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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 (ie. 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, push 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