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NAME | SYNOPSIS | DESCRIPTION | OBJECTS | REFERENCES | THE INDEX | REFLOGS | GIT | COLOPHON |
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GITDATAMODEL(7) Git Manual GITDATAMODEL(7)
gitdatamodel - Git's core data model
gitdatamodel
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".
Git’s core operations use 4 kinds of data:
1. Objects: commits, trees, blobs, and tag objects
2. References: branches, tags, remote-tracking branches, etc
3. The index, also known as the staging area
4. Reflogs: logs of changes to references ("ref log")
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
1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a.
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.
Every object has:
1. an ID (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
1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a.
2. a type. There are 4 types of objects: commits, trees, blobs,
and tag objects.
3. contents. The structure of the contents depends on the type.
Here’s how each type of object is structured:
commit
A commit contains these required fields (though there are
other optional fields):
1. The full directory structure of all the files in that
version of the repository and each file’s contents, stored
as the tree ID of the commit’s top-level directory
2. Its parent commit ID(s). The first commit in a repository
has 0 parents, regular commits have 1 parent, merge
commits have 2 or more parents
3. An author and the time the commit was authored
4. A committer and the time the commit was committed
5. A commit message
Here’s how an example commit is stored:
tree 1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a
parent 4ccb6d7b8869a86aae2e84c56523f8705b50c647
author Maya <maya@example.com> 1759173425 -0400
committer Maya <maya@example.com> 1759173425 -0400
Add README
Like all other objects, commits can never be changed after
they’re created. For example, "amending" a commit with git
commit --amend creates a new commit with the same parent.
Git does not store the diff for a commit: when you ask Git
to show the commit with git-show(1), it calculates the
diff from its parent on the fly.
tree
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:
1. The filename, for example hello.py
2. The file type, which must be one of these five types:
• regular file
• executable file
• symbolic link
• directory
• gitlink (for use with submodules)
3. The object ID with the contents of the file, directory, or
gitlink.
For example, this is how a tree containing one directory
(src) and one file (README.md) is stored:
100644 blob 8728a858d9d21a8c78488c8b4e70e531b659141f README.md
040000 tree 89b1d2e0495f66d6929f4ff76ff1bb07fc41947d src
Note
In the output above, Git displays the file type of each tree
entry using a format that’s loosely modelled on Unix file
modes (100644 is "regular file", 100755 is "executable file",
120000 is "symbolic link", 040000 is "directory", and 160000
is "gitlink"). It also displays the object’s type: blob for
files and symlinks, tree for directories, and commit for
gitlinks.
blob
A blob object contains a file’s contents.
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.
tag object
Tag objects contain these required fields (though there are
other optional fields):
1. The ID of the object it references
2. The type of the object it references
3. The tagger and tag date
4. A tag message, similar to a commit message
Here’s how an example tag object is stored:
object 750b4ead9c87ceb3ddb7a390e6c7074521797fb3
type commit
tag v1.0.0
tagger Maya <maya@example.com> 1759927359 -0400
Release version 1.0.0
Note
All of the examples in this section were generated with git
cat-file -p <object-id>.
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 turtle branch"
than "the changes are in commit bb69721404348e". Git often uses
"ref" as shorthand for "reference".
References can either refer to:
1. An object ID, usually a commit ID
2. Another reference. This is called a "symbolic reference"
References are stored in a hierarchy, and Git handles references
differently based on where they are in the hierarchy. Most
references are under refs/. Here are the main types:
branches: refs/heads/<name>
A branch refers to a commit ID. That commit is the latest
commit on the branch.
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.
tags: refs/tags/<name>
A tag refers to a commit ID, tag object ID, or other object
ID. There are two types of tags:
1. "Annotated tags", which reference a tag object ID which
contains a tag message
2. "Lightweight tags", which reference a commit, blob, or
tree ID directly
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 current branch to point to the new commit. Tags are
usually not changed after they’re created.
HEAD: HEAD
HEAD is where Git stores your current branch, if there is a
current branch. HEAD can either be:
1. A symbolic reference to your current branch, for example
ref: refs/heads/main if your current branch is main.
2. 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 git-checkout(1) for more.
remote-tracking branches: refs/remotes/<remote>/<branch>
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. git fetch updates remote-tracking branches. When
git status says "you’re up to date with origin/main", it’s
looking at this.
refs/remotes/<remote>/HEAD is a symbolic reference to the
remote’s default branch. This is the branch that git clone
checks out by default.
Other references
Git tools may create references anywhere under refs/. For
example, git-stash(1), git-bisect(1), and git-notes(1) all
create their own references in refs/stash, refs/bisect, etc.
Third-party Git tools may also create their own references.
Git may also create references other than HEAD at the base of
the hierarchy, like ORIG_HEAD.
Note
Git may delete objects that aren’t "reachable" from any
reference or reflog. 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 git commit
--amend, there will no longer be a branch that points at the
old commit. The old commit is recorded in the current branch’s
reflog, so it is still "reachable", but when the reflog entry
expires it may become unreachable and get deleted. Reachable
objects will never be deleted.
The index, also known as the "staging area", is a list of files
and the contents of each file, stored as a blob. You can add files
to the index or update the contents of a file in the index with
git-add(1). This is called "staging" the file for commit.
Unlike a tree, the index is a flat list of files. When you commit,
Git converts the list of files in the index to a directory tree
and uses that tree in the new commit.
Each index entry has 4 fields:
1. The file type, which must be one of:
• regular file
• executable file
• symbolic link
• gitlink (for use with submodules)
2. The blob ID of the file, or (rarely) the commit ID of the
submodule
3. The stage number, 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.
4. The file path, for example src/hello.py
It’s extremely uncommon to look at the index directly: normally
you’d run git status to see a list of changes between the index
and HEAD. But you can use git ls-files --stage to see the index.
Here’s the output of git ls-files --stage in a repository with 2
files:
100644 8728a858d9d21a8c78488c8b4e70e531b659141f 0 README.md
100644 665c637a360874ce43bf74018768a96d2d4d219a 0 src/hello.py
Every time a branch, remote-tracking branch, or HEAD is updated,
Git updates a log called a "reflog" for that reference. This means
that if you make a mistake and "lose" a commit, you can generally
recover the commit ID by running git reflog <reference>.
A reflog is a list of log entries. Each entry has:
1. The commit ID
2. Timestamp when the change was made
3. Log message, for example pull: Fast-forward
Reflogs only log changes made in your local repository. They are
not shared with remotes.
You can view a reflog with git reflog <reference>. For example,
here’s the reflog for a main branch which has changed twice:
$ 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
Part of the git(1) suite
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2026-01-16. (At that time,
the date of the most recent commit that was found in the
repository was 2026-01-15.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Git 2.53.0.rc0 2026-01-15 GITDATAMODEL(7)
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HTML rendering created 2026-01-16 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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