{"id":30850,"date":"2022-07-24T14:44:57","date_gmt":"2022-07-24T14:44:57","guid":{"rendered":"https:\/\/www.devopsschool.com\/blog\/?p=30850"},"modified":"2022-12-23T05:48:01","modified_gmt":"2022-12-23T05:48:01","slug":"git-tutorial-objects-references-the-index","status":"publish","type":"post","link":"https:\/\/www.devopsschool.com\/blog\/git-tutorial-objects-references-the-index\/","title":{"rendered":"Git Tutorial: Objects, References, The Index"},"content":{"rendered":"\n

To understand the core of Git internals, there are 3 things to we should know: objects, references, the index.<\/p>\n\n\n\n

I find this model is elegant. It fits well in a small diagram, as well as in my head.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Objects<\/strong><\/h1>\n\n\n\n
\n\n\n\n

All files that you commited into a Git repository, including the commit info are stored as objects<\/strong> in .git\/objects\/.<\/p>\n\n\n\n

An object is identified by a 40-character-long string \u2013 SHA1 hash of the object\u2019s content.<\/p>\n\n\n\n

There are 4 types<\/strong> of objects:<\/p>\n\n\n\n

  1. blob – stores file content.<\/li>
  2. tree – stores direcotry layouts and filenames.<\/li>
  3. commit – stores commit info and forms the Git commit graph.<\/li>
  4. tag – stores annotated tag.<\/li><\/ol>\n\n\n\n

    The example will illustrate how these objects relate to each others.<\/p>\n\n\n\n

    References<\/strong><\/h1>\n\n\n\n
    \n\n\n\n

    A branch, remote branch or a tag (also called lightweight tag) in Git, is just a pointer to an object<\/strong>, usually a commit object.<\/p>\n\n\n\n

    They are stored as plain text files in .git\/refs\/.<\/p>\n\n\n\n

    Symbolic References<\/h2>\n\n\n\n
    \n\n\n\n

    Git has a special kind of reference, called symbolic reference. It doesn\u2019t point to an object directly. Instead, it points to another reference<\/strong>.<\/p>\n\n\n\n

    For instance, .git\/HEAD is a symbolic reference. It points to the current branch you are working on.<\/p>\n\n\n\n

    The Index<\/strong><\/h1>\n\n\n\n
    \n\n\n\n

    The index is a staging area, stored as a binary file in .git\/index.<\/p>\n\n\n\n

    When git add a file, Git adds the file info to the index. When git commit, Git only commits what\u2019s listed in the index.<\/p>\n\n\n\n

    Examples<\/h1>\n\n\n\n

    Let’s walkthrough a simple example, to create a Git repository, commit some files and see what happened behind the scene in .git directory.<\/p>\n\n\n\n

    Initialize New Repository<\/h2>\n\n\n\n
    \n\n\n\n
    1<\/td>$ git init canai<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    \"\"<\/figure>\n\n\n\n

    What happened:<\/p>\n\n\n\n

    • Empty .git\/objects\/ and .git\/refs\/ created.<\/li>
    • No index file yet.<\/li>
    • HEAD symbolic reference created. $ cat .git\/HEAD ref: refs\/heads\/master<\/li><\/ul>\n\n\n\n

      Add New File<\/h2>\n\n\n\n
      \n\n\n\n
      1 2<\/td>$ echo “A roti canai project.” >> README $ git add README<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
      \"\"<\/figure>\n\n\n\n

      What happened:<\/p>\n\n\n\n

      • Index file created.
        It has a SHA1 hash that points to a blob object.<\/li>
      • Blob object created.
        The content of README file is stored in this blob.<\/li><\/ul>\n\n\n\n

        First Commit<\/h2>\n\n\n\n
        \n\n\n\n
        1 2 3 4<\/td>$ git commit -m’first commit’ [master (root-commit) d9976cf] first commit 1 files changed, 1 insertions(+), 0 deletions(-) create mode 100644 README<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
        \"\"<\/figure>\n\n\n\n

        What happened:<\/p>\n\n\n\n

        • Branch ‘master’ reference created.
          It points to the lastest commit object in ‘master’ branch.<\/li>
        • First commit object created. It points to the root tree object.<\/li>
        • Tree object created.
          This tree represents the ‘canai’ directory.<\/li><\/ul>\n\n\n\n

          Add Modified File<\/h2>\n\n\n\n
          \n\n\n\n
          1 2<\/td>$ echo “Welcome everyone.” >> README $ git add README<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
          \"\"<\/figure>\n\n\n\n

          What happened:<\/p>\n\n\n\n

          • Index file updated.
            Notice it points to a new blob?<\/li>
          • Blob object created.
            The entire README content is stored as a new blob.<\/li><\/ul>\n\n\n\n

            Add File into Subdirectory<\/h2>\n\n\n\n
            \n\n\n\n
            1 2 3<\/td>$ mkdir doc $ echo “[[TBD]] manual toc” >> doc\/manual.txt $ git add doc<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
            \"\"<\/figure>\n\n\n\n

            What happened:<\/p>\n\n\n\n

            • Index file updated.<\/li>
            • Blob object created.<\/li><\/ul>\n\n\n\n

              Second Commit<\/h2>\n\n\n\n
              \n\n\n\n
              1 2 3 4<\/td>$ git commit -m’second commit’ [master 556eaf3] second commit 2 files changed, 2 insertions(+), 0 deletions(-) create mode 100644 doc\/manual.txt<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
              \"\"<\/figure>\n\n\n\n

              What happened:<\/p>\n\n\n\n

              • Branch ‘master’ reference updated.
                It points to a lastest commit in this branch.<\/li>
              • Second commit object created. Notice its \u2018parent\u2019 points to the first commit object. This forms a commit graph.<\/li>
              • New root tree object created.<\/li>
              • New subdir tree object created.<\/li><\/ul>\n\n\n\n

                Add Annotated Tag<\/h2>\n\n\n\n
                \n\n\n\n
                1<\/td>$ git tag -a -m’this is annotated tag’ v0.1 d9976<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
                \"\"
                What happened:
                Tag reference created.
                It points to a commit object.
                1 2
                $ cat .git\/refs\/tags\/root-commit d9976cfe0430557885d162927dd70186d0f521e8<\/figcaption><\/figure>\n","protected":false},"excerpt":{"rendered":"

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