GITSUBMODULES(7)          (08/17/2021)           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 superprojectcqs working
          directory, and a .git file at the root of the submodulecqs
          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 submodulecqs working
          directory to be at.

          The section submodule.foo.* in the .gitmodules file gives
          additional hints to Gitcqs 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.

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           2. Splitting a (logically single) project into multiple
              repositories and tying them back together. This can be
              used to overcome current limitations of Gitcqs
              implementation to have finer grained access:

              +o   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.

              +o   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.

              +o   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):

          +o   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.

          +o   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 submodulecqs .gitignore
              file would be observed when you run git status
              --ignore-submodules=none in the superproject. This
              collects information from the submodulecqs working
              directory by running status in the submodule while
              paying attention to the .gitignore file of the
              submodule.

              The submodulecqs $GIT_DIR/config file would come into
              play when running git push --recurse-submodules=check in

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              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.

          +o   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.

          +o   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 submodulecqs name and its path.

              This file mainly serves as the mapping between the name
              and path of submodules in the superproject, such that
              the submodulecqs 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:

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

          +o   "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
              submodulecqs Git directory is automatically moved to
              $GIT_DIR/modules/<name>/ of the superproject.

          +o   Deinitialized submodule: A gitlink, and a .gitmodules
              entry, but no submodule working directory. The
              submodulecqs Git directory may be there as after
              deinitializing the Git directory is kept around. The

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              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 superprojectcqs
              $GIT_DIR/config file, so the superprojectcqs history is
              not affected. This can be undone using git submodule
              init.

          +o   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 superprojectcqs tracking data,
              which are both the gitlink entry and the section in the
              .gitmodules file. The submodulecqs 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 submodulecqs 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

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          In the above config only the submodule bar and baz are
          active, bar due to (1) and baz due to (3). foo is inactive
          because (1) takes precedence over (3)

          Note that (3) is a historical artefact and will be ignored
          if the (1) and (2) specify that the submodule is not active.
          In other words, if we have a submodule.<name>.active set to
          false or if the submodulecqs path is excluded in the pathspec
          in submodule.active, the url doesncqt 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 most other commands below, clone still needs
              # its own recurse flag:

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              git clone --recurse <URL> <directory>
              cd <directory>

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

              Note

              git ls-files also requires its own --recurse-submodules
              flag.

              # 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 (note that
          submodule.recurse also affects other Git commands, see git-
          config(1) for a complete list).

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

     GIT
          Part of the git(1) suite



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