Documentation/gitsubmodules.txt - git - Git at Google

 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
	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