Simple Use of Git for OpenSim Development

Git is a source code version control supporting contributions from a number of developers. Git tutorial and reference material is available via including a complete online “Pro Git” book.

Contributions are made by “non-core” developers who can submit a “patch” attached to the OpenSim Mantis issue tracking system at These are reviewed by one or more OpenSim core developers who can, if appropriate, then approve and insert the patch to appear in the OpenSim Git development “master” at

The notes here are the process I used, after a few experiments, to establish a simple Git environment and workflow on a Windows 10 desktop to allow me to create patches in the required form to potentially contribute to OpenSim.

Setting up the Git Tools, a link to the OpenSim Master Repository and Making a Branch for your Changes


  1. Create an account on if you don’t already have one. That will allow you to create a simple profile including the name, e-mail address and thumbnail image you want to use when making contributions to projects.
  2. Obtain and install GitHub Desktop (Windows or Mac) via This creates two desktop icons, one for the GitHub GUI and one for a Git Shell (using Windows Power Shell) which work in harmony.
  3. Configure Git on launch to add in your GitHub username and e-mail which will be used to label contributions or patches you make. This creates [user] section “name” and “email” lines in your .gitconfig file. It can also be done using the Git Shell and using the commands…
    git config "Your Name"
    git config
  4. If you have not already done so, launch the Git Shell and you can set the editor you wish to use to add messages into patches to one that can handle Unix line endings (which Windows Notepad does not). Note the editor executable must be on the Windows PATH. E.g.
    git config --global core.editor emacs
  5. OpenSim wants patches that do not add Windows style line endings. Automatic translation between the Unix line ending reporitory files and local Windows files is possible, but to be certain, I explicitly set this via…
    git config --global core.autocrlf false
  6. After these steps your C:\Users\username\.gitconfig file should have something like these entries… amongst others…
    	name = Your Name
    	email =
    	autocrlf = false
    	editor = emacs
  7. Now, as a potential non-core OpenSim contributor you can establish a (read-only) “clone” of the main OpenSim Master Repository. I found the easiest way to do that was using the Git Shell” and typing in…

    This also sets all the files as “tracked”. Changes to any of these files are monitored so they can be added to any commit. Newly added files though need to be explicitly handled (see later).

  8. Note that git:// at is a mirror of the main repository at git:// but you should use the main repository as it will be up to date.
  9. With the default location for GitHub files of C:\Users\username\Documents\GitHub this will create a directory in C:\Users\username\Documents\GitHub which contains the OpenSim files and a special .git directory which supports all the Git operations, your commits and patch creation operations.
  10. The Git Shell by default starts in C:\Users\username\Documents\GitHub. You can change to work in a Github managed project simply by moving into its directory. E.g. into the “opensim” directory using…
    cd opensim

    Note that this will also show the current branch you are set to (initially “[master]“).

  11. While in Git Shell and set to the “master” you can also do a “pull” of the current OpenSim Master Repository content to your local filestore with…
    git pull

    You can use also this Git command at any time when set to the “master” branch to “sync” the OpenSim Master Repository files with your local copy.

  12. Also while in Git Shell create a branch off the opensim “master” called, for example, “mods” with…
    git branch mods
  13. You can change between the “master” and “mods” branches to work in using the Git Shell command…
    git checkout mods
  14. You can check the current status of your branch and whether there are outstanding modifications which are not yet committed using…
    git status


Now you are all set to make changes, and to create patches from the changes you make. I found that the “GitHub” GUI tool was most useful from this point right up to the preparation of a patch, for which I swapped back to the Git Shell. Git settings and branch changes made in the Git Shell and GitHub Tool for any GitHub project are synchronised (the magic happens in the projects .git directory).

Make your Changes

At any time, independent of whether you are using GitShell or the GitHub Tool, you can make any changes you wish to the files in C:\Users\username\Documents\GitHub\opensim by directly editing them. It is suggested you use an editor (e.g. emacs) that does not change the Unix style OpenSim file line endings

Using the GitHub Tool

  1. Select “opensim” Repository – When you run the GitHub tool you can choose one of the repository is you have set up… the opensim repository will show in the left column, if you have performed a “clone” operation. More can be added via the “+” icon in the top left corner. But for some reason “remote” does not allow for connections to repositories that are not hosted on itself.
  2. Select “master” branch and “Sync” – You can select the “master” branch in the branches drop down menu near the top left. A “Sync” button in the top right corner allows you to synchronise the OpenSim Master repository files with the local copy. The “Sync” on the “master” branch in the GitHub Tool is the same as using “git pull” on the masdter branch in Git Shell.
  3. Select “mods” branch and “Update from master” – Select the “mods” branch you made for your own changes and perform an “Update from master” to bring in any changes made recently to the “master” repository so they are available in the version you will change.
  4. Check if there are uncommitted changes – On the “mods” branch you can see a button in the top right saying if there are “No uncommitted changes” unless you have made changes you have made to the “tracked” files that are not yet notified to Git via a commit.
  5. Commit N uncommitted changes(s) – When you do have changes and want to create a single commit for all the changes.. hit the button and a panel opens to allow you to insert a short “Summary” and a longer “Description” of the changes you have made.

Making a Commit

  1. Ensure you are on the “mods” branch by selecting the “mods” branch in the GitHub Tool or in Git Shell use…
    git checkout mods
  2. Bring the branch up to date with “master” using the “Update from master” button in the top left of the GitHub Tool or in Git Shell use…
    git merge master
  3. Commit changes to all “tracked” files that are modified (so excluding any newly added files) using the “N uncommitted changes” button in the top right of the GitHub Tool and then using the form provided in the GitHub Tool or in Git Shell use…
    git commit -a
  4. Note that by default Git does not add all modified files during a commit. To add new files use:
    git add file1 file2 ...
    git commit

Creating a Patch

OpenSim development asks for patches to be prepared and attached to an OpenSimulator Mantis issue. The following commands create a patch for the last commit (-1) or changes between the current “master” – known as “origin/master” – and the commit labelled #hashtag is the one for the commit, usually 8 or so characters are sufficient to identify it) … and places the patch in a directory one up from the opensim repository files…

git format-patch -1 -o ..\tmp
git format-patch -o ..\tmp -1
git format-patch -o ..\tmp origin/master #f0e70490

Note that using –stdout > patch.txt does not work for opensim use as the line endings are Windows form)..

The results will be in C:\Users\username\Documents\GitHub\tmp

Inspect the patch to check it looks okay… an example follows…


Submitting a Patch to the OpenSimulator Community

  1. Go to the OpenSimulator “Mantis” issue tracking web site at and create an account there if you have not already done so, or login if you have.
  2. If your modifications relate to an existing Mantis issue add a comment and attach your patch file. If its a new issue create that and attach the patch file. In both cases amend the status to “patch attached” to draw the attention of the OpenSim core developers to your contribution. For a new issue, by convention, add the text [PATCH] to the front of the Mantis issue summary.
  3. After the issue has been created or modified, so a final check that the patch looks okay expand the patch to “Show Contents” and especially watching out for extra blank lines, indicating that Windows CR/LF extra symbols have wrongly been added.

OpenSim Core Developer Sign Off of a Contributed Patch

An OpenSim Core developer with appropriate permissions to commit directly to the remote master copy of OpenSim at may use a command such as the following to “sign off” a contribution…

git.exe am --signoff --ignore-space-change --keep-cr patch-filename

Reverting the Changes Made to Sync with Master – Better to Delete the Branch

As explained above, as a non-core developer you cannot directly commit changes to the remote master version of OpenSimulator. You are contributing potentially useful patches which might or might not be adopted. This can mean you have to constantly keep changing your “mods” branch to keep it in line with other changes in master. I tried to maintain a simple setup by using “revert” on each commit on the “mods” branch after creating the patch. A revert of the last commit (or any commit in fact) on the “mods” branch can be made very simply by selecting the commit and using the “Revert” button. But it turns out that this process does not allow “clean” patches to be successfully generated. The patches when applied try to refer to the commits/reverts that were in the branch history but not passed to the remote repository and give this message…

sha1 information is lacking or useless

Instead, I found its best to DELETE the “mods” branch entirely, using… (the capitalised -D flag forces the delete, so ensure you want to do that)…

git branch -D mods

Then go back and cerate a new branch “¬£mods” and work afresh in that for the next patch you wish to contribute.

Alternative workflow using Git Shell for actions performed in the GitHub GUI Tool

  • Resetting local master copy to remote contents. Ensure you are set to the opensim “master” branch.
    cd opensim
    git checkout master
    git pull
  • You can edit the message associated with the last commit using the GitHub Tool by clicking on the commit or using the Git Shell command:
    git commit --amend
  • You can see the master and any branches that exists using the GitHub Tool or using the Git Shell command:
    git branch
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