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Contributing to Checkmk

We welcome contributions to Checkmk on Github.

Most contributions to Checkmk are small bug-fixes, enhancements of existing features, or completely new plugins. The guidelines below address these types of contributions.

If you would like to make a major change to Checkmk, please contact us first. Let's talk about what you want to do. Somebody else may already be working on it, or there are certain topics you should know before implementing the change.

We love to work with community contributors and want to make sure contributions and time investments are as effective as possible. That's why it is important to us to discuss major changes you might be planning in order to jointly agree on the best solution approach to the problem at hand. This preempts potential issues during the code reviews of pull requests.

Contributing code

In general, we follow the standard GitHub workflow which roughly works like this:

  1. Fork the repository on GitHub
  2. Clone the forked repository to your own machine
  3. Commit changes to your own feature branch
  4. Push your work back up to your forked repository
  5. Submit a Pull request (PR) so that we can review your changes

Have a careful look at the following chapters if your are working like this for the first time. The following document also includes a lot of details about our coding standards, needed tools and so on.

How to prepare for contributing?

We are developing Checkmk on Ubuntu Linux systems. It's not a hard requirement, but most helper scripts are optimized for this, so we highly recommend it for best experience.

To set up the development environment do the following:

  • Setup a Checkmk source code working copy on your computer

    First you need a local fork of the Checkmk git. To create it, press the fork button on the Checkmk GitHub page. This will create a copy of the repository in your own GitHub account and you'll see a note that it’s been forked as: YourName/checkmk.

    You now need to clone your own copy to your computer. This can be done using the command line on your computer:

    We do this from ~/git directory. This will result in a ~/git/checkmk directory.

    $ mkdir ~/git
    $ cd ~/git
    $ git clone
  • Then change to the just created project directory.

    $ cd checkmk
  • Install development dependencies

    Before you can start working on Checkmk, you will have to install some additional software, like tools for development and testing. Execute this in the project directory:

    $ make setup

    This is optimized for Ubuntu, but you may also get all the required programs on other platforms.

    After the dependencies have been installed, you could execute the shipped tests to ensure everything is working fine before start making changes to Checkmk. If you like to do this, please have a look at the How to execute tests? chapter.

Once done, you are ready for the next chapter.

How to change Checkmk?

The number one rule is to put each piece of work on its own branch. In most of the cases your development will be based on the master branch. So lets start like this:

$ git checkout master
$ git checkout -b my-feature-branch

The first command ensures you start with the master branch. The second command created the branch my-feature-branch. Pick some descriptive name you can remember later.

Let's check if everything worked fine:

$ git status
On branch my-feature-branch

Now start developing and create one or multiple commits.

Important: Do one thing in one commit, e.g. don't mix code reorganization and changes of the moved lines. Separate this in two commits.

Make sure that you commit in logical blocks and write good commit messages.

If you have finished your work, it's a good time to execute the tests locally to ensure you did not break anything.

Once you are done with the commits and tests in your feature branch you could push them to your own GitHub fork like this.

$ git push -u origin my-feature-branch

In the output of this command you will see a URL which you can open to create a pull request from your feature branch.

On GitHub in your browser, submit a pull request from your my-feature-branch to the official Checkmk branch you forked from.

The Travis CI bot will start testing your commits for issues. In case there are issues, it will send you a mail and ask you to fix the issues.

Help: I have a conflict

If you are working on a change in a file while the same file changes in the official repository, this will produce a merge conflict once you try to upstream your change.

To avoid that, it is recommended to rebase your own changes often on top of the current upstream branch.

To be able to do this, you need to prepare your project directory once with this command:

git remote add upstream

From now, you can always update your feature branches with this command:

git pull --rebase upstream master

Using rebase instead of merge gives us a clean git history.

Help: I need to change my commits

In case Travis notifies you about issues or the reviewer asks you to change your code, you will have to rework your commits. Be sure, we don't want to upset you :-).

There are several ways to update your changes in Git. We want to have as clean as possible commits, so the best is to apply the changes in a new commit and then meld them together with the previous commit.

This article on how to amend a commit may help you.

How to execute tests?

The public repository of Checkmk is integrated with Travis CI. Each time a Pull request is submitted, Travis will have a look at the changes.

Important: We only review PRs that are confirmed to be OK by Travis.

It is recommended to run all tests locally before submitting a PR. If you want to execute the full test suite, you can do this by executing these commands in the project base directory:

$ make -C tests test-pylint
$ make -C tests test-bandit
$ make -C tests test-unit
$ make -C tests test-python-futurize
$ make -C tests test-format-python

$ make -C tests-py3 test-pylint
$ make -C tests-py3 test-unit
$ make -C tests-py3 test-mypy-raw

Some of these commands take several minutes, for example the command test-format-python because it tests the formatting of the whole code base. Normally you only change a small set of files in your commits. If you execute yapf -i [filename] to format the changed code, this should be enough and you don't need to execute the formatting test at all.

We highly recommend to integrate yapf, pylint and mypy into the editor you work with. Most editors will notify you about issues in the moment you edit the code.

You could also push your changed to your forked repository and wait for Travis to execute the tests for you, but that takes several minutes for each try.

Style guide: Guidelines for coding check plug-ins

Respect the Guidelines for coding check plug-ins.

Style guide: Commit messages

  • Use the present tense ("Add feature" not "Added feature")
  • Use the imperative mood ("Move cursor to..." not "Moves cursor to...")
  • The first line is a short title (limit to 72 characters or less)
  • Reference issues and pull requests liberally after the first line
  • Write good commit messages

Style guide: Python

The guidelines listed here are binding for the development at Checkmk in Python.

This list is intended to grow in practice and does not claim to be exhaustive.

First orientate yourself on the existing code. If it doesn't fit these guidelines at all, it might be better to first orientate yourself on the existing code and adapt the code to these guidelines separately from any content changes.

Zen of Python.

Checkmk is mostly written in Python. At the moment the most of the code base is using Python 2.7. We are already preparing to change to Python 3, but this will take some time. We plan to finish this until 2020. For the moment Python 2.7 is the language to use.

Only rely on non-standard modules that are mentioned in the Pipfile.

Agent plugins: Supported Python versions

The agent plugins need to be executed on older Linux systems which may have very old Python versions available. For this reason we need to use the old Python 2.5 compatible syntax here.

On the monitored host we use Python for some popular agent plugins (like mk_logwatch). These are currently built to support Python 2.5 to Python 2.7.

Python plugins that are incompatible to 2.5, for example because some 3rd party library is not available with 2.5, need to be syntax compatible with 2.5 for the moment, but are allowed to terminate with a helpful error message about this incompatibility.

Use #!/usr/bin/env python as shebang.

Completely new plugins should be written to be compatible with Python 2.7 and Python 3.

In case you want to explicitly create a Python 3 agent plugin, use #!/usr/bin/env python3 as shebang.


Don't use star import like from module import *. They make it hard to understand which names are really available and needed in the current namespace.

Exception handling

  • Easier to ask for forgiveness than permission

    def get_status(file):
        if not os.path.exists(file):
            print "file not found"
        return open(file).readline()


    def get_status(file):
            return open(file).readline()
        except EnvironmentError as e:
            print "Unable to open file: %s" % e
  • Be as specific as possible when catching exceptions

  • Keep try-blocks as small as possible

  • Don't use except: (Slightly better for special cases: except Exception)

Paths and files

  • Use pathlib2 / pathlib (in Python 3). To be more future-proof, import like this:

    from pathlib2 import Path
  • Use context-managers (the with keyword) to open files.

  • You are welcome to refactor old style file IO to pathlib (with tests :-))

String formatting

  • Use classic format strings (%s) for the time being. We'll move over to the new format() syntax in the future, but for the moment we'd like to stay consistent.

Sub processes

  • Use mechanisms that are natively available in Python instead of subprocess/command line tools. Example: Don't use tar command line tools. Use the tarfile module instead. There may be good reasons to go with the command line tools in special situations.

  • Use secure methods for calling external programs to prevent shell injections

    • Use the subprocess module instead of os.system() or os.popen()
    • Use shell=False and close_fds=True with subprocess.
    • Use pipes.quote() in case you need to create a command line string

Argument parsing

  • Use argparse. In Checkmk where we have Python 2.7. In agent plugins, which have to support Python <2.5, use optparse.


  • Use logger or cmk.log, cmk.gui.log as base for logging
  • Add a logger object to self._logger to all classes (Use either a class/object specific child logger, the module level logger or cmk.log.logger)
  • Don't use format strings for log messages. Use:'Hello, %s', world)

HTTP requests

  • Use requests, it's great!
  • Work with requests sessions in case you need to perform multiple requests

Times and Dates

  • Use top level functionality, e.g.: datetime, dateutil


  • Document the non-obvious. Don't document the how, do document the why.
  • Use doc-strings for classes and methods.

Code structuring

  • Use the right data structure for combining data. For more complex code it is important to carefully think about this. This may help you find the right data structure for your data. In increasing order of preference:
    • Dictionaries: Worst possible representation. One has no clue whatsoever about the valid keys nor the valid values for a given key. In addition, they are mutable, which might not be what one wants. In effect, we are abusing a useful mapping type to simulate a struct.
    • Tuples of varying length: Basically the same as dictionaries with all the downsides.
    • Tuples of a fixed length: Slightly better, they have a fixed number of slots and are immutable. Still, one has no clue what a slot should mean.
    • collectons.namedtuple: A bit better than tuples of a fixed length, at least the slots have names now. Still no clue about the valid values of a slot.
    • typing.NamedTuple: Kind of OK, slots have names and a type now. Still not really OO, because it is still a dumb data container, but at least we have reached a state where Pascal was in the 70s.
    • A class: This is almost what we want. Note: Classes with tons of static/class methods are actually //not// a class, they are a namespace in disguise, so we should not use them like that.
    • A class with mypy type annotations: This is the optimum. Now we're talking OO and mypy can help us tremendously during e.g. refactorings.
  • Don't use global variables unless you have to and can do so thread-safe.
  • Don't assign attributes to function objects.
  • Use abc for specifying abstract classes, methods and properties and add raise NotImplementedError() in abstract methods and properties)
  • Make class attributes explicit in the constructor or helper functions (Don't add them dynamically e.g. via a dict argument and ''getattr()'')
  • Extensive getter/setters: In Python it is acceptable to simply access class or object members directly. In case you want to protect things from external access make use of @property
  • Use @staticmethod and @classmethod decorators for methods without references to cls or self
  • Use early exits in your functions.

Module: cmk

  • The entire Python code of Checkmk should be under the main module cmk in the future
  • Below cmk.utils there is a module that provides functionalities for all components. These can be imported from anywhere. e.g. below is cmk.utils.log for logging functionalities.
  • At the first module level, the component modules are split up, e.g.:
    • cmk.base
    • cmk.gui
  • All names that a component declares on its main level may be loaded by other components.
    • Another approach may be to explicitly declare the exports in a dedicated sub module, e.g.
    • e.g. if the name load_ec_rule_packs is registered in cmk/ec/, the GUI code may access
    • Names from submodules must not be imported from other components.
  • For the CME/CEE there is a module hierarchy under cmk/cee or cmk/cme. The same rules apply as for cmk itself.

Code formatting

  • We supply an .editorconfig file, which is used to automatically configure your editor to adhere to the most basic formatting style, like indents or line-lengths. If your editor doesn't already come with Editorconfig support, install one of the available plugins.

  • We use YAPF for automatic formatting of the Python code. Have a look below for further information.

  • Multi line imports: Use braces instead of continuation character

    from germany import bmw, \
        mercedes, \


    from germany import (

Automatic formatting

The style definition file, .style.yapf, lives in the root directory of the project repository, where YAPF picks it up automatically. YAPF itself lives in a virtualenv managed by pipenv in check_mk/virtual-envs/2.7/.venv, you can run it with make format-python or scripts/run-pipenv run yapf.

Manual invocation: Single file

yapf -i []

Manual invocation: Whole code base

If you want to format all Python files in the repository, you can run:

make format-python

Integration with CI

Our CI executes the following formatting test on the whole code base:

make -C tests test-format-python

Our review tests jobs prevent un-formatted code from being added to the repository.

Editor integration: *macs

  • plugins for vim and emacs with installation instructions can be found here:
  • in Spacemacs yapfify-buffer is available in the Python layer; formatting on save can be enabled by setting ''python-enable-yapf-format-on-save'' to ''t''
  • In Emacs with elpy call the function 'elpy-yapf-fix-code'. Because there are many large files you may want to increase the timeout for rpc calls by setting ''elpy-rpc-timeout'' to ''20''

Editor integration: vim

  • It is recommended to use yapf as fixer for ALE

Configure YAPF as fixer in your ~/vimrc. This way the file gets fixed on every save:

let g:ale_fixers = {'python': ['isort']}
let g:ale_fix_on_save = 1
  • for vim formatting on save should work with autocmds

Type checking: mypy

Code can be checked manually with make -C tests-py3 test-mypy.

The configuration file is mypy.ini and lives in the root directory of the Checkmk repository. For info about how to type hint refer to mypy docs - Type hints cheat sheet (Python 2).


This is where ALE comes in again. To include mypy there adjust the following things in the .vimrc:

  • Add mypy to the liners. With me it looks like this:

    let g:ale_linters = {
    \ 'python': ['pylint', 'mypy'],
    \ 'javascript': ['eslint'],
  • Then tell the linter how to run mypy:

    let g:ale_python_mypy_executable = 'scripts/run-mypy'
    let g:ale_python_mypy_options = '--config-file=../mypy.ini'

The mypy-Checker should run with this. With ":ALEInfo" you get information about the error diagnosis below, if it doesn't work.

Editor integration: *macs

  • The mypy.ini should be found by Flycheck without further configuration.

  • To use the correct mypy executable a .dir-locals.el in the root directory of the Checkmk repository is used.

  • Flycheck by default does not execute multiple checkers. To enable the mypy checker after the pylint checker the following snippet can be used e.g. in the dotspacemacs/user-config:

      (with-eval-after-load 'flycheck
        (flycheck-add-next-checker 'python-pylint 'python-mypy))
  • To disable the risky variable warning that is triggered by setting the mypy executable the safe-local-variables variable has to be extended by:

    (eval setq flycheck-python-mypy-executable
                (projectile-locate-dominating-file default-directory dir-locals-file)
  • An example value of the safe-local-variables variable is e.g.:

    ((eval setq flycheck-python-mypy-executable
            (projectile-locate-dominating-file default-directory dir-locals-file)
     (py-indent-offset . 4)
     (encoding . utf-8))

Style guide: Shell scripts

The Linux / Unix agents and several plugins are written in shell code for best portability and transparency. In case you want to change something respect the following things:

  • Bash scripts are written for Bash version 3.1 or newer

  • Use shellcheck for your changes before submitting patches.

    The best results are achieved with a direct integration into the editor, so that you are immediately informed about possible problems during programming. The agent itself is not clean at the moment, but we aim to clean this up in the near future.

  • Format the code according to the Google's Shell Style Guidelines with these exceptions:

    • Line length up to 100 characters is allowed
    • Use 4 spaces for indentation
  • You may use shfmt to help with formatting.

    If you don't have a Go environment ready, the easiest way is to use it is using a prepared docker image (See bottom of project README). We have a shortcut to this, which is also used by our CI system.

    Execute this in Checkmk git directory:

  sudo docker run --rm -v "$(pwd):/sh" -w /sh peterdavehello/shfmt shfmt -i 4 -ci -w agents/check_mk_agent.linux


The User interface of Checkmk can be localized. Currently we maintain a German localization of Checkmk for all users. We are open to support other languages when the localization is in a good state and nearly complete.

If you are interested: We can use for upstream localizations. Please contact us if you are interested.

Translation of technical terms

Technical terms outside of Checkmk like "container" may be translated according to the common usage for that technology.

There are several terms in Checkmk that may be kept for a better understanding. Some of them are:

  • Host
  • Service
  • Check
  • Item


Be consistent in the terms you use for a thing. E.g. in case for a server one could say something like "host", "system", "server" or "device". Decide to use one name for one thing and use it consistently in all translations.

Copyright and Licensing

The open source part of Checkmk is licensed under the terms of the GNU GPLv2 License. Any code brought in must be compatible with those terms.

You need to make sure that the code you send us in your pull request is GPLv2 compatible.

You can’t perform that action at this time.