p><code>refs/remotes/</code><em><remote></em><code>/HEAD</code> is a symbolic reference to the remote’s | |
| default branch. This is the branch that <code>git</code> <code>clone</code> checks out by default.</p> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div id="other-refs" class="dlist"> | |
| <dl> | |
| <dt class="hdlist1">Other references</dt> | |
| <dd> | |
| <p>Git tools may create references anywhere under <code>refs/</code>. | |
| For example, <a href="git-stash.html">git-stash(1)</a>, <a href="git-bisect.html">git-bisect(1)</a>, | |
| and <a href="git-notes.html">git-notes(1)</a> all create their own references | |
| in <code>refs/stash</code>, <code>refs/bisect</code>, etc. | |
| Third-party Git tools may also create their own references.</p> | |
| <div class="paragraph"> | |
| <p>Git may also create references other than <code>HEAD</code> at the base of the | |
| hierarchy, like <code>ORIG_HEAD</code>.</p> | |
| </div> | |
| </dd> | |
| </dl> | |
| </div> | |
| <div class="admonitionblock note"> | |
| <table> | |
| <tr> | |
| <td class="icon"> | |
| <div class="title">Note</div> | |
| </td> | |
| <td class="content"> | |
| Git may delete objects that aren’t "reachable" from any reference | |
| or <a href="#reflogs">reflog</a>. | |
| An object is "reachable" if we can find it by following tags to whatever | |
| they tag, commits to their parents or trees, and trees to the trees or | |
| blobs that they contain. | |
| For example, if you amend a commit with <code>git</code> <code>commit</code> <code>--amend</code>, | |
| there will no longer be a branch that points at the old commit. | |
| The old commit is recorded in the current branch’s <a href="#reflogs">reflog</a>, | |
| so it is still "reachable", but when the reflog entry expires it may | |
| become unreachable and get deleted. | |
| Reachable objects will never be deleted. | |
| </td> | |
| </tr> | |
| </table> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="index">THE INDEX</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>The index, also known as the "staging area", is a list of files and | |
| the contents of each file, stored as a <a href="#blob">blob</a>. | |
| You can add files to the index or update the contents of a file in the | |
| index with <a href="git-add.html">git-add(1)</a>. This is called "staging" the file for commit.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Unlike a <a href="#tree">tree</a>, the index is a flat list of files. | |
| When you commit, Git converts the list of files in the index to a | |
| directory <a href="#tree">tree</a> and uses that tree in the new <a href="#commit">commit</a>.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Each index entry has 4 fields:</p> | |
| </div> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>The <strong>file type</strong>, which must be one of:</p> | |
| <div class="ulist"> | |
| <ul> | |
| <li> | |
| <p><strong>regular file</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>executable file</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>symbolic link</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>gitlink</strong> (for use with submodules)</p> | |
| </li> | |
| </ul> | |
| </div> | |
| </li> | |
| <li> | |
| <p>The <strong><a href="#blob">blob</a></strong> ID of the file, | |
| or (rarely) the <strong><a href="#commit">commit</a></strong> ID of the submodule</p> | |
| </li> | |
| <li> | |
| <p>The <strong>stage number</strong>, either 0, 1, 2, or 3. This is normally 0, but if | |
| there’s a merge conflict there can be multiple versions of the same | |
| filename in the index.</p> | |
| </li> | |
| <li> | |
| <p>The <strong>file path</strong>, for example <code>src/hello.py</code></p> | |
| </li> | |
| </ol> | |
| </div> | |
| <div class="paragraph"> | |
| <p>It’s extremely uncommon to look at the index directly: normally you’d | |
| run <code>git</code> <code>status</code> to see a list of changes between the index and <a href="#HEAD">HEAD</a>. | |
| But you can use <code>git</code> <code>ls-files</code> <code>--stage</code> to see the index. | |
| Here’s the output of <code>git</code> <code>ls-files</code> <code>--stage</code> in a repository with 2 files:</p> | |
| </div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre>100644 8728a858d9d21a8c78488c8b4e70e531b659141f 0 README.md | |
| 100644 665c637a360874ce43bf74018768a96d2d4d219a 0 src/hello.py</pre> | |
| </div> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="reflogs">REFLOGS</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>Every time a branch, remote-tracking branch, or HEAD is updated, Git | |
| updates a log called a "reflog" for that <a href="#references">reference</a>. | |
| This means that if you make a mistake and "lose" a commit, you can | |
| generally recover the commit ID by running <code>git</code> <code>reflog</code> <em><reference></em>.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>A reflog is a list of log entries. Each entry has:</p> | |
| </div> | |
| <div class="olist arabic"> | |
| <ol class="arabic"> | |
| <li> | |
| <p>The <strong>commit ID</strong></p> | |
| </li> | |
| <li> | |
| <p><strong>Timestamp</strong> when the change was made</p> | |
| </li> | |
| <li> | |
| <p><strong>Log message</strong>, for example <code>pull:</code> <code>Fast-forward</code></p> | |
| </li> | |
| </ol> | |
| </div> | |
| <div class="paragraph"> | |
| <p>Reflogs only log changes made in your local repository. | |
| They are not shared with remotes.</p> | |
| </div> | |
| <div class="paragraph"> | |
| <p>You can view a reflog with <code>git</code> <code>reflog</code> <em><reference></em>. | |
| For example, here’s the reflog for a <code>main</code> branch which has changed twice:</p> | |
| </div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre>$ git reflog main --date=iso --no-decorate | |
| 750b4ea main@{2025-09-29 15:17:05 -0400}: commit: Add README | |
| 4ccb6d7 main@{2025-09-29 15:16:48 -0400}: commit (initial): Initial commit</pre> | |
| </div> | |
| </div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_git">GIT</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"> | |
| <p>Part of the <a href="git.html">git(1)</a> suite</p> | |
| </div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footer"> | |
| <div id="footer-text"> | |
| Last updated 2025-12-14 03:08:20 -0800 | |
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