GITSUBMODULES(7) Git Manual GITSUBMODULES(7)NAMEgitsubmodules - 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 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 ALSOgit-submodule(1), gitmodules(5).
GIT
Part of the git(1) suite
Git 2.15.1 12/23/2017 GITSUBMODULES(7)
