Git - Git References

<!DOCTYPE html> <html lang="en"> <head> <meta charset='utf-8'> <meta content='IE=edge,chrome=1' http-equiv='X-UA-Compatible'> <meta name="viewport" content="width=device-width, initial-scale=1"> <title>Git - Git References</title> <link href="/favicon.ico" rel='shortcut icon' type='image/x-icon'> <link rel="stylesheet" href="/application.min.css"> <script src="/js/modernizr.js"></script> <script src="/js/modernize.js"></script> </head> <body id="documentation"> <div class="inner"> <header> <a href="/"><img src="/images/logo@2x.png" width="110" height="46" alt="Git" /></a> <span id="tagline"></span> <script type="text/javascript"> const taglines = [ "fast-version-control", "everything-is-local", "distributed-even-if-your-workflow-isnt", "local-branching-on-the-cheap", "distributed-is-the-new-centralized" ]; var tagline = taglines[Math.floor(Math.random() * taglines.length)]; document.getElementById('tagline').innerHTML = '--' + tagline; </script> <form id="search" 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translation available in <table> <tr><td><a href="/book/az/v2/Git%e2%80%99in-Daxili-%c4%b0%c5%9fl%c9%99ri-Git-Referanslar%c4%b1">azərbaycan dili</a>,</td></tr> <tr><td><a href="/book/bg/v2/%d0%9f%d1%80%d0%b8%d0%bb%d0%be%d0%b6%d0%b5%d0%bd%d0%b8%d0%b5-B:-%d0%92%d0%b3%d1%80%d0%b0%d0%b6%d0%b4%d0%b0%d0%bd%d0%b5-%d0%bd%d0%b0-Git-%d0%b2-%d0%bf%d1%80%d0%b8%d0%bb%d0%be%d0%b6%d0%b5%d0%bd%d0%b8%d1%8f-JGit">български език</a>,</td></tr> <tr><td><a href="/book/de/v2/Git-Interna-Git-Referenzen">Deutsch</a>,</td></tr> <tr><td><a href="/book/es/v2/Los-entresijos-internos-de-Git-Referencias-Git">Español</a>,</td></tr> <tr><td><a href="/book/fr/v2/Les-tripes-de-Git-R%c3%a9f%c3%a9rences-Git">Français</a>,</td></tr> <tr><td><a href="/book/gr">Ελληνικά</a>,</td></tr> <tr><td><a href="/book/ja/v2/Git%e3%81%ae%e5%86%85%e5%81%b4-Git%e3%81%ae%e5%8f%82%e7%85%a7">日本語</a>,</td></tr> <tr><td><a href="/book/ko/v2/Git%ec%9d%98-%eb%82%b4%eb%b6%80-Git-Refs">한국어</a>,</td></tr> <tr><td><a href="/book/nl/v2/Git-Binnenwerk-Git-Referenties">Nederlands</a>,</td></tr> <tr><td><a href="/book/ru/v2/Git-%d0%b8%d0%b7%d0%bd%d1%83%d1%82%d1%80%d0%b8-%d0%a1%d1%81%d1%8b%d0%bb%d0%ba%d0%b8-%d0%b2-Git">Русский</a>,</td></tr> <tr><td><a href="/book/sl/v2/Notranjost-Gita-Reference-Git">Slovenščina</a>,</td></tr> <tr><td><a href="/book/tl/v2/Mga-Panloob-ng-GIT-Git-References">Tagalog</a>,</td></tr> <tr><td><a href="/book/uk/v2/Git-%d0%b7%d1%81%d0%b5%d1%80%d0%b5%d0%b4%d0%b8%d0%bd%d0%b8-%d0%9f%d0%be%d1%81%d0%b8%d0%bb%d0%b0%d0%bd%d0%bd%d1%8f-Git">Українська</a></td></tr> <tr><td><a href="/book/zh/v2/Git-%e5%86%85%e9%83%a8%e5%8e%9f%e7%90%86-Git-%e5%bc%95%e7%94%a8">简体中文</a>,</td></tr> </table> </p> <p> Partial translations available in <table> <tr><td><a href="/book/cs/v2/Git-Internals-Git-References">Čeština</a>,</td></tr> <tr><td><a href="/book/mk/v2/%d0%92%d0%bd%d0%b0%d1%82%d1%80%d0%b5%d1%88%d0%bd%d0%be%d1%81%d1%82%d0%b0-%d0%bd%d0%b0-Git-Git-References">Македонски</a>,</td></tr> <tr><td><a href="/book/pl/v2/Mechanizmy-wewn%c4%99trzne-w-Git-Referencje-w-Git">Polski</a>,</td></tr> <tr><td><a href="/book/sr/v2/%d0%93%d0%b8%d1%82-%d0%b8%d0%b7%d0%bd%d1%83%d1%82%d1%80%d0%b0-%d0%93%d0%b8%d1%82-%d1%80%d0%b5%d1%84%d0%b5%d1%80%d0%b5%d0%bd%d1%86%d0%b5">Српски</a>,</td></tr> <tr><td><a href="/book/uz/v2/Git-Internals-Git-References">Ўзбекча</a>,</td></tr> <tr><td><a href="/book/zh-tw/v2/Git-Internals-Git-References">繁體中文</a>,</td></tr> </table> </p> <p> Translations started for <table> <tr><td><a href="/book/be/v2/Git-Internals-Git-References">Беларуская</a>,</td></tr> <tr><td><a href="/book/fa/v2/Git-Internals-Git-References" dir="rtl">فارسی</a>,</td></tr> <tr><td><a href="/book/id/v2/Git-Internals-Git-References">Indonesian</a>,</td></tr> <tr><td><a href="/book/it/v2/Git-Internals-Git-References">Italiano</a>,</td></tr> <tr><td><a href="/book/ms/v2/Git-Internals-Git-References">Bahasa Melayu</a>,</td></tr> <tr><td><a href="/book/pt-br/v2/Funcionamento-Interno-do-Git-Refer%c3%aancias-do-Git">Português (Brasil)</a>,</td></tr> <tr><td><a href="/book/pt-pt/v2/Internos-do-Git-Git-References">Português (Portugal)</a>,</td></tr> <tr><td><a href="/book/sv/v2/Git-Internals-Git-References">Svenska</a>,</td></tr> <tr><td><a href="/book/tr/v2/Dahili-Git-%c3%96geleri-Git-Referanslar%c4%b1">Türkçe</a>.</td></tr> </table> </p> <hr class="sidebar"/> <p> The source of this book is <a href="https://github.com/progit/progit2">hosted on GitHub.</a></br> Patches, suggestions and comments are welcome. </p> </nav> </aside> <div id="content"> <div id="book-chapters"> <a class="dropdown-trigger" id="book-chapters-trigger" data-panel-id="chapters-dropdown" href="#">Chapters ▾</a> <div class='dropdown-panel' id='chapters-dropdown'> <div class='three-column'> <div class="column-left"> <ol class='book-toc'> <li class='chapter'> <h2>1. <a href="/book/en/v2/Getting-Started-About-Version-Control">Getting Started</a></h2> <ol> <li> 1.1 <a href="/book/en/v2/Getting-Started-About-Version-Control">About Version Control</a> </li> <li> 1.2 <a href="/book/en/v2/Getting-Started-A-Short-History-of-Git">A Short History of Git</a> </li> <li> 1.3 <a href="/book/en/v2/Getting-Started-What-is-Git%3F">What is Git?</a> </li> <li> 1.4 <a href="/book/en/v2/Getting-Started-The-Command-Line">The Command Line</a> </li> <li> 1.5 <a href="/book/en/v2/Getting-Started-Installing-Git">Installing Git</a> </li> <li> 1.6 <a href="/book/en/v2/Getting-Started-First-Time-Git-Setup">First-Time Git Setup</a> </li> <li> 1.7 <a href="/book/en/v2/Getting-Started-Getting-Help">Getting Help</a> </li> <li> 1.8 <a href="/book/en/v2/Getting-Started-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>2. <a href="/book/en/v2/Git-Basics-Getting-a-Git-Repository">Git Basics</a></h2> <ol> <li> 2.1 <a href="/book/en/v2/Git-Basics-Getting-a-Git-Repository">Getting a Git Repository</a> </li> <li> 2.2 <a href="/book/en/v2/Git-Basics-Recording-Changes-to-the-Repository">Recording Changes to the Repository</a> </li> <li> 2.3 <a href="/book/en/v2/Git-Basics-Viewing-the-Commit-History">Viewing the Commit History</a> </li> <li> 2.4 <a href="/book/en/v2/Git-Basics-Undoing-Things">Undoing Things</a> </li> <li> 2.5 <a href="/book/en/v2/Git-Basics-Working-with-Remotes">Working with Remotes</a> </li> <li> 2.6 <a href="/book/en/v2/Git-Basics-Tagging">Tagging</a> </li> <li> 2.7 <a href="/book/en/v2/Git-Basics-Git-Aliases">Git Aliases</a> </li> <li> 2.8 <a href="/book/en/v2/Git-Basics-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>3. <a href="/book/en/v2/Git-Branching-Branches-in-a-Nutshell">Git Branching</a></h2> <ol> <li> 3.1 <a href="/book/en/v2/Git-Branching-Branches-in-a-Nutshell">Branches in a Nutshell</a> </li> <li> 3.2 <a href="/book/en/v2/Git-Branching-Basic-Branching-and-Merging">Basic Branching and Merging</a> </li> <li> 3.3 <a href="/book/en/v2/Git-Branching-Branch-Management">Branch Management</a> </li> <li> 3.4 <a href="/book/en/v2/Git-Branching-Branching-Workflows">Branching Workflows</a> </li> <li> 3.5 <a href="/book/en/v2/Git-Branching-Remote-Branches">Remote Branches</a> </li> <li> 3.6 <a href="/book/en/v2/Git-Branching-Rebasing">Rebasing</a> </li> <li> 3.7 <a href="/book/en/v2/Git-Branching-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>4. <a href="/book/en/v2/Git-on-the-Server-The-Protocols">Git on the Server</a></h2> <ol> <li> 4.1 <a href="/book/en/v2/Git-on-the-Server-The-Protocols">The Protocols</a> </li> <li> 4.2 <a href="/book/en/v2/Git-on-the-Server-Getting-Git-on-a-Server">Getting Git on a Server</a> </li> <li> 4.3 <a href="/book/en/v2/Git-on-the-Server-Generating-Your-SSH-Public-Key">Generating Your SSH Public Key</a> </li> <li> 4.4 <a href="/book/en/v2/Git-on-the-Server-Setting-Up-the-Server">Setting Up the Server</a> </li> <li> 4.5 <a href="/book/en/v2/Git-on-the-Server-Git-Daemon">Git Daemon</a> </li> <li> 4.6 <a href="/book/en/v2/Git-on-the-Server-Smart-HTTP">Smart HTTP</a> </li> <li> 4.7 <a href="/book/en/v2/Git-on-the-Server-GitWeb">GitWeb</a> </li> <li> 4.8 <a href="/book/en/v2/Git-on-the-Server-GitLab">GitLab</a> </li> <li> 4.9 <a href="/book/en/v2/Git-on-the-Server-Third-Party-Hosted-Options">Third Party Hosted Options</a> </li> <li> 4.10 <a href="/book/en/v2/Git-on-the-Server-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>5. <a href="/book/en/v2/Distributed-Git-Distributed-Workflows">Distributed Git</a></h2> <ol> <li> 5.1 <a href="/book/en/v2/Distributed-Git-Distributed-Workflows">Distributed Workflows</a> </li> <li> 5.2 <a href="/book/en/v2/Distributed-Git-Contributing-to-a-Project">Contributing to a Project</a> </li> <li> 5.3 <a href="/book/en/v2/Distributed-Git-Maintaining-a-Project">Maintaining a Project</a> </li> <li> 5.4 <a href="/book/en/v2/Distributed-Git-Summary">Summary</a> </li> </ol> </li> </ol> </div> <div class='column-middle'> <ol class='book-toc'> <li class='chapter'> <h2>6. <a href="/book/en/v2/GitHub-Account-Setup-and-Configuration">GitHub</a></h2> <ol> <li> 6.1 <a href="/book/en/v2/GitHub-Account-Setup-and-Configuration">Account Setup and Configuration</a> </li> <li> 6.2 <a href="/book/en/v2/GitHub-Contributing-to-a-Project">Contributing to a Project</a> </li> <li> 6.3 <a href="/book/en/v2/GitHub-Maintaining-a-Project">Maintaining a Project</a> </li> <li> 6.4 <a href="/book/en/v2/GitHub-Managing-an-organization">Managing an organization</a> </li> <li> 6.5 <a href="/book/en/v2/GitHub-Scripting-GitHub">Scripting GitHub</a> </li> <li> 6.6 <a href="/book/en/v2/GitHub-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>7. <a href="/book/en/v2/Git-Tools-Revision-Selection">Git Tools</a></h2> <ol> <li> 7.1 <a href="/book/en/v2/Git-Tools-Revision-Selection">Revision Selection</a> </li> <li> 7.2 <a href="/book/en/v2/Git-Tools-Interactive-Staging">Interactive Staging</a> </li> <li> 7.3 <a href="/book/en/v2/Git-Tools-Stashing-and-Cleaning">Stashing and Cleaning</a> </li> <li> 7.4 <a href="/book/en/v2/Git-Tools-Signing-Your-Work">Signing Your Work</a> </li> <li> 7.5 <a href="/book/en/v2/Git-Tools-Searching">Searching</a> </li> <li> 7.6 <a href="/book/en/v2/Git-Tools-Rewriting-History">Rewriting History</a> </li> <li> 7.7 <a href="/book/en/v2/Git-Tools-Reset-Demystified">Reset Demystified</a> </li> <li> 7.8 <a href="/book/en/v2/Git-Tools-Advanced-Merging">Advanced Merging</a> </li> <li> 7.9 <a href="/book/en/v2/Git-Tools-Rerere">Rerere</a> </li> <li> 7.10 <a href="/book/en/v2/Git-Tools-Debugging-with-Git">Debugging with Git</a> </li> <li> 7.11 <a href="/book/en/v2/Git-Tools-Submodules">Submodules</a> </li> <li> 7.12 <a href="/book/en/v2/Git-Tools-Bundling">Bundling</a> </li> <li> 7.13 <a href="/book/en/v2/Git-Tools-Replace">Replace</a> </li> <li> 7.14 <a href="/book/en/v2/Git-Tools-Credential-Storage">Credential Storage</a> </li> <li> 7.15 <a href="/book/en/v2/Git-Tools-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>8. <a href="/book/en/v2/Customizing-Git-Git-Configuration">Customizing Git</a></h2> <ol> <li> 8.1 <a href="/book/en/v2/Customizing-Git-Git-Configuration">Git Configuration</a> </li> <li> 8.2 <a href="/book/en/v2/Customizing-Git-Git-Attributes">Git Attributes</a> </li> <li> 8.3 <a href="/book/en/v2/Customizing-Git-Git-Hooks">Git Hooks</a> </li> <li> 8.4 <a href="/book/en/v2/Customizing-Git-An-Example-Git-Enforced-Policy">An Example Git-Enforced Policy</a> </li> <li> 8.5 <a href="/book/en/v2/Customizing-Git-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>9. <a href="/book/en/v2/Git-and-Other-Systems-Git-as-a-Client">Git and Other Systems</a></h2> <ol> <li> 9.1 <a href="/book/en/v2/Git-and-Other-Systems-Git-as-a-Client">Git as a Client</a> </li> <li> 9.2 <a href="/book/en/v2/Git-and-Other-Systems-Migrating-to-Git">Migrating to Git</a> </li> <li> 9.3 <a href="/book/en/v2/Git-and-Other-Systems-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>10. <a href="/book/en/v2/Git-Internals-Plumbing-and-Porcelain">Git Internals</a></h2> <ol> <li> 10.1 <a href="/book/en/v2/Git-Internals-Plumbing-and-Porcelain">Plumbing and Porcelain</a> </li> <li> 10.2 <a href="/book/en/v2/Git-Internals-Git-Objects">Git Objects</a> </li> <li> 10.3 <a href="/book/en/v2/Git-Internals-Git-References" class="active">Git References</a> </li> <li> 10.4 <a href="/book/en/v2/Git-Internals-Packfiles">Packfiles</a> </li> <li> 10.5 <a href="/book/en/v2/Git-Internals-The-Refspec">The Refspec</a> </li> <li> 10.6 <a href="/book/en/v2/Git-Internals-Transfer-Protocols">Transfer Protocols</a> </li> <li> 10.7 <a href="/book/en/v2/Git-Internals-Maintenance-and-Data-Recovery">Maintenance and Data Recovery</a> </li> <li> 10.8 <a href="/book/en/v2/Git-Internals-Environment-Variables">Environment Variables</a> </li> <li> 10.9 <a href="/book/en/v2/Git-Internals-Summary">Summary</a> </li> </ol> </li> </ol> </div> <div class='column-right'> <ol class='book-toc'> <li class='chapter'> <h2>A1. <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Graphical-Interfaces">Appendix A: Git in Other Environments</a></h2> <ol> <li> A1.1 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Graphical-Interfaces">Graphical Interfaces</a> </li> <li> A1.2 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-Visual-Studio">Git in Visual Studio</a> </li> <li> A1.3 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-Visual-Studio-Code">Git in Visual Studio Code</a> </li> <li> A1.4 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-IntelliJ-/-PyCharm-/-WebStorm-/-PhpStorm-/-RubyMine">Git in IntelliJ / PyCharm / WebStorm / PhpStorm / RubyMine</a> </li> <li> A1.5 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-Sublime-Text">Git in Sublime Text</a> </li> <li> A1.6 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-Bash">Git in Bash</a> </li> <li> A1.7 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-Zsh">Git in Zsh</a> </li> <li> A1.8 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Git-in-PowerShell">Git in PowerShell</a> </li> <li> A1.9 <a href="/book/en/v2/Appendix-A:-Git-in-Other-Environments-Summary">Summary</a> </li> </ol> </li> <li class='chapter'> <h2>A2. <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-Command-line-Git">Appendix B: Embedding Git in your Applications</a></h2> <ol> <li> A2.1 <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-Command-line-Git">Command-line Git</a> </li> <li> A2.2 <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-Libgit2">Libgit2</a> </li> <li> A2.3 <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-JGit">JGit</a> </li> <li> A2.4 <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-go-git">go-git</a> </li> <li> A2.5 <a href="/book/en/v2/Appendix-B:-Embedding-Git-in-your-Applications-Dulwich">Dulwich</a> </li> </ol> </li> <li class='chapter'> <h2>A3. <a href="/book/en/v2/Appendix-C:-Git-Commands-Setup-and-Config">Appendix C: Git Commands</a></h2> <ol> <li> A3.1 <a href="/book/en/v2/Appendix-C:-Git-Commands-Setup-and-Config">Setup and Config</a> </li> <li> A3.2 <a href="/book/en/v2/Appendix-C:-Git-Commands-Getting-and-Creating-Projects">Getting and Creating Projects</a> </li> <li> A3.3 <a href="/book/en/v2/Appendix-C:-Git-Commands-Basic-Snapshotting">Basic Snapshotting</a> </li> <li> A3.4 <a href="/book/en/v2/Appendix-C:-Git-Commands-Branching-and-Merging">Branching and Merging</a> </li> <li> A3.5 <a href="/book/en/v2/Appendix-C:-Git-Commands-Sharing-and-Updating-Projects">Sharing and Updating Projects</a> </li> <li> A3.6 <a href="/book/en/v2/Appendix-C:-Git-Commands-Inspection-and-Comparison">Inspection and Comparison</a> </li> <li> A3.7 <a href="/book/en/v2/Appendix-C:-Git-Commands-Debugging">Debugging</a> </li> <li> A3.8 <a href="/book/en/v2/Appendix-C:-Git-Commands-Patching">Patching</a> </li> <li> A3.9 <a href="/book/en/v2/Appendix-C:-Git-Commands-Email">Email</a> </li> <li> A3.10 <a href="/book/en/v2/Appendix-C:-Git-Commands-External-Systems">External Systems</a> </li> <li> A3.11 <a href="/book/en/v2/Appendix-C:-Git-Commands-Administration">Administration</a> </li> <li> A3.12 <a href="/book/en/v2/Appendix-C:-Git-Commands-Plumbing-Commands">Plumbing Commands</a> </li> </ol> </li> </ol> </div> </div> </div> <span class="light" id="edition"> 2nd Edition </span> </div> <div id="main" data-pagefind-filter="category:book" data-pagefind-meta="category:Book" data-pagefind-weight="0.05" data-pagefind-body class="book edition2"> <h1>10.3 Git Internals - Git References</h1> <div> <h2 id="_git_refs">Git References</h2> <div class="paragraph"> <p>If you were interested in seeing the history of your repository reachable from commit, say, <code>1a410e</code>, you could run something like <code>git log 1a410e</code> to display that history, but you would still have to remember that <code>1a410e</code> is the commit you want to use as the starting point for that history. Instead, it would be easier if you had a file in which you could store that SHA-1 value under a simple name so you could use that simple name rather than the raw SHA-1 value.</p> </div> <div class="paragraph"> <p>In Git, these simple names are called “references” or “refs”; you can find the files that contain those SHA-1 values in the <code>.git/refs</code> directory. In the current project, this directory contains no files, but it does contain a simple structure:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ find .git/refs .git/refs .git/refs/heads .git/refs/tags $ find .git/refs -type f</code></pre> </div> </div> <div class="paragraph"> <p>To create a new reference that will help you remember where your latest commit is, you can technically do something as simple as this:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ echo 1a410efbd13591db07496601ebc7a059dd55cfe9 > .git/refs/heads/master</code></pre> </div> </div> <div class="paragraph"> <p>Now, you can use the head reference you just created instead of the SHA-1 value in your Git commands:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git log --pretty=oneline master 1a410efbd13591db07496601ebc7a059dd55cfe9 Third commit cac0cab538b970a37ea1e769cbbde608743bc96d Second commit fdf4fc3344e67ab068f836878b6c4951e3b15f3d First commit</code></pre> </div> </div> <div class="paragraph"> <p>You aren’t encouraged to directly edit the reference files; instead, Git provides the safer command <code>git update-ref</code> to do this if you want to update a reference:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git update-ref refs/heads/master 1a410efbd13591db07496601ebc7a059dd55cfe9</code></pre> </div> </div> <div class="paragraph"> <p>That’s basically what a branch in Git is: a simple pointer or reference to the head of a line of work. To create a branch back at the second commit, you can do this:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git update-ref refs/heads/test cac0ca</code></pre> </div> </div> <div class="paragraph"> <p>Your branch will contain only work from that commit down:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git log --pretty=oneline test cac0cab538b970a37ea1e769cbbde608743bc96d Second commit fdf4fc3344e67ab068f836878b6c4951e3b15f3d First commit</code></pre> </div> </div> <div class="paragraph"> <p>Now, your Git database conceptually looks something like this:</p> </div> <div class="imageblock"> <div class="content"> <img src="/book/en/v2/images/data-model-4.png" alt="Git directory objects with branch head references included"> </div> <div class="title">Figure 176. Git directory objects with branch head references included</div> </div> <div class="paragraph"> <p>When you run commands like <code>git branch <branch></code>, Git basically runs that <code>update-ref</code> command to add the SHA-1 of the last commit of the branch you’re on into whatever new reference you want to create.</p> </div> <div class="sect3"> <h3 id="ref_the_ref">The HEAD</h3> <div class="paragraph"> <p>The question now is, when you run <code>git branch <branch></code>, how does Git know the SHA-1 of the last commit? The answer is the HEAD file.</p> </div> <div class="paragraph"> <p>Usually the HEAD file is a symbolic reference to the branch you’re currently on. By symbolic reference, we mean that unlike a normal reference, it contains a pointer to another reference.</p> </div> <div class="paragraph"> <p>However in some rare cases the HEAD file may contain the SHA-1 value of a Git object. This happens when you checkout a tag, commit, or remote branch, which puts your repository in <a href="https://git-scm.com/docs/git-checkout#_detached_head" target="_blank" rel="noopener">"detached HEAD"</a> state.</p> </div> <div class="paragraph"> <p>If you look at the file, you’ll normally see something like this:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ cat .git/HEAD ref: refs/heads/master</code></pre> </div> </div> <div class="paragraph"> <p>If you run <code>git checkout test</code>, Git updates the file to look like this:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ cat .git/HEAD ref: refs/heads/test</code></pre> </div> </div> <div class="paragraph"> <p>When you run <code>git commit</code>, it creates the commit object, specifying the parent of that commit object to be whatever SHA-1 value the reference in HEAD points to.</p> </div> <div class="paragraph"> <p>You can also manually edit this file, but again a safer command exists to do so: <code>git symbolic-ref</code>. You can read the value of your HEAD via this command:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git symbolic-ref HEAD refs/heads/master</code></pre> </div> </div> <div class="paragraph"> <p>You can also set the value of HEAD using the same command:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git symbolic-ref HEAD refs/heads/test $ cat .git/HEAD ref: refs/heads/test</code></pre> </div> </div> <div class="paragraph"> <p>You can’t set a symbolic reference outside of the refs style:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git symbolic-ref HEAD test fatal: Refusing to point HEAD outside of refs/</code></pre> </div> </div> </div> <div class="sect3"> <h3 id="_tags">Tags</h3> <div class="paragraph"> <p>We just finished discussing Git’s three main object types (<em>blobs</em>, <em>trees</em> and <em>commits</em>), but there is a fourth. The <em>tag</em> object is very much like a commit object — it contains a tagger, a date, a message, and a pointer. The main difference is that a tag object generally points to a commit rather than a tree. It’s like a branch reference, but it never moves — it always points to the same commit but gives it a friendlier name.</p> </div> <div class="paragraph"> <p>As discussed in <a href="/book/en/v2/ch00/ch02-git-basics-chapter">Git Basics</a>, there are two types of tags: annotated and lightweight. You can make a lightweight tag by running something like this:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git update-ref refs/tags/v1.0 cac0cab538b970a37ea1e769cbbde608743bc96d</code></pre> </div> </div> <div class="paragraph"> <p>That is all a lightweight tag is — a reference that never moves. An annotated tag is more complex, however. If you create an annotated tag, Git creates a tag object and then writes a reference to point to it rather than directly to the commit. You can see this by creating an annotated tag (using the <code>-a</code> option):</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git tag -a v1.1 1a410efbd13591db07496601ebc7a059dd55cfe9 -m 'Test tag'</code></pre> </div> </div> <div class="paragraph"> <p>Here’s the object SHA-1 value it created:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ cat .git/refs/tags/v1.1 9585191f37f7b0fb9444f35a9bf50de191beadc2</code></pre> </div> </div> <div class="paragraph"> <p>Now, run <code>git cat-file -p</code> on that SHA-1 value:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git cat-file -p 9585191f37f7b0fb9444f35a9bf50de191beadc2 object 1a410efbd13591db07496601ebc7a059dd55cfe9 type commit tag v1.1 tagger Scott Chacon <schacon@gmail.com> Sat May 23 16:48:58 2009 -0700 Test tag</code></pre> </div> </div> <div class="paragraph"> <p>Notice that the object entry points to the commit SHA-1 value that you tagged. Also notice that it doesn’t need to point to a commit; you can tag any Git object. In the Git source code, for example, the maintainer has added their GPG public key as a blob object and then tagged it. You can view the public key by running this in a clone of the Git repository:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git cat-file blob junio-gpg-pub</code></pre> </div> </div> <div class="paragraph"> <p>The Linux kernel repository also has a non-commit-pointing tag object — the first tag created points to the initial tree of the import of the source code.</p> </div> </div> <div class="sect3"> <h3 id="_remotes">Remotes</h3> <div class="paragraph"> <p>The third type of reference that you’ll see is a remote reference. If you add a remote and push to it, Git stores the value you last pushed to that remote for each branch in the <code>refs/remotes</code> directory. For instance, you can add a remote called <code>origin</code> and push your <code>master</code> branch to it:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ git remote add origin git@github.com:schacon/simplegit-progit.git $ git push origin master Counting objects: 11, done. Compressing objects: 100% (5/5), done. Writing objects: 100% (7/7), 716 bytes, done. Total 7 (delta 2), reused 4 (delta 1) To git@github.com:schacon/simplegit-progit.git a11bef0..ca82a6d master -> master</code></pre> </div> </div> <div class="paragraph"> <p>Then, you can see what the <code>master</code> branch on the <code>origin</code> remote was the last time you communicated with the server, by checking the <code>refs/remotes/origin/master</code> file:</p> </div> <div class="listingblock"> <div class="content"> <pre class="highlight"><code class="language-console" data-lang="console">$ cat .git/refs/remotes/origin/master ca82a6dff817ec66f44342007202690a93763949</code></pre> </div> </div> <div class="paragraph"> <p>Remote references differ from branches (<code>refs/heads</code> references) mainly in that they’re considered read-only. You can <code>git checkout</code> to one, but Git won’t symbolically reference HEAD to one, so you’ll never update it with a <code>commit</code> command. Git manages them as bookmarks to the last known state of where those branches were on those servers.</p> </div> </div> <div id="nav"><a href="/book/en/v2/Git-Internals-Git-Objects">prev</a> | <a href="/book/en/v2/Git-Internals-Packfiles">next</a></div> </div> </div> </div> </div> <footer> <div class="site-source"> <a href="/site">About this site</a><br> Patches, suggestions, and comments are welcome. </div> <div class="sfc-member"> Git is a member of <a href="/sfc">Software Freedom Conservancy</a> </div> </footer> <a href="#top" class="no-js scrollToTop" id="scrollToTop" data-label="Scroll to top"> <img src="/images/icons/chevron-up@2x.png" width="20" height="20" alt="scroll-to-top"/> </a> <script src="/js/jquery-1.7.1.min.js"></script> <script src="/js/jquery-ui-1.8.18.custom.min.js"></script> <script src="/js/jquery.defaultvalue.js"></script> <script src="/js/session.min.js"></script> <script src="/js/application.min.js"></script> </div> </body> </html>

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