merge(third_party/git): Merge squashed git subtree at v2.23.0
Merge commit '1b593e1ea4' as 'third_party/git'
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Vincent Ambo
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283
third_party/git/Documentation/gitsubmodules.txt
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third_party/git/Documentation/gitsubmodules.txt
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gitsubmodules(7)
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================
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NAME
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----
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gitsubmodules - mounting one repository inside another
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SYNOPSIS
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--------
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.gitmodules, $GIT_DIR/config
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------------------
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git submodule
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git <command> --recurse-submodules
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------------------
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DESCRIPTION
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-----------
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A submodule is a repository embedded inside another repository.
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The submodule has its own history; the repository it is embedded
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in is called a superproject.
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On the filesystem, a submodule usually (but not always - see FORMS below)
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consists of (i) a Git directory located under the `$GIT_DIR/modules/`
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directory of its superproject, (ii) a working directory inside the
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superproject's working directory, and a `.git` file at the root of
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the submodule's working directory pointing to (i).
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Assuming the submodule has a Git directory at `$GIT_DIR/modules/foo/`
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and a working directory at `path/to/bar/`, the superproject tracks the
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submodule via a `gitlink` entry in the tree at `path/to/bar` and an entry
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in its `.gitmodules` file (see linkgit:gitmodules[5]) of the form
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`submodule.foo.path = path/to/bar`.
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The `gitlink` entry contains the object name of the commit that the
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superproject expects the submodule's working directory to be at.
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The section `submodule.foo.*` in the `.gitmodules` file gives additional
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hints to Git's porcelain layer. For example, the `submodule.foo.url`
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setting specifies where to obtain the submodule.
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Submodules can be used for at least two different use cases:
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1. Using another project while maintaining independent history.
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Submodules allow you to contain the working tree of another project
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within your own working tree while keeping the history of both
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projects separate. Also, since submodules are fixed to an arbitrary
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version, the other project can be independently developed without
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affecting the superproject, allowing the superproject project to
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fix itself to new versions only when desired.
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2. Splitting a (logically single) project into multiple
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repositories and tying them back together. This can be used to
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overcome current limitations of Git's implementation to have
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finer grained access:
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* Size of the Git repository:
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In its current form Git scales up poorly for large repositories containing
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content that is not compressed by delta computation between trees.
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For example, you can use submodules to hold large binary assets
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and these repositories can be shallowly cloned such that you do not
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have a large history locally.
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* Transfer size:
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In its current form Git requires the whole working tree present. It
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does not allow partial trees to be transferred in fetch or clone.
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If the project you work on consists of multiple repositories tied
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together as submodules in a superproject, you can avoid fetching the
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working trees of the repositories you are not interested in.
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* Access control:
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By restricting user access to submodules, this can be used to implement
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read/write policies for different users.
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The configuration of submodules
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-------------------------------
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Submodule operations can be configured using the following mechanisms
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(from highest to lowest precedence):
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* The command line for those commands that support taking submodules
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as part of their pathspecs. Most commands have a boolean flag
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`--recurse-submodules` which specify whether to recurse into submodules.
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Examples are `grep` and `checkout`.
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Some commands take enums, such as `fetch` and `push`, where you can
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specify how submodules are affected.
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* The configuration inside the submodule. This includes `$GIT_DIR/config`
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in the submodule, but also settings in the tree such as a `.gitattributes`
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or `.gitignore` files that specify behavior of commands inside the
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submodule.
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+
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For example an effect from the submodule's `.gitignore` file
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would be observed when you run `git status --ignore-submodules=none` in
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the superproject. This collects information from the submodule's working
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directory by running `status` in the submodule while paying attention
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to the `.gitignore` file of the submodule.
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+
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The submodule's `$GIT_DIR/config` file would come into play when running
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`git push --recurse-submodules=check` in the superproject, as this would
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check if the submodule has any changes not published to any remote. The
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remotes are configured in the submodule as usual in the `$GIT_DIR/config`
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file.
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* The configuration file `$GIT_DIR/config` in the superproject.
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Git only recurses into active submodules (see "ACTIVE SUBMODULES"
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section below).
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+
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If the submodule is not yet initialized, then the configuration
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inside the submodule does not exist yet, so where to
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obtain the submodule from is configured here for example.
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* The `.gitmodules` file inside the superproject. A project usually
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uses this file to suggest defaults for the upstream collection
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of repositories for the mapping that is required between a
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submodule's name and its path.
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+
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This file mainly serves as the mapping between the name and path of submodules
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in the superproject, such that the submodule's Git directory can be
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located.
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+
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If the submodule has never been initialized, this is the only place
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where submodule configuration is found. It serves as the last fallback
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to specify where to obtain the submodule from.
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FORMS
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-----
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Submodules can take the following forms:
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* The basic form described in DESCRIPTION with a Git directory,
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a working directory, a `gitlink`, and a `.gitmodules` entry.
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* "Old-form" submodule: A working directory with an embedded
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`.git` directory, and the tracking `gitlink` and `.gitmodules` entry in
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the superproject. This is typically found in repositories generated
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using older versions of Git.
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+
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It is possible to construct these old form repositories manually.
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+
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When deinitialized or deleted (see below), the submodule's Git
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directory is automatically moved to `$GIT_DIR/modules/<name>/`
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of the superproject.
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* Deinitialized submodule: A `gitlink`, and a `.gitmodules` entry,
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but no submodule working directory. The submodule's Git directory
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may be there as after deinitializing the Git directory is kept around.
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The directory which is supposed to be the working directory is empty instead.
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+
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A submodule can be deinitialized by running `git submodule deinit`.
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Besides emptying the working directory, this command only modifies
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the superproject's `$GIT_DIR/config` file, so the superproject's history
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is not affected. This can be undone using `git submodule init`.
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* Deleted submodule: A submodule can be deleted by running
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`git rm <submodule path> && git commit`. This can be undone
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using `git revert`.
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+
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The deletion removes the superproject's tracking data, which are
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both the `gitlink` entry and the section in the `.gitmodules` file.
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The submodule's working directory is removed from the file
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system, but the Git directory is kept around as it to make it
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possible to checkout past commits without requiring fetching
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from another repository.
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+
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To completely remove a submodule, manually delete
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`$GIT_DIR/modules/<name>/`.
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ACTIVE SUBMODULES
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-----------------
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A submodule is considered active,
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1. if `submodule.<name>.active` is set to `true`
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+
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or
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2. if the submodule's path matches the pathspec in `submodule.active`
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+
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or
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3. if `submodule.<name>.url` is set.
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and these are evaluated in this order.
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For example:
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[submodule "foo"]
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active = false
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url = https://example.org/foo
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[submodule "bar"]
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active = true
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url = https://example.org/bar
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[submodule "baz"]
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url = https://example.org/baz
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In the above config only the submodule 'bar' and 'baz' are active,
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'bar' due to (1) and 'baz' due to (3). 'foo' is inactive because
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(1) takes precedence over (3)
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Note that (3) is a historical artefact and will be ignored if the
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(1) and (2) specify that the submodule is not active. In other words,
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if we have a `submodule.<name>.active` set to `false` or if the
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submodule's path is excluded in the pathspec in `submodule.active`, the
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url doesn't matter whether it is present or not. This is illustrated in
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the example that follows.
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[submodule "foo"]
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active = true
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url = https://example.org/foo
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[submodule "bar"]
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url = https://example.org/bar
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[submodule "baz"]
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url = https://example.org/baz
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[submodule "bob"]
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ignore = true
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[submodule]
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active = b*
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active = :(exclude) baz
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In here all submodules except 'baz' (foo, bar, bob) are active.
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'foo' due to its own active flag and all the others due to the
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submodule active pathspec, which specifies that any submodule
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starting with 'b' except 'baz' are also active, regardless of the
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presence of the .url field.
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Workflow for a third party library
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----------------------------------
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# add a submodule
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git submodule add <url> <path>
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# occasionally update the submodule to a new version:
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git -C <path> checkout <new version>
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git add <path>
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git commit -m "update submodule to new version"
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# See the list of submodules in a superproject
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git submodule status
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# See FORMS on removing submodules
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Workflow for an artificially split repo
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--------------------------------------
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# Enable recursion for relevant commands, such that
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# regular commands recurse into submodules by default
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git config --global submodule.recurse true
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# Unlike the other commands below clone still needs
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# its own recurse flag:
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git clone --recurse <URL> <directory>
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cd <directory>
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# Get to know the code:
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git grep foo
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git ls-files
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# Get new code
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git fetch
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git pull --rebase
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# change worktree
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git checkout
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git reset
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Implementation details
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----------------------
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When cloning or pulling a repository containing submodules the submodules
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will not be checked out by default; You can instruct 'clone' to recurse
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into submodules. The 'init' and 'update' subcommands of 'git submodule'
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will maintain submodules checked out and at an appropriate revision in
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your working tree. Alternatively you can set 'submodule.recurse' to have
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'checkout' recursing into submodules.
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SEE ALSO
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--------
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linkgit:git-submodule[1], linkgit:gitmodules[5].
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GIT
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---
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Part of the linkgit:git[1] suite
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