29 May 2008

sharing work between computers with a usb flash drive and git

I couldn't find anything exactly matching this on the net when I was figuring it out, so here's what I did.

This is working against a remote svn (subversion) server, but applies even without one.

On the first computer, grab your git working copy from svn with git-svn clone (or clone a git repo, or just start a new one).

mkdir ~/project.git
cd ~/project.git
git-svn clone svn://project-server/trunk
git repack #for good measure

Plugin your usb flash drive/stick/external harddrive, I'll presume it's a vfat/fat32/fat16 formated device mounted at /media/flash.
Create an empty repository on the drive, I'll use a bare one as there's no need to keep the working copy as well.

mkdir /media/flash/project.git
git --bare init /media/flash/project.git

Then add the flash drive git repo as a remote source in your local git repo. "flash" is the name I've given to the remote branch reference, you can call it whatever you like.

git remote add flash /media/flash/project.git

If you push immediately it will fail (as I discovered) because fat doesn't support the execute flag on files, so all the hooks are automatically active. I deleted all the hooks as I wasn't planning on using them, this may be wrong so no promises, but it seems ok so far for me. So remove the hooks with:

rm /media/flash/project.git/hooks/*

Then push your current local copy to the flash drive with:

git push flash

This will copy all your committed work onto the flash drive, even if you haven't pushed it upstream to the svn server with "git-svn dcommit" yet. Bonus! It won't copy any of your branches across though, so you if you want them you can add those independently with:

git push flash mybranch

Now move over to the second computer and plug the flash drive in. I'm making the same assumptions on paths and devices. Do another completely independent svn checkout as above:

mkdir ~/project.git
cd ~/project.git
git-svn clone svn://project-server/trunk
git repack #for good measure

Then add the flash drive's repo to the git repo on the second pc and pull all changes from the flash drive, optionally including any branches:

git remote add flash /media/flash/project.git
git pull flash master
git pull flash mybranch #if you like

When you've committed changes to git or pulled the latest changes from svn on either pc, you can then update the flash drive with the simple command:

git push flash

Which pushes all your changes on your master branch on to the flash drive. You are now ready to run the pull command on the other computer to get back in sync:

git pull flash master

If you don't push changes to the flash drive before committing to svn then things will be very simple. If you push changes to the flash drive, and then commit them to the svn server you will need to do a little more work. This is because when you run "git svn dcommit" it pushes your latest git commits to the svn server, deletes your locally committed changes, and then fetches them back from the svn server. This means that git won't recognise your local changes as being the same as the ones on the flash drive because they have different commit message and SHA1 hash. Attempting to push to the flash drive fails with the message "! [rejected] master -> master (non-fast forward)" as the old copy of the commits are still there.

To resolve this you need to throw away the matching set of changes on the flash drive. To do this you can use git reset as follows, where HEAD~1 should be the number of commits you need to throw away (eg HEAD~3 to throw away the last 3 commits that were pushed to the flash drive):

cd /media/flash/project.git
git --bare log #to see how many changes don't have svn information
git --bare reset HEAD~1

You can then push your changes as above.

cd ~/project.git
git push flash

I've glossed over subtleties with fetch vs pull, but hopefully you will find this useful.

This howto makes use of git's ability to pull from multiple sources, and I've found that git quite happily copes with changes that were checked in to svn coming via the flash drive, even when later running "git-svn rebase".

Please do comment or contact me with any problems, errors, extra info and feedback, and let me know if it was useful.


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Joseph Morse said...

Thanks, was searching for a answer for this this morning since i figured why keep pushing and pulling form github when I now knew you could do it locally from a networked computer or flash drive.

I skipped the step you talked about with rm the hooks dir though as I figured as with github you have to specify the master branch the first time you push and and it worked.

So now I can just keep a up to date copy locally and only have to push to github when i have changes I want to push there.

@Shun Chang completely defeats the purpose of working with version control

Tim Abell said...

You're welcome. I believe the example hooks have been renamed by the git developers so they no longer cause problems on filesystems that don't have a +x flag.