archive: handle commits with an empty tree

[git/jnareb-git.git] / contrib / subtree / git-subtree.txt

git-subtree(1)
==============

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


SYNOPSIS
--------
[verse]
'git subtree' add   -P <prefix> <commit>
'git subtree' pull  -P <prefix> <repository> <refspec...>
'git subtree' push  -P <prefix> <repository> <refspec...>
'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 <refspec> or <repository> and remote <refspec>.
        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 commit 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 refspec. 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 incldued 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>


GIT
---
Part of the linkgit:git[1] suite