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Date: Sat, 5 Jan 2008 20:17:40 -0500
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From: Sean <seanlkml@sympatico.ca>
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To: Miklos Vajna <vmiklos@frugalware.org>
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Cc: git@vger.kernel.org
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Subject: how to use git merge -s subtree?
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Abstract: In this article, Sean demonstrates how one can use the subtree merge
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strategy.
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Content-type: text/asciidoc
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Message-ID: <BAYC1-PASMTP12374B54BA370A1E1C6E78AE4E0@CEZ.ICE>
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How to use the subtree merge strategy
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=====================================
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There are situations where you want to include contents in your project
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from an independently developed project. You can just pull from the
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other project as long as there are no conflicting paths.
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The problematic case is when there are conflicting files. Potential
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candidates are Makefiles and other standard filenames. You could merge
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these files but probably you do not want to. A better solution for this
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problem can be to merge the project as its own subdirectory. This is not
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supported by the 'recursive' merge strategy, so just pulling won't work.
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What you want is the 'subtree' merge strategy, which helps you in such a
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situation.
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In this example, let's say you have the repository at `/path/to/B` (but
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it can be a URL as well, if you want). You want to merge the 'master'
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branch of that repository to the `dir-B` subdirectory in your current
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branch.
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Here is the command sequence you need:
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----------------
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$ git remote add -f Bproject /path/to/B <1>
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$ git merge -s ours --no-commit Bproject/master <2>
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$ git read-tree --prefix=dir-B/ -u Bproject/master <3>
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$ git commit -m "Merge B project as our subdirectory" <4>
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$ git pull -s subtree Bproject master <5>
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----------------
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<1> name the other project "Bproject", and fetch.
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<2> prepare for the later step to record the result as a merge.
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<3> read "master" branch of Bproject to the subdirectory "dir-B".
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<4> record the merge result.
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<5> maintain the result with subsequent merges using "subtree"
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The first four commands are used for the initial merge, while the last
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one is to merge updates from 'B project'.
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Comparing 'subtree' merge with submodules
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-----------------------------------------
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- The benefit of using subtree merge is that it requires less
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administrative burden from the users of your repository. It works with
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older (before Git v1.5.2) clients and you have the code right after
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clone.
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- However if you use submodules then you can choose not to transfer the
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submodule objects. This may be a problem with the subtree merge.
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- Also, in case you make changes to the other project, it is easier to
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submit changes if you just use submodules.
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Additional tips
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---------------
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- If you made changes to the other project in your repository, they may
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want to merge from your project. This is possible using subtree -- it
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can shift up the paths in your tree and then they can merge only the
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relevant parts of your tree.
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- Please note that if the other project merges from you, then it will
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connect its history to yours, which can be something they don't want
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to.
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