Types of git objects

If you have read Curious git, you know that git stores different types of objects in .git/objects. The object types are:

  • commit;
  • tree;
  • blob;
  • annotated tag.

Here we make examples of each of these object types in a new repository.

First we make the working tree and initialize the repository:

$ mkdir example_repo
$ cd example_repo
$ git init
Initialized empty Git repository in /Users/mb312/dev_trees/curious-git/working/example_repo/.git/

Next we make an example commit:

$ echo "An example file" > example_file.txt
$ git add example_file.txt
$ git commit -m "An example commit"
[master (root-commit) cf1fdb2] An example commit
 1 file changed, 1 insertion(+)
 create mode 100644 example_file.txt

From Curious git, we expect there will now be three objects in the directory .git/objects, one storing the backup of example_file.txt, one storing the directory listing for the commit, and one storing the commit message:

objects
├── 2f
│   └── 781156939ad540b2434d012446154321e41e03 [32B]
├── 83
│   └── 207f0274383b4a79ff6d6c297e95204ba961bc [60B]
├── cf
│   └── 1fdb215f948795523cde2987107944d1374777 [135B]
├── info
└── pack

Commit object type

The commit object contains the directory tree object hash, parent commit hash, author, committer, date and message.

Git log will show us the hash for the commit message:

$ git log
commit cf1fdb215f948795523cde2987107944d1374777
Author: Matthew Brett <matthew.brett@gmail.com>
Date:   Mon Feb 13 16:56:24 2017 +0000

    An example commit

Note

I’ll use git cat-file to show the contents of the hashed files in .git/objects, but cat-file is a relatively obscure git command that you will probably not need in your daily git work.

git cat-file -t shows us the type of the object represented by a particular hash:

$ git cat-file -t cf1fdb215f948795523cde2987107944d1374777
commit

git cat-file -p shows the contents of the file associated with this hash:

$ git cat-file -p cf1fdb215f948795523cde2987107944d1374777
tree 83207f0274383b4a79ff6d6c297e95204ba961bc
author Matthew Brett <matthew.brett@gmail.com> 1487004984 +0000
committer Matthew Brett <matthew.brett@gmail.com> 1487004984 +0000

An example commit

Tree object type

The commit contents gave us the hash of the directory listing for the commit. If we inspect this object, we find it is of type “tree” and contains the directory listing for the commit:

$ git cat-file -t 83207f0274383b4a79ff6d6c297e95204ba961bc
tree
$ git cat-file -p 83207f0274383b4a79ff6d6c297e95204ba961bc
100644 blob 2f781156939ad540b2434d012446154321e41e03	example_file.txt

The tree object contains one line per file or subdirectory, with each line giving file permissions, object type, object hash and filename. Object type is usually one of “blob” for a file or “tree” for a subdirectory [1].

Blob object type

The directory listing gave us the hash of the stored of example_file.txt. This object is of type “blob” and contains the file snapshot:

$ git cat-file -t 2f781156939ad540b2434d012446154321e41e03
blob
$ git cat-file -p 2f781156939ad540b2434d012446154321e41e03
An example file

Blob is an abbreviation for “binary large object”. When we git add a file such as example_file.txt, git creates a blob object containing the contents of the file. Blobs are therefore the git object type for storing files.

Tag object type

There is also a git type for annotated tags. We don’t have one of those yet, so let’s make one:

$ git tag -a first-commit -m "Tag pointing to first commit"

This gives us a new object in .git/objects:

objects
├── 2f
│   └── 781156939ad540b2434d012446154321e41e03 [32B]
├── 83
│   └── 207f0274383b4a79ff6d6c297e95204ba961bc [60B]
├── c7
│   └── e94cc3a0d893a23dab7bb2f601dc8e682a42cc [146B]
├── cf
│   └── 1fdb215f948795523cde2987107944d1374777 [135B]
├── info
└── pack

The object is of type “tag”:

$ git cat-file -t c7e94cc3a0d893a23dab7bb2f601dc8e682a42cc
tag

The tag object type contains the hash of the tagged object, the type of tagged object (usually a commit), the tag name, author, date and message:

$ git cat-file -p c7e94cc3a0d893a23dab7bb2f601dc8e682a42cc
object cf1fdb215f948795523cde2987107944d1374777
type commit
tag first-commit
tagger Matthew Brett <matthew.brett@gmail.com> 1487004984 +0000

Tag pointing to first commit

Notice that the “object” the tag points to, via its hash, is the commit object, as we were expecting.

Footnotes

[1]The object types in a directory listing are almost invariably either “blob” or “tree”, but can also be “commit” for recording the commit of a git submodule - see How do git submodules work?.