gitsubmodules(7) (original) (raw)

A submodule is a repository embedded inside another repository. The submodule has its own history; the repository it is embedded in is called a superproject.

On the filesystem, a submodule usually (but not always - see FORMS below) consists of (i) a Git directory located under the $GIT_DIR/modules/directory of its superproject, (ii) a working directory inside the superproject’s working directory, and a .git file at the root of the submodule’s working directory pointing to (i).

Assuming the submodule has a Git directory at $GIT_DIR/modules/foo/and a working directory at path/to/bar/, the superproject tracks the submodule via a gitlink entry in the tree at path/to/bar and an entry in its .gitmodules file (see gitmodules(5)) of the formsubmodule.foo.path = path/to/bar.

The gitlink entry contains the object name of the commit that the superproject expects the submodule’s working directory to be at.

The section submodule.foo.* in the .gitmodules file gives additional hints to Git’s porcelain layer. For example, the submodule.foo.urlsetting specifies where to obtain the submodule.

Submodules can be used for at least two different use cases:

  1. Using another project while maintaining independent history. Submodules allow you to contain the working tree of another project within your own working tree while keeping the history of both projects separate. Also, since submodules are fixed to an arbitrary version, the other project can be independently developed without affecting the superproject, allowing the superproject project to fix itself to new versions only when desired.
  2. Splitting a (logically single) project into multiple repositories and tying them back together. This can be used to overcome current limitations of Git’s implementation to have finer grained access:
    • Size of the Git repository: In its current form Git scales up poorly for large repositories containing content that is not compressed by delta computation between trees. For example, you can use submodules to hold large binary assets and these repositories can be shallowly cloned such that you do not have a large history locally.
    • Transfer size: In its current form Git requires the whole working tree present. It does not allow partial trees to be transferred in fetch or clone. If the project you work on consists of multiple repositories tied together as submodules in a superproject, you can avoid fetching the working trees of the repositories you are not interested in.
    • Access control: By restricting user access to submodules, this can be used to implement read/write policies for different users.