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git-subtree - Merge subtrees together and split repository into subtrees


git subtree add -P <prefix> <commit>
git subtree add -P <prefix> <repository> <ref>
git subtree pull -P <prefix> <repository> <ref>
git subtree push -P <prefix> <repository> <ref>
git subtree merge -P <prefix> <commit>
git subtree split -P <prefix> [OPTIONS] [<commit>]


Subtrees allow subprojects to be included within a subdirectory of the main project,
optionally including the subproject’s entire history.

For example, you could include the source code for a library as a subdirectory of your

Subtrees are not to be confused with submodules, which are meant for the same task. Unlike
submodules, subtrees do not need any special constructions (like .gitmodule files or
gitlinks) be present in your repository, and do not force end-users of your repository to
do anything special or to understand how subtrees work. A subtree is just a subdirectory
that can be committed to, branched, and merged along with your project in any way you

They are also not to be confused with using the subtree merge strategy. The main
difference is that, besides merging the other project as a subdirectory, you can also
extract the entire history of a subdirectory from your project and make it into a
standalone project. Unlike the subtree merge strategy you can alternate back and forth
between these two operations. If the standalone library gets updated, you can
automatically merge the changes into your project; if you update the library inside your
project, you can "split" the changes back out again and merge them back into the library

For example, if a library you made for one application ends up being useful elsewhere, you
can extract its entire history and publish that as its own git repository, without
accidentally intermingling the history of your application project.

In order to keep your commit messages clean, we recommend that people split their
commits between the subtrees and the main project as much as possible. That is, if you
make a change that affects both the library and the main application, commit it in two
pieces. That way, when you split the library commits out later, their descriptions
will still make sense. But if this isn’t important to you, it’s not necessary. git
subtree will simply leave out the non-library-related parts of the commit when it
splits it out into the subproject later.


Create the <prefix> subtree by importing its contents from the given <commit> or
<repository> and remote <ref>. A new commit is created automatically, joining the
imported project’s history with your own. With --squash, imports only a single commit
from the subproject, rather than its entire history.

Merge recent changes up to <commit> into the <prefix> subtree. As with normal git
merge, this doesn’t remove your own local changes; it just merges those changes into
the latest <commit>. With --squash, creates only one commit that contains all the
changes, rather than merging in the entire history.

If you use --squash, the merge direction doesn’t always have to be forward; you can
use this command to go back in time from v2.5 to v2.4, for example. If your merge
introduces a conflict, you can resolve it in the usual ways.

Exactly like merge, but parallels git pull in that it fetches the given ref from the
specified remote repository.

Does a split (see below) using the <prefix> supplied and then does a git push to push
the result to the repository and ref. This can be used to push your subtree to
different branches of the remote repository.

Extract a new, synthetic project history from the history of the <prefix> subtree. The
new history includes only the commits (including merges) that affected <prefix>, and
each of those commits now has the contents of <prefix> at the root of the project
instead of in a subdirectory. Thus, the newly created history is suitable for export
as a separate git repository.

After splitting successfully, a single commit id is printed to stdout. This
corresponds to the HEAD of the newly created tree, which you can manipulate however
you want.

Repeated splits of exactly the same history are guaranteed to be identical (i.e. to
produce the same commit ids). Because of this, if you add new commits and then
re-split, the new commits will be attached as commits on top of the history you
generated last time, so git merge and friends will work as expected.

Note that if you use --squash when you merge, you should usually not just --rejoin
when you split.


-q, --quiet
Suppress unnecessary output messages on stderr.

-d, --debug
Produce even more unnecessary output messages on stderr.

-P <prefix>, --prefix=<prefix>
Specify the path in the repository to the subtree you want to manipulate. This option
is mandatory for all commands.

-m <message>, --message=<message>
This option is only valid for add, merge and pull (unsure). Specify <message> as the
commit message for the merge commit.


This option is only valid for add, merge, and pull commands.

Instead of merging the entire history from the subtree project, produce only a single
commit that contains all the differences you want to merge, and then merge that new
commit into your project.

Using this option helps to reduce log clutter. People rarely want to see every change
that happened between v1.0 and v1.1 of the library they’re using, since none of the
interim versions were ever included in their application.

Using --squash also helps avoid problems when the same subproject is included multiple
times in the same project, or is removed and then re-added. In such a case, it doesn’t
make sense to combine the histories anyway, since it’s unclear which part of the
history belongs to which subtree.

Furthermore, with --squash, you can switch back and forth between different versions
of a subtree, rather than strictly forward. git subtree merge --squash always adjusts
the subtree to match the exactly specified commit, even if getting to that commit
would require undoing some changes that were added earlier.

Whether or not you use --squash, changes made in your local repository remain intact
and can be later split and send upstream to the subproject.


This option is only valid for the split command.

When generating synthetic history, add <annotation> as a prefix to each commit
message. Since we’re creating new commits with the same commit message, but possibly
different content, from the original commits, this can help to differentiate them and
avoid confusion.

Whenever you split, you need to use the same <annotation>, or else you don’t have a
guarantee that the new re-created history will be identical to the old one. That will
prevent merging from working correctly. git subtree tries to make it work anyway,
particularly if you use --rejoin, but it may not always be effective.

-b <branch>, --branch=<branch>
This option is only valid for the split command.

After generating the synthetic history, create a new branch called <branch> that
contains the new history. This is suitable for immediate pushing upstream. <branch>
must not already exist.

This option is only valid for the split command.

If you use --rejoin, git subtree attempts to optimize its history reconstruction to
generate only the new commits since the last --rejoin. --ignore-join disables this
behaviour, forcing it to regenerate the entire history. In a large project, this can
take a long time.

This option is only valid for the split command.

If your subtree was originally imported using something other than git subtree, its
history may not match what git subtree is expecting. In that case, you can specify the
commit id <onto> that corresponds to the first revision of the subproject’s history
that was imported into your project, and git subtree will attempt to build its history
from there.

If you used git subtree add, you should never need this option.

This option is only valid for the split command.

After splitting, merge the newly created synthetic history back into your main
project. That way, future splits can search only the part of history that has been
added since the most recent --rejoin.

If your split commits end up merged into the upstream subproject, and then you want to
get the latest upstream version, this will allow git’s merge algorithm to more
intelligently avoid conflicts (since it knows these synthetic commits are already part
of the upstream repository).

Unfortunately, using this option results in git log showing an extra copy of every new
commit that was created (the original, and the synthetic one).

If you do all your merges with --squash, don’t use --rejoin when you split, because
you don’t want the subproject’s history to be part of your project anyway.


Let’s assume that you have a local repository that you would like to add an external
vendor library to. In this case we will add the git-subtree repository as a subdirectory
of your already existing git-extensions repository in ~/git-extensions/:

$ git subtree add --prefix=git-subtree --squash \
git://github.com/apenwarr/git-subtree.git master

master needs to be a valid remote ref and can be a different branch name

You can omit the --squash flag, but doing so will increase the number of commits that are
included in your local repository.

We now have a ~/git-extensions/git-subtree directory containing code from the master
branch of git://github.com/apenwarr/git-subtree.git in our git-extensions repository.


Let’s use the repository for the git source code as an example. First, get your own copy
of the git.git repository:

$ git clone git://git.kernel.org/pub/scm/git/git.git test-git
$ cd test-git

gitweb (commit 1130ef3) was merged into git as of commit 0a8f4f0, after which it was no
longer maintained separately. But imagine it had been maintained separately, and we wanted
to extract git’s changes to gitweb since that time, to share with the upstream. You could
do this:

$ git subtree split --prefix=gitweb --annotate='(split) ' \
0a8f4f0^.. --onto=1130ef3 --rejoin \
--branch gitweb-latest
$ gitk gitweb-latest
$ git push [email protected]:whatever/gitweb.git gitweb-latest:master

(We use 0a8f4f0^.. because that means "all the changes from 0a8f4f0 to the current
version, including 0a8f4f0 itself.")

If gitweb had originally been merged using git subtree add (or a previous split had
already been done with --rejoin specified) then you can do all your splits without having
to remember any weird commit ids:

$ git subtree split --prefix=gitweb --annotate='(split) ' --rejoin \
--branch gitweb-latest2

And you can merge changes back in from the upstream project just as easily:

$ git subtree pull --prefix=gitweb \
[email protected]:whatever/gitweb.git master

Or, using --squash, you can actually rewind to an earlier version of gitweb:

$ git subtree merge --prefix=gitweb --squash gitweb-latest~10

Then make some changes:

$ date >gitweb/myfile
$ git add gitweb/myfile
$ git commit -m 'created myfile'

And fast forward again:

$ git subtree merge --prefix=gitweb --squash gitweb-latest

And notice that your change is still intact:

$ ls -l gitweb/myfile

And you can split it out and look at your changes versus the standard gitweb:

git log gitweb-latest..$(git subtree split --prefix=gitweb)


Suppose you have a source directory with many files and subdirectories, and you want to
extract the lib directory to its own git project. Here’s a short way to do it:

First, make the new repository wherever you want:

$ <go to the new location>
$ git init --bare

Back in your original directory:

$ git subtree split --prefix=lib --annotate="(split)" -b split

Then push the new branch onto the new empty repository:

$ git push <new-repo> split:master

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