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| Here are quick and easy steps to track your changes to an open-source project.
| | == Purpose == |
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| Once you get a handle on git, you'll find it very powerful. But getting started can be the biggest challenge. Distributed source control requires a different way of thinking than using a central repository. git's terminology is a little bent from what you're used to. Use this guide to get past the first hurdles.
| | Here are some of the projects on which I have overlaid my code, with the help of git: |
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| == Requirements ==
| | *[http://www.mythtv.org/ mythtv] |
| | *[http://ampache.org/ ampache] |
| | *[http://mu.wordpress.org/ wordpressmu] (now integrated into [http://codex.wordpress.org/Create_A_Network wordpress]) |
| | *[http://recessframework.org/ recess] |
| | *a proprietary project hosted privately behind an openvpn connection |
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| Before you can start making changes to a project, you should get the code and make sure you can compile and run it. In this guide, we assume that the project is currently under subversion(svn) control. Check out a copy and get it up and running.
| | All of these currently use subversion, which git can read from and write to. Some of them (read: "mythtv") are insanely active. Git can help you remain sane while you try to overlay your changes on top of a moving target. |
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| The next set of requirements is pretty simple:
| | == Overview == |
| * I need my own branch off of the svn branch.
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| * I need the ability to overlay the latest svn changes, over time, on top of changes in my branch.
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| Notice how simple this is - we're not even asking to commit anything yet. These are fundamental requirements that any developer on a non-trivial open-source project would have. You <strong>need</strong> to do this if you are going to bang on an open-source project.
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| Now let's add one more requirement. I'm going to have this code on at least three different machines, so I need to be able to have a common repository for my changes.
| | Historically, source control always kept your code in a central location, and each developer checked out file(s) when they wanted to do modifications. Sometimes you could not get access to a file because someone else had checked it out with exclusive access. |
| *ability to work from several locations, pushing/pulling my changes to a central repo
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| Another basic source control requirement, and we're done.
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| But <strong>try</strong> to meet this set of requirements with cvs or svn. No longer simple. Enter git. It was born to do this job. If you're with me so far, and it sounds like we're on the right track, I promise you: step through the rest of this guide and you'll be happy you did. Let's get to it.
| | Open-source culture has no tolerance for this. Typical source control for open-source projects does not rely on checking out files. Anyone can grab the files and modify them at any time. Then conflicts are dealt with when you want to commit your files back into the central repository. The headaches are left for when you merge changes back into the project that has completely changed from under you. |
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| == Setup ==
| | git was made to ease life within this environment. But you have to step back and adjust your way of thinking. Code flows in every direction, and every developer is her own central repository. |
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| NOTE: I'm going to use mythtv in this example, because that's what I wanted to work on.
| | == Implementation == |
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| Create a local git repository of the project's subversion repository, using git-svn. There are two ways to do this. If you do not specify a svn revision number, git will grab the entire history of the project as it is available in the svn repo. WARNING: this may be HUGE for older bigger projects!
| | I have used three approaches, in reverse chronological order: |
| git svn clone http://svn.mythtv.org/svn/trunk/mythtv mythtv
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| If you specify a revision number, git will grab just that version, and then we can grab all changes after that version, too. It's probably worth digging into the project history to find a reasonable revision number.
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| git svn clone -r15502 http://svn.mythtv.org/svn/trunk/mythtv mythtv
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| We now have a repo of the project under git control. Simple, eh? git uses branches, and named our svn grab the "master" branch in this repo. See the list of branches:
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| git branch
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| Now we can easily update our master branch with the latest svn commits at any time. Do this now to make sure you're completely up-to-date:
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| git svn rebase
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| Now we create a branch of our own, derived from the svn base. git loves branches!
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| git checkout -b mybranch
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| git branch
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| Now that you have your branch checked out, you can bang away and change anything you want to (except the top-level .git directory). Eventually you'll want to commit your changes. Time for some terminology (I swear I'll be brief - on a "need to know" basis). As you edit, you're only changing local files. git refers to the locally modified files as your "working set". git doesn't track anything about your working set - you're free to abuse it as you see fit. If you want to keep changes you've made, you need to add each file you've changed or added to the git "index" (also called the "cache"). Then, when you're ready, you commit all the indexed changes to the repository in one single commit. So git provides the "index" as a space in which you can manage your next commit.
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| Let's say you've changed the configure script. Here's how to commit the change to your git branch:
| | #[[Managed publishing of your personal git repository]] - this is how I currently work, publishing my code in a clean controlled way to a gitorious repo. |
| git add configure
| | #[[Work on an open-source project from anywhere]] - this was a popular approach for a while, using an SVN-based master repo. |
| git commit -m "I updated the configure script."
| | #[[Setting up your own public repo on top of an open-source project]] - this is a technique useful with an SVN-based master repo but probably overly complex. |
| The index is an extra layer that you don't usually have with cvs/svn. It's totally a benefit, and you can ignore it by doing the add and commit in one step. This will add every file that is in the repo and has been modified to the index, then commit, in one step ("cvs/svn style"):
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| git commit -a -m "I changed this to that."
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| So over time, you'll have a series of commits to your branch. Later, you'll want to get the latest svn changes and incorporate them. We're going to use rebase again. Previously, we used it to update from svn, let's do that again. Make sure you switch to the svn branch first.
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| git checkout master
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| git svn rebase
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| Now it gets interesting. We're going to switch back to our branch, and once again use rebase. To be more specific, rebase means to take your current set of commits all the way back to the last rebase, REMOVE them from the branch, "rebase" the branch as specified, and reapply all our commits. This is good stuff!
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| git checkout mybranch
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| git rebase master
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| git has now reapplied your changes on top of the latest svn. I told you it was cool!
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| We've now handled our first set of requirements.
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| tbc...
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Purpose
Here are some of the projects on which I have overlaid my code, with the help of git:
All of these currently use subversion, which git can read from and write to. Some of them (read: "mythtv") are insanely active. Git can help you remain sane while you try to overlay your changes on top of a moving target.
Overview
Historically, source control always kept your code in a central location, and each developer checked out file(s) when they wanted to do modifications. Sometimes you could not get access to a file because someone else had checked it out with exclusive access.
Open-source culture has no tolerance for this. Typical source control for open-source projects does not rely on checking out files. Anyone can grab the files and modify them at any time. Then conflicts are dealt with when you want to commit your files back into the central repository. The headaches are left for when you merge changes back into the project that has completely changed from under you.
git was made to ease life within this environment. But you have to step back and adjust your way of thinking. Code flows in every direction, and every developer is her own central repository.
Implementation
I have used three approaches, in reverse chronological order:
- Managed publishing of your personal git repository - this is how I currently work, publishing my code in a clean controlled way to a gitorious repo.
- Work on an open-source project from anywhere - this was a popular approach for a while, using an SVN-based master repo.
- Setting up your own public repo on top of an open-source project - this is a technique useful with an SVN-based master repo but probably overly complex.