git/Documentation/gitsubmodules.txt

gitsubmodules(7)
================

NAME
----
gitsubmodules - mounting one repository inside another

SYNOPSIS
--------
 .gitmodules, $GIT_DIR/config
------------------
git submodule
git <command> --recurse-submodules
------------------

DESCRIPTION
-----------

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 linkgit:gitmodules[5]) of the form
`submodule.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 Gits porcelain layer such as where to obtain the submodule via
the `submodule.foo.url` setting.

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 Gits 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.
      However you can also use submodules to e.g. hold large binary assets
      and these repositories are then 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.
    * Access control:
      By restricting user access to submodules, this can be used to implement
      read/write policies for different users.

The configuration of submodules
-------------------------------

Submodule operations can be configured using the following mechanisms
(from highest to lowest precedence):

 * The command line for those commands that support taking submodule specs.
   Most commands have a boolean flag '--recurse-submodules' whether to
   recurse into submodules. Examples are `ls-files` or `checkout`.
   Some commands take enums, such as `fetch` and `push`, where you can
   specify how submodules are affected.

 * The configuration inside the submodule. This includes `$GIT_DIR/config`
   in the submodule, but also settings in the tree such as a `.gitattributes`
   or `.gitignore` files that specify behavior of commands inside the
   submodule.
+
For example an effect from the submodule's `.gitignore` file
would be observed when you run `git status --ignore-submodules=none` in
the superproject. This collects information from the submodule's working
directory by running `status` in the submodule, which does pay attention
to its `.gitignore` file.
+
The submodule's `$GIT_DIR/config` file would come into play when running
`git push --recurse-submodules=check` in the superproject, as this would
check if the submodule has any changes not published to any remote. The
remotes are configured in the submodule as usual in the `$GIT_DIR/config`
file.

 * The configuration file `$GIT_DIR/config` in the superproject.
   Typical configuration at this place is controlling if a submodule
   is recursed into at all via the `active` flag for example.
+
If the submodule is not yet initialized, then the configuration
inside the submodule does not exist yet, so configuration where to
obtain the submodule from is configured here for example.

 * the `.gitmodules` file inside the superproject. Additionally to the
   required mapping between submodule's name and path, a project usually
   uses this file to suggest defaults for the upstream collection
   of repositories.
+
This file mainly serves as the mapping between name and path in
the superproject, such that the submodule's git directory can be
located.
+
If the submodule has never been initialized, this is the only place
where submodule configuration is found. It serves as the last fallback
to specify where to obtain the submodule from.

FORMS
-----

Submodules can take the following forms:

 * The basic form described in DESCRIPTION with a Git directory,
a working directory, a `gitlink`, and a `.gitmodules` entry.

 * "Old-form" submodule: A working directory with an embedded
`.git` directory, and the tracking `gitlink` and `.gitmodules` entry in
the superproject. This is typically found in repositories generated
using older versions of Git.
+
It is possible to construct these old form repositories manually.
+
When deinitialized or deleted (see below), the submodule’s Git
directory is automatically moved to `$GIT_DIR/modules/<name>/`
of the superproject.

 * Deinitialized submodule: A `gitlink`, and a `.gitmodules` entry,
but no submodule working directory. The submodule’s git directory
may be there as after deinitializing the git directory is kept around.
The directory which is supposed to be the working directory is empty instead.
+
A submodule can be deinitialized by running `git submodule deinit`.
Besides emptying the working directory, this command only modifies
the superproject’s `$GIT_DIR/config` file, so the superproject’s history
is not affected. This can be undone using `git submodule init`.

 * Deleted submodule: A submodule can be deleted by running
`git rm <submodule path> && git commit`. This can be undone
using `git revert`.
+
The deletion removes the superproject’s tracking data, which are
both the `gitlink` entry and the section in the `.gitmodules` file.
The submodule’s working directory is removed from the file
system, but the Git directory is kept around as it to make it
possible to checkout past commits without requiring fetching
from another repository.
+
To completely remove a submodule, manually delete
`$GIT_DIR/modules/<name>/`.

Workflow for a third party library
----------------------------------

  # add a submodule
  git submodule add <url> <path>

  # occasionally update the submodule to a new version:
  git -C <path> checkout <new version>
  git add <path>
  git commit -m "update submodule to new version"

  # See the list of submodules in a superproject
  git submodule status

  # See FORMS on removing submodules


Workflow for an artificially split repo
--------------------------------------

  # Enable recursion for relevant commands, such that
  # regular commands recurse into submodules by default
  git config --global submodule.recurse true

  # Unlike the other commands below clone still needs
  # its own recurse flag:
  git clone --recurse <URL> <directory>
  cd <directory>

  # Get to know the code:
  git grep foo
  git ls-files

  # Get new code
  git fetch
  git pull --rebase

  # change worktree
  git checkout
  git reset

Implementation details
----------------------

When cloning or pulling a repository containing submodules the submodules
will not be checked out by default; You can instruct 'clone' to recurse
into submodules. The 'init' and 'update' subcommands of 'git submodule'
will maintain submodules checked out and at an appropriate revision in
your working tree. Alternatively you can set 'submodule.recurse' to have
'checkout' recursing into submodules.


SEE ALSO
--------
linkgit:git-submodule[1], linkgit:gitmodules[5].

GIT
---
Part of the linkgit:git[1] suite