zobie's blog I create software, I like music, and I'm mildly(?) OCD.

14Jan/09

What’s so great about git?

Over the last few days, I have been asked several times why I like git. Many explanations and detailed comparisons exist that discuss how git compares to other version control systems. I'm not going to try and duplicate any of that work. These are just some thoughts about features in git that allow me to work more productively.

I didn't immediately grok git because it is fundamentally different from subversion. Subversion tracks the history of files whereas Git tracks the history of content. That difference may seem minor, but the shift brings a lot of flexibility. You can expect a small learning curve, but don't give up! If git works as well for you as it does for me, you will not want to go back to subversion.

So, what are my favorite things about git?

  • Branching: Coming from a CVS/SVN background, I didn't think git's "cheap branching" would matter to me. I am not exaggerating when I say that it has dramatically changed the way I work. Because branching and merging are so easy, I create a new branch for each feature or bug I'm working on. I am never tempted to check in broken code just because I want a checkpoint.
  • The index: It is really nice to be able to just check in a portion of the changes in my code rather than having to commit the entire file.
  • git stash: If I'm in the middle of some big change and need to fix a small bug, I can hide my outstanding changes, make the fix and restore my changes without a checkin.
  • git rebase: Sometimes it is really handy to be able to rewrite history.
  • Git is a DVCS: My work isn't crippled when I am not connected to the network. I can check code in and out. I can branch and merge. I have a full history of the repository. Awesome!
  • Very flexible workflow: With git I can push/pull to a central server like CVS or SVN. Alternatively, changes can bypass the server and be sent directly between working repositories.
  • Git is fast: Neither CVS nor SVN come close to git's speed.

Is git perfect? No. Off the top of my head, here are some things that I would really like to have in git:

  • git checkout -i: Sometimes I want to rollback a few changes in a file. I imagine this would work very similarly to 'git add -i'.
  • git stash -i: Sometimes I only want to stash some of my changes. This too would work like 'git add -i'.
  • Local tags: Local branches are great; local tagging would be too.
  • Track empty directories: I understand why this doesn't currently happen, but it can be obnoxious.
  • Comments on branches: It is easy to make a lot of branches but unless your branch names are super descriptive (and therefore super long), it is also easy to forget what each of them is for.
  • Better/more documentation: In general I am able to find what I'm looking for, but I still don't know what the "fetch-all" in git svn fetch --fetch-all actually does.
  • Windows support: Not that I've actually tried it, I've just heard that it doesn't work well.
  • GUI options: Most of the time I work in the cli but sometimes a gui is more convienent. I've heard multiple people ask for a tool that would allow them to branch and tag without checking out code. gitk and gitx just don't offer all the flexibility that some other tools do (e.g. WinCVS, SmartSVN, TortoiseSVN, etc.).

Why do you like (or hate) git?

2Dec/08

Migrating from svn to git

About a year ago I decided that it was time to try git. As a long-time user of SVN, CVS and even VSS *shudder*, I wanted to see what all the fuss was about. At first I didn't get it; having never used a DVCS before I just didn't grok the concept but I keep with it and one day it clicked. After a few weeks I was completely hooked and have since migrated most of my personal repositories from SVN (I haven't tried it under Windows so I haven't migrated my VisualStudio projects yet).

Not long after my conversion, we decided to start using git for a few projects at work. Since then we've been using both subversion and git on a self-hosted server. As anyone who gets git can tell you, after using a DVCS subversion can feel very limiting so we recently decided to migrate everything from subversion to git. We decided to host all of our repositories on github.

This is a long way to say that, over the past year I've migrated ~20 subversion repositories, of varying complexity, over to git. The process is not terribly difficult but there are multiple steps that you have to remember. I've kept a list of each step and thought that it might be useful to someone else (or perhaps myself when I need to do this again in a few months 😉 ).

To migrate the subversion repository my_great_app to git first you'll create an empty git repository. Note that this just does the setup, no import happens at this point.

$ git svn init -s svn://svn.zobie.com/svn/my_great_app my_great_app
$ cd my_great_app

Now, if you want your new git history to look pretty, you need to create a text file that maps subversion users to git users. For this example I'll name the file ~/users.text; it should look something like this (svn user on the left, git user on the right):

(no author) = Unknown Author <unknown@zobie.com>
nate = Nate Zobrist <zobie@zobie.com>

Then you tell git about your authors file and start the import:

$ git config svn.authorsfile ~/users.text
$ git svn fetch

Honestly, I don't really understand the "fetch-all" option but on one of the larger repositories that I migrated, the initial import didn't seem to be complete. I started completely over and when I ran "git svn fetch" I included the "--fetch-all" option. This significantly increased the amount of time that the import took but it seemed to do the trick.

$ git svn fetch --fetch-all

At this point your entire subversion repository has been imported into git and we want to create a tag in git that corresponds to each of the tags in subversion:

$ git branch -r | sed -rne 's, *tags/([^@]+)$,\1,p' | while read tag; do echo "git tag $tag 'tags/${tag}^'; git branch -r -d tags/$tag"; done | sh

Then, for each svn branch, we create a branch in git:

$ git branch -r | grep -v tags | sed -rne 's, *([^@]+)$,\1,p' | while read branch; do echo "git branch $branch $branch"; done | sh

These two commands will import every tag and branch from subversion. Sometimes git finds branches that you didn't know existed. Just delete whatever branches and tags that you don't want persisted.

Now that the repository is setup like we want, we do some housework to shrink and compact the repository.

$ git repack -d -f -a --depth=50 --window=100

Then we remove the meta-data that was used by git-svn:

$ git config --remove-section svn-remote.svn
$ git config --remove-section svn
$ rm -r .git/svn

At this point you have successfully migrated your subversion repository into git! If you aren't going to push this repository to a centrally hosted server you can stop at this point and enjoy using your new git repository.

If you want to keep a copy of your repository on a self-hosted server then create a bare clone and copy it up. Then configure your new server as "origin." You'll be happiest if you make sure that the master branch is checked out before cloning the repository.

$ git checkout master
$ cd ..
$ git clone --bare my_great_app my_great_app.git
$ scp -r my_great_app.git zobie@git.zobie.com:/git/.
$ rm -rf my_great_app.git
$ cd my_great_app
$ git remote add origin ssh://git.zobie.com/git/my_great_app.git

Alternatively, if you want to push the repository to github, use the the clone url that they provide:

$ git remote add origin git@github.com:zobie/my_great_app.git
$ git push --all && git push --tags

The last thing that you need to do is to point your "master" branch to "origin/master". There is probably a way to do this from the command line but I don't know it. I can't give exact instructions for modifying the config but the process isn't difficult. Open .git/config and point your master branch to origin; it should look something like this:

[branch "master"]
	remote = origin
	merge = refs/heads/master

If you have other local branches that should track their remote counterpart, you will need to make a similar change for each.

You can test that everything is working:

$ git pull
$ git push

Assuming you don't get any errors, you're done!

Update 2009-01-13: Corrected an error in the sed regex. The '1' should be escaped.