This chapter will get you started with Borg development.

Borg is written in Python (with a little bit of Cython and C for the performance critical parts).


… are welcome!

Some guidance for contributors:

  • Discuss changes on the GitHub issue tracker, on IRC or on the mailing list.

  • Make your PRs on the master branch (see Branching Model for details).

  • Do clean changesets:

    • Focus on some topic, resist changing anything else.

    • Do not do style changes mixed with functional changes.

    • Try to avoid refactorings mixed with functional changes.

    • If you need to fix something after commit/push:

      • If there are ongoing reviews: do a fixup commit you can squash into the bad commit later.

      • If there are no ongoing reviews or you did not push the bad commit yet: amend the commit to include your fix or merge the fixup commit before pushing.

    • Have a nice, clear, typo-free commit comment.

    • If you fixed an issue, refer to it in your commit comment.

    • Follow the style guide (see below).

  • If you write new code, please add tests and docs for it.

  • Run the tests, fix any issues that come up.

  • Make a pull request on GitHub.

  • Wait for review by other developers.

Branching model

Borg development happens on the master branch and uses GitHub pull requests (if you don’t have GitHub or don’t want to use it you can send smaller patches via the borgbackup mailing list to the maintainers).

Stable releases are maintained on maintenance branches named x.y-maint, eg. the maintenance branch of the 1.0.x series is 1.0-maint.

Most PRs should be filed against the master branch. Only if an issue affects only a particular maintenance branch a PR should be filed against it directly.

While discussing / reviewing a PR it will be decided whether the change should be applied to maintenance branches. Each maintenance branch has a corresponding backport/x.y-maint label, which will then be applied.

Changes that are typically considered for backporting:

  • Data loss, corruption and inaccessibility fixes.

  • Security fixes.

  • Forward-compatibility improvements.

  • Documentation corrections.

Maintainer part

From time to time a maintainer will backport the changes for a maintenance branch, typically before a release or if enough changes were collected:

  1. Notify others that you’re doing this to avoid duplicate work.

  2. Branch a backporting branch off the maintenance branch.

  3. Cherry pick and backport the changes from each labelled PR, remove the label for each PR you’ve backported.

    To preserve authorship metadata, do not follow the git cherry-pick instructions to use git commit after resolving conflicts. Instead, stage conflict resolutions and run git cherry-pick --continue, much like using git rebase.

    To avoid merge issues (a cherry pick is a form of merge), use these options (similar to the git merge options used previously, the -x option adds a reference to the original commit):

    git cherry-pick --strategy recursive -X rename-threshold=5% -x
  4. Make a PR of the backporting branch against the maintenance branch for backport review. Mention the backported PRs in this PR, e.g.:

    Includes changes from #2055 #2057 #2381

    This way GitHub will automatically show in these PRs where they were backported.

Historic model

Previously (until release 1.0.10) Borg used a “merge upwards” model where most minor changes and fixes where committed to a maintenance branch (eg. 1.0-maint), and the maintenance branch(es) were regularly merged back into the main development branch. This became more and more troublesome due to merges growing more conflict-heavy and error-prone.

Code and issues

Code is stored on GitHub, in the Borgbackup organization. Issues and pull requests should be sent there as well. See also the Support section for more details.

Style guide

We generally follow pep8, with 120 columns instead of 79. We do not use form-feed (^L) characters to separate sections either. Compliance is tested automatically when you run the tests.

Continuous Integration

All pull requests go through GitHub Actions, which runs the tests on misc. Python versions and on misc. platforms as well as some additional checks.

Output and Logging

When writing logger calls, always use correct log level (debug only for debugging, info for informative messages, warning for warnings, error for errors, critical for critical errors/states).

When directly talking to the user (e.g. Y/N questions), do not use logging, but directly output to stderr (not: stdout, it could be connected to a pipe).

To control the amount and kinds of messages output emitted at info level, use flags like --stats or --list, then create a topic logger for messages controlled by that flag. See _setup_implied_logging() in borg/ for the entry point to topic logging.

Building a development environment

First, just install borg into a virtual env as described before.

To install some additional packages needed for running the tests, activate your virtual env and run:

pip install -r requirements.d/development.txt

This project utilizes pre-commit to lint code before it is committed. Although pre-commit is installed when running the command above, the pre-commit hooks will have to be installed separately. Run this command to install the pre-commit hooks:

pre-commit install

Running the tests

The tests are in the borg/testsuite package.

To run all the tests, you need to have fakeroot installed. If you do not have fakeroot, you still will be able to run most tests, just leave away the fakeroot -u from the given command lines.

To run the test suite use the following command:

fakeroot -u tox  # run all tests

Some more advanced examples:

# verify a changed tox.ini (run this after any change to tox.ini):
fakeroot -u tox --recreate

fakeroot -u tox -e py38  # run all tests, but only on python 3.8

fakeroot -u tox borg.testsuite.locking  # only run 1 test module

fakeroot -u tox borg.testsuite.locking -- -k '"not Timer"'  # exclude some tests

fakeroot -u tox borg.testsuite -- -v  # verbose py.test

Important notes:

  • When using -- to give options to py.test, you MUST also give borg.testsuite[.module].

Adding a compression algorithm

If you want to add a new compression algorithm, please refer to #1633 and leave a post there in order to discuss about the proposal.


Generated files

Usage documentation (found in docs/usage/) and man pages (docs/man/) are generated automatically from the command line parsers declared in the program and their documentation, which is embedded in the program (see These are committed to git for easier use by packagers downstream.

When a command is added, a command line flag changed, added or removed, the usage docs need to be rebuilt as well:

python build_usage
python build_man

However, we prefer to do this as part of our Creating a new release preparations, so it is generally not necessary to update these when submitting patches that change something about the command line.

Building the docs with Sphinx

The documentation (in reStructuredText format, .rst) is in docs/.

To build the html version of it, you need to have Sphinx installed (in your Borg virtualenv with Python 3):

pip install -r requirements.d/docs.txt

Now run:

cd docs/
make html

Then point a web browser at docs/_build/html/index.html.

The website is updated automatically by ReadTheDocs through GitHub web hooks on the main repository.

Using Vagrant

We use Vagrant for the automated creation of testing environments and borgbackup standalone binaries for various platforms.

For better security, there is no automatic sync in the VM to host direction. The plugin vagrant-scp is useful to copy stuff from the VMs to the host.

The “windows10” box requires the reload plugin (vagrant plugin install vagrant-reload).


# To create and provision the VM:
vagrant up OS
# same, but use 6 VM cpus and 12 workers for pytest:
VMCPUS=6 XDISTN=12 vagrant up OS
# To create an ssh session to the VM:
vagrant ssh OS
# To execute a command via ssh in the VM:
vagrant ssh OS -c "command args"
# To shut down the VM:
vagrant halt OS
# To shut down and destroy the VM:
vagrant destroy OS
# To copy files from the VM (in this case, the generated binary):
vagrant scp OS:/vagrant/borg/borg.exe .

Creating standalone binaries

Make sure you have everything built and installed (including fuse stuff). When using the Vagrant VMs, pyinstaller will already be installed.

With virtual env activated:

pip install pyinstaller  # or git checkout master
pyinstaller -F -n borg-PLATFORM borg/
for file in dist/borg-*; do gpg --armor --detach-sign $file; done

If you encounter issues, see also our Vagrantfile for details.


Standalone binaries built with pyinstaller are supposed to work on same OS, same architecture (x86 32bit, amd64 64bit) without external dependencies.

Creating a new release


  • Make sure all issues for this milestone are closed or moved to the next milestone.

  • Check if there are any pending fixes for security issues.

  • Find and fix any low hanging fruit left on the issue tracker.

  • Check that GitHub Actions CI is happy.

  • Update CHANGES.rst, based on git log $PREVIOUS_RELEASE...

  • Check version number of upcoming release in CHANGES.rst.

  • Render CHANGES.rst via make html and check for markup errors.

  • Verify that and are complete.

  • python build_usage ; python build_man and commit.

  • Tag the release:

    git tag -s -m "tagged/signed release X.Y.Z" X.Y.Z
  • Create a clean repo and use it for the following steps:

    git clone borg borg-clean

    This makes sure no uncommitted files get into the release archive. It will also reveal uncommitted required files. Moreover, it makes sure the vagrant machines only get committed files and do a fresh start based on that.

  • Run tox and/or binary builds on all supported platforms via vagrant, check for test failures.

  • Create sdist, sign it, upload release to (test) PyPi:

    scripts/sdist-sign X.Y.Z
    scripts/upload-pypi X.Y.Z test
    scripts/upload-pypi X.Y.Z

    Note: the signature is not uploaded to PyPi any more, but we upload it to github releases.

  • Put binaries into dist/borg-OSNAME and sign them:

    scripts/sign-binaries 201912312359
  • Close the release milestone on GitHub.

  • Update with the new version number and release date.

  • Announce on:

  • Mailing list.

  • Twitter.

  • IRC channel (change /topic).

  • Create a GitHub release, include:

    • pypi dist package and signature

    • Standalone binaries (see above for how to create them).

      • For macOS, document the macFUSE version in the README of the binaries. macFUSE uses a kernel extension that needs to be compatible with the code contained in the binary.

    • A link to CHANGES.rst.