git-subtree(1)
NAME
git-subtree - Merge subtrees together and split repository into subtrees
SYNOPSIS
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>]
DESCRIPTION
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 application.
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 want.
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 project.
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.
Tip
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.
COMMANDS
add
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
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.
pull
Exactly like merge, but parallels git pull in that it fetches the
given ref from the specified remote repository.
push
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.
split
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.
OPTIONS
-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.
OPTIONS FOR ADD, MERGE, PUSH, PULL
--squash
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.
OPTIONS FOR SPLIT
--annotate=<annotation>
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.
--ignore-joins
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.
--onto=<onto>
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.
--rejoin
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.
EXAMPLE 1. ADD COMMAND
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.
EXAMPLE 2. EXTRACT A SUBTREE USING COMMIT, MERGE AND PULL
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 git@github.com: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 \
git@github.com: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)
EXAMPLE 3. EXTRACT A SUBTREE USING BRANCH
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
AUTHOR
Written by Avery Pennarun <apenwarr@gmail.com[1]>
GIT
Part of the git(1) suite
NOTES
1. apenwarr@gmail.com
mailto:apenwarr@gmail.com
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