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An introduction to git-svn for Subversion/SVK users and deser...

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Saved by 15 people (4 private), first by anonymouse user on 2007-08-19


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This article is aimed at people who want to contribute to projects which are using Subversion as their code-wiki

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Subversion users can skip SVK and move straight onto git-svn with this tutorial.

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People who are responsible for Subversion servers and are converting them to git in order to lay them down to die are advised to consider the one-off git-svnimport, which is useful for bespoke conversions where you don't necessarily want to leave SVN/CVS/etc breadcrumbs behind. I'll mention bespoke conversions at the end of the tutorial, and the sort of thing that you end up doing with them.

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A lot of this tutorial is dedicated to advocacy, sadly necessary. Those who would rather just cut to the chase will probably want to skip straight to

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Another way of looking at it is to say that it's really a content- addressable filesystem, used to track directory trees.

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we've got a simple and efficient filesystem which competes with RevML but is XML free

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Subversion added nothing to CVS' development model.

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Yes, it's a bunch of small programs that do one thing and do it well, get over it, they're being unified

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There's also a pure Java

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I used to push strongly for SVK, but got brow-beaten by people who were getting far more out of their version control system than I knew possible until I saw what they were talking about.

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SVK could easily use git as a backing filesystem and drop the dependency on Subversion altogether. So could bzr or hg.

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The repository model (see right) is also simple enough that there are complete git re-implementations you can draw upon, in a variety of languages.

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git is first and foremost a toolkit for writing VCS systems

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Writing a tool to do something that you want is often quite a simple matter of plugging together a few core commands.

It's simple enough that once a few basic concepts are there, you begin to feel comfortable knowing that the repository just can't wedge, changes can be discarded yet not lost unless you request them to be cleaned up, etc.

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I really haven't seen a nicer tool than gitk for browsing a repository.

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gitk does some really cool things but is most useful when looking at projects that have cottoned onto feature branches (see feature branches, below). If you're looking at a project where everyone commits largely unrelated changes to one branch it just ends up a straight line, and not very interesting.

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You can easily publish your changes for others who are switched on to git to pull. At a stretch, you can just throw the .git directory on an HTTP server somewhere and publish the path.

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There's the git-daemon for more efficient serving of repositories (at least, in terms of network use), and gitweb.cgi to provide a visualisation of a git repository.

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With Subversion, everyone has to commit their changes back to the central wiki, I mean repository, to share them.

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With Git (actually this is completely true for other distributed systems), it's trivial to push and pull changes between each other. If what you're pulling has common history then git will just pull the differences.

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If the person publishes their repository as described above, using the git-daemon(1), http or anything else that you can get your kernel to map to its VFS, then you can set it up as a "remote" and pull from it

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Most people say "but I don't want branches". But users of darcs report that they didn't know how much they really did want branches, but never knew until darcs made it so easy. In essence every change can behave as a branch, and this isn't painful.

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Because you can easily separate your repositories into stable branches, temporary branches, etc, then you can easily set up programs that only let commits through if they meet criteria of your choosing.

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Because you can readily work on branches without affecting the stable branch, it is perfectly acceptable for a stable branch to be updated by a single maintainer only

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Some repositories, for instance the Linux kernel, run a policy of no commit may break the build. What this means is that if you have a problem, you can use bisection to work out which patch introduced the bug.

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You might use a continual integration server that is responsible for promoting branches to trunk should they pass the strictures that you set.

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There is an awful lot less to keep in your head, and you don't have to do things like plan branching in advance.

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Good feature branches mean you end up prototyping well-developed changes; the emphasis shifts away from making atomic commits. If you forgot to add a file, or made some other little mistake, it's easy to go back and change it. If you haven't even pushed your changes anywhere, that's not only fine, but appreciated by everyone involved. Review and revise before you push is the counter-balance to frequent commits.

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Not only is the implementation fast locally, it's very network efficient, and the protocol for exchanging revisions is also very good at figuring out what needs to be transferred quickly. This is a huge difference - one repository hosted on Debian's Alioth SVN server took 2 days to synchronise because the protocol is so chatty. Now it fits in 3 megs and would not take that long to synchronise over a 150 baud modem.

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Disk might be cheap, but my /home is always full - git has a separate step for compacting repositories, which means that delta compression can be far more effective. If you're a compression buff, think of it as having an arbitrarily sized window, because when delta compressing git is able to match strings anywhere else in the repository - not just the file which is the notional ancestor of the new revision.

This space efficiency affects everything - the virtual memory footprint in your buffercache while mining information from the repository, how much data needs to be transferred during "push" and "pull" operations, and so on. Compare that to Subversion, which even when merging between branches is incapable of using the same space for the changes hitting the target branch. The results speak for themselves - I have observed an average of 10 to 1 space savings going from Subversion FSFS to git.

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Perhaps somebody has already made a conversion of the project and put it somewhere

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git-svn fetch

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But people who use git are used to treating their repositories as a revision data warehouse which they use to mine useful information when they are trying to understand a codebase.

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importing the whole repository from Subversion

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git svn init

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git svn fetch

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If you like, you can skip early revisions using the -r option to git-fetch.

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make a local branch for development

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The name "foo" is completely private; it's just a local name you're assigning to the piece of work you're doing. Eventually you will learn to group related commits onto branches, called "topic branches", as described in the introduction.

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Say you want to take a project, and work on it somewhere else in a different direction, you can just make a copy using cp or your favourite file manager. Contrast this with Subversion, where you have to fiddle around with branches/ paths, svn cp, svn switch, etc

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Each of those copies is fully independent, and can diverge freely. You can easily push and pull changes between them without tearing your hair out.

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Each time you have a new idea, make a new branch and work in that.

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But anyway, that copying was too slow and heavy. We don't want to copy 70MB each time we want to work on a new idea. We want to create new branches at the drop of a hat. Maybe you don't want to copy the actual repository, just make another checkout. We can use git-clone again

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The -l option to git-clone told git to hardlink the objects together, so not only are these two sharing the same repository but they can still be moved around independently. Cool. I now have two checkouts I can work with, build software in, etc.

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But all that's a lot of work and most of the time I don't care to create lots of different directories for all my branches. I can just make a new branch and switch to it immediately with git-checkout:

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Once you have some edits you want to commit, you can use git-commit to commit them. Nothing (not even file changes) gets committed by default; you'll probably find yourself using git-commit -a to get similar semantics to svn commit.

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There is also a GUI for preparing commits in early (but entirely functional) stages of development.

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People used to darcs or SVK's interactive commit will like to try git add -i

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correcting changes in your local branch

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If it's the top commit, you can just add --amend to your regular git-commit command to, well, amend the last commit. If you explored the git-gui interface, you might have noticed the "Amend Last Commit" switch as well.

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You can also uncommit. The command for this is git-reset

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HEAD~1 is a special syntax that means "one commit before the reference called HEAD". HEAD^ is a slightly shorter shorthand for the same thing. I could have also put a complete revision number, a partial (non-ambiguous) revision number, or something like remotes/trunk. See git-rev-parse(1) for the full list of ways in which you can specify revisions.

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I sometimes write commands like `gitk --all `git-fsck | awk '/dangling commit/ {print $3}'`' to see all the commits in the repository, not just the ones with "post-it notes" (aka references) stuck to them.

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"Another" way to revise commits is to make a branch from the point a few commits ago, then make a new series of commits that is revised in the way that you want. This is the same scenario as before.

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I've introduced a new command there - git-cherry-pick. This takes a commit and tries to copy its changes to the branch you've currently got checked out. This technique is called rebasing commits. There is also a git-rebase command which probably would have been fewer commands than the above. But that's my way.

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Using Git opens the door to a bazaar of VCS tools rather than sacrificing your projects at the altar of one.

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keep your local branch up to date with Subversion

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The recommended way to do this for people familiar with Subversion is to use git-svn rebase.

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Note: before you do this, you should have a "clean" working tree - no local uncommitted changes. You can use git-stash (git 1.5.3+) to hide away local uncommitted changes for later.

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This command is doing something similar to the above commands that used git-cherry-pick; it's copying the changes from one point on the revision tree to another

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Better still is to bunch up your in-progress working copy changes into a set of unfinished commits, using git add -i (or git-gui / git-citool). Then try the rebase. You'll end up this time with more commits on top of the SVN tree than just one, so using Stacked Git you can "stg uncommit -n 4" (if you broke your changes into 4 commits), then use "stg pop" / "stg push" to wind around the stack (as well as "stg refresh" when finished making changes) to finish them - see

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Once you grok that, you'll only need to use stg and git-svn fetch.

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in my experience stg is the best tool for rebasing

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Ok, so you've already gone and made the commits locally that you wanted to publish back to the Subversion server. Perhaps you've even made a collection of changes, revising each change to be clearly understandable, making a single small change well such that the entire series of changes can be easily reviewed by your fellow project contributors. It is now time to publish your changes back to Subversion.

The command to use is git svn dcommit. The d stands for delta

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git-svn won't let the server merge revisions on the fly; if there were updates since you fetched / rebased, you'll have to do that again.

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People are not used to this, thinking somehow that if somebody commits something to file A, then somebody else commits something to file B, the server should make a merged version with both changes, despite neither of the people committing actually having a working tree with both changes. This suffers from the same fundamental problem that darcs' patch calculus does - that just because patches apply 'cleanly' does not imply that they make sense - such a decision can only be automatically made with a dedicated continual integration (smoke) server.

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This is normally what I use in preference to rebase.

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This will merge all the commits that aren't in your ancestry, but are in the ancestry of the branch trunk (try setting rightmost drop-down in gitk to 'ancestor' and clicking around to get a feel for what this means), and make a new commit which has two parents - your old HEAD, and whatever commit trunk is up to.

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there are many shortfallings in git.

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Sadly, this model is in use by virtually every Subversion hosted project out there. And that is going to be hard to undo.

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It is possible to use git in this way (see the figure to the right) - but it's not trivial, and not default. In fact git itself is developed in this way, using feature branches, aka topic branches.

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Left: what darcs thinks when you start committing without marking tag points.

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Right: Subversion has a somewhat smaller brain...

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bzr comes with some great utilities like the Patch Queue Manager which helps show you your feature branches. With PQM, you just create a branch with a description of what you're trying to do, make it work against the version that you branched off, and then you're done. The branch can be updated to reflect changes in trunk, and eventually merged and closed.

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Windows support is good. Consistent implementation. Experience with the distributed development model. Friendly and approachable author and core team.

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Actually the models of git and bzr are similar enough that bzr could be fitted atop of the git repository model

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Mercurial is missing lightweight branches that makes git so powerful, and there is no content hashing, so it doesn't really do the whole "revision protocol" thing like git.

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If you're on Windows it's probably a lot easier to get going.

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