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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 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 Git’s
       porcelain layer. For example, the submodule.foo.url setting 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.

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 submodules as part of their
           pathspecs. Most commands have a boolean flag --recurse-submodules which specify
           whether to recurse into submodules. Examples are grep and 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 while paying
           attention to the .gitignore file of the submodule.

           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. Git only recurses into
           active submodules (see "ACTIVE SUBMODULES" section below).

           If the submodule is not yet initialized, then the configuration inside the submodule
           does not exist yet, so where to obtain the submodule from is configured here for
           example.

       ·   The .gitmodules file inside the superproject. A project usually uses this file to
           suggest defaults for the upstream collection of repositories for the mapping that is
           required between a submodule’s name and its path.

           This file mainly serves as the mapping between the name and path of submodules 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>/.

ACTIVE SUBMODULES

       A submodule is considered active,

        1. if submodule.<name>.active is set to true

           or

        2. if the submodule’s path matches the pathspec in submodule.active

           or

        3. if submodule.<name>.url is set.

       and these are evaluated in this order.

       For example:

           [submodule "foo"]
             active = false
             url = https://example.org/foo
           [submodule "bar"]
             active = true
             url = https://example.org/bar
           [submodule "baz"]
             url = https://example.org/baz

       In the above config only the submodule bar and baz are active, bar due to (a) and baz due
       to (c). foo is inactive because (a) takes precedence over (c)

       Note that (c) is a historical artefact and will be ignored if the (a) and (b) specify that
       the submodule is not active. In other words, if we have a submodule.<name>.active set to
       false or if the submodule’s path is excluded in the pathspec in submodule.active, the url
       doesn’t matter whether it is present or not. This is illustrated in the example that
       follows.

           [submodule "foo"]
             active = true
             url = https://example.org/foo
           [submodule "bar"]
             url = https://example.org/bar
           [submodule "baz"]
             url = https://example.org/baz
           [submodule "bob"]
             ignore = true
           [submodule]
             active = b*
             active = :(exclude) baz

       In here all submodules except baz (foo, bar, bob) are active. foo due to its own active
       flag and all the others due to the submodule active pathspec, which specifies that any
       submodule starting with b except baz are also active, regardless of the presence of the
       .url field.

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

       git-submodule(1), gitmodules(5).

GIT

       Part of the git(1) suite