Command Runner guide

Introduction

The ansible_collections.community.general.plugins.module_utils.cmd_runner module util provides the CmdRunner class to help execute external commands. The class is a wrapper around the standard AnsibleModule.run_command() method, handling command arguments, localization setting, output processing output, check mode, and other features.

It is even more useful when one command is used in multiple modules, so that you can define all options in a module util file, and each module uses the same runner with different arguments.

For the sake of clarity, throughout this guide, unless otherwise specified, we use the term option when referring to Ansible module options, and the term argument when referring to the command line arguments for the external command.

Quickstart

CmdRunner defines a command and a set of coded instructions on how to format the command-line arguments, in which specific order, for a particular execution. It relies on ansible.module_utils.basic.AnsibleModule.run_command() to actually execute the command. There are other features, see more details throughout this document.

To use CmdRunner you must start by creating an object. The example below is a simplified version of the actual code in community.general.ansible_galaxy_install:

from ansible_collections.community.general.plugins.module_utils.cmd_runner import CmdRunner, cmd_runner_fmt

runner = CmdRunner(
    module,
    command="ansible-galaxy",
    arg_formats=dict(
        type=cmd_runner_fmt.as_func(lambda v: [] if v == 'both' else [v]),
        galaxy_cmd=cmd_runner_fmt.as_list(),
        upgrade=cmd_runner_fmt.as_bool("--upgrade"),
        requirements_file=cmd_runner_fmt.as_opt_val('-r'),
        dest=cmd_runner_fmt.as_opt_val('-p'),
        force=cmd_runner_fmt.as_bool("--force"),
        no_deps=cmd_runner_fmt.as_bool("--no-deps"),
        version=cmd_runner_fmt.as_fixed("--version"),
        name=cmd_runner_fmt.as_list(),
    )
)

This is meant to be done once, then every time you need to execute the command you create a context and pass values as needed:

# Run the command with these arguments, when values exist for them
with runner("type galaxy_cmd upgrade force no_deps dest requirements_file name", output_process=process) as ctx:
    ctx.run(galaxy_cmd="install", upgrade=upgrade)

# version is fixed, requires no value
with runner("version") as ctx:
    dummy, stdout, dummy = ctx.run()

# Another way of expressing it
dummy, stdout, dummy = runner("version").run()

Note that you can pass values for the arguments when calling run(), otherwise CmdRunner uses the module options with the exact same names to provide values for the runner arguments. If no value is passed and no module option is found for the name specified, then an exception is raised, unless the argument is using cmd_runner_fmt.as_fixed as format function like the version in the example above. See more about it below.

In the first example, values of type, force, no_deps and others are taken straight from the module, whilst galaxy_cmd and upgrade are passed explicitly.

That generates a resulting command line similar to (example taken from the output of an integration test):

[
    "<venv>/bin/ansible-galaxy",
    "collection",
    "install",
    "--upgrade",
    "-p",
    "<collection-install-path>",
    "netbox.netbox",
]

Argument formats

As seen in the example, CmdRunner expects a parameter named arg_formats defining how to format each CLI named argument. An “argument format” is nothing but a function to transform the value of a variable into something formatted for the command line.

Argument format function

An arg_format function should be of the form:

def func(value):
    return ["--some-param-name", value]

The parameter value can be of any type - although there are convenience mechanisms to help handling sequence and mapping objects.

The result is expected to be of the type Sequence[str] type (most commonly list[str] or tuple[str]), otherwise it is considered to be a str, and it is coerced into list[str]. This resulting sequence of strings is added to the command line when that argument is actually used.

For example, if func returns:

  • ["nee", 2, "shruberries"], the command line adds arguments "nee" "2" "shruberries".

  • 2 == 2, the command line adds argument True.

  • None, the command line adds argument None.

  • [], the command line adds no command line argument for that particular argument.

Convenience format methods

In the same module as CmdRunner there is a class cmd_runner_fmt which provides a set of convenience methods that return format functions for common cases. In the first block of code in the Quickstart section you can see the importing of that class:

from ansible_collections.community.general.plugins.module_utils.cmd_runner import CmdRunner, cmd_runner_fmt

The same example shows how to make use of some of them in the instantiation of the CmdRunner object. A description of each one of the convenience methods available and examples of how to use them is found below. In these descriptions value refers to the single parameter passed to the formatting function.

  • cmd_runner_fmt.as_list()

    This method does not receive any parameter, function returns value as-is.

    • Creation:

      cmd_runner_fmt.as_list()

    • Example:

      Value

      Outcome

      ["foo", "bar"]

      ["foo", "bar"]

      "foobar"

      ["foobar"]
  • cmd_runner_fmt.as_bool()

    This method receives two different parameters: args_true and args_false, latter being optional. If the boolean evaluation of value is True, the format function returns args_true. If the boolean evaluation is False, then the function returns args_false if it was provided, or [] otherwise.

    • Creation:

      cmd_runner_fmt.as_bool("--force")

    • Example:

      Value

      Outcome

      True

      ["--force"]

      False

      []
  • cmd_runner_fmt.as_bool_not()

    This method receives one parameter, which is returned by the function when the boolean evaluation of value is False.

    • Creation:

      cmd_runner_fmt.as_bool_not("--no-deps")

    • Example:

      Value

      Outcome

      True

      []

      False

      ["--no-deps"]
  • cmd_runner_fmt.as_optval()

    This method receives one parameter arg, the function returns the string concatenation of arg and value.

    • Creation:

      cmd_runner_fmt.as_optval("-i")

    • Example:

      Value

      Outcome

      3

      ["-i3"]

      foobar

      ["-ifoobar"]
  • cmd_runner_fmt.as_opt_val()

    This method receives one parameter arg, the function returns [arg, value].

    • Creation:

      cmd_runner_fmt.as_opt_val("--name")

    • Example:

      Value

      Outcome

      abc

      ["--name", "abc"]
  • cmd_runner_fmt.as_opt_eq_val()

    This method receives one parameter arg, the function returns the string of the form {arg}={value}.

    • Creation:

      cmd_runner_fmt.as_opt_eq_val("--num-cpus")

    • Example:

      Value

      Outcome

      10

      ["--num-cpus=10"]
  • cmd_runner_fmt.as_fixed()

    This method receives one parameter arg, the function expects no value - if one is provided then it is ignored. The function returns arg as-is.

    • Creation:

      cmd_runner_fmt.as_fixed("--version")

    • Example:

      Value

      Outcome

      ["--version"]

      57

      ["--version"]
    • Note:

      This is the only special case in which a value can be missing for the formatting function. The example also comes from the code in Quickstart. In that case, the module has code to determine the command’s version so that it can assert compatibility. There is no value to be passed for that CLI argument.

  • cmd_runner_fmt.as_map()

    This method receives one parameter arg which must be a dictionary, and an optional parameter default. The function returns the evaluation of arg[value]. If value not in arg, then it returns default if defined, otherwise [].

    • Creation:

      cmd_runner_fmt.as_map(dict(a=1, b=2, c=3), default=42)

    • Example:

      Value

      Outcome

      "b"

      ["2"]

      "yabadabadoo"

      ["42"]
    • Note:

      If default is not specified, invalid values return an empty list, meaning they are silently ignored.

  • cmd_runner_fmt.as_func()

    This method receives one parameter arg which is itself is a format function and it must abide by the rules described above.

    • Creation:

      cmd_runner_fmt.as_func(lambda v: [] if v == 'stable' else ['--channel', '{0}'.format(v)])

    • Note:

      The outcome for that depends entirely on the function provided by the developer.

Other features for argument formatting

Some additional features are available as decorators:

  • cmd_runner_fmt.unpack args()

    This decorator unpacks the incoming value as a list of elements.

    For example, in ansible_collections.community.general.plugins.module_utils.puppet, it is used as:

    @cmd_runner_fmt.unpack_args
def execute_func(execute, manifest):
    if execute:
        return ["--execute", execute]
    else:
        return [manifest]

runner = CmdRunner(
    module,
    command=_prepare_base_cmd(),
    path_prefix=_PUPPET_PATH_PREFIX,
    arg_formats=dict(
        # ...
        _execute=cmd_runner_fmt.as_func(execute_func),
        # ...
    ),
)

    Then, in community.general.puppet it is put to use with:

    with runner(args_order) as ctx:
    rc, stdout, stderr = ctx.run(_execute=[p['execute'], p['manifest']])
  • cmd_runner_fmt.unpack_kwargs()

    Conversely, this decorator unpacks the incoming value as a dict-like object.

  • cmd_runner_fmt.stack()

    This decorator assumes value is a sequence and concatenates the output of the wrapped function applied to each element of the sequence.

    For example, in community.general.django_check, the argument format for database is defined as:

    arg_formats = dict(
    # ...
    database=cmd_runner_fmt.stack(cmd_runner_fmt.as_opt_val)("--database"),
    # ...
)

    When receiving a list ["abc", "def"], the output is:

    ["--database", "abc", "--database", "def"]

Command Runner

Settings that can be passed to the CmdRunner constructor are:

  • module: AnsibleModule

    Module instance. Mandatory parameter.

  • command: str | list[str]

    Command to be executed. It can be a single string, the executable name, or a list of strings containing the executable name as the first element and, optionally, fixed parameters. Those parameters are used in all executions of the runner.

  • arg_formats: dict

    Mapping of argument names to formatting functions.

  • default_args_order: str

    As the name suggests, a default ordering for the arguments. When this is passed, the context can be created without specifying args_order. Defaults to ().

  • check_rc: bool

    When True, if the return code from the command is not zero, the module exits with an error. Defaults to False.

  • path_prefix: list[str]

    If the command being executed is installed in a non-standard directory path, additional paths might be provided to search for the executable. Defaults to None.

  • environ_update: dict

    Pass additional environment variables to be set during the command execution. Defaults to None.

  • force_lang: str

    It is usually important to force the locale to one specific value, so that responses are consistent and, therefore, parseable. Please note that using this option (which is enabled by default) overwrites the environment variables LANGUAGE and LC_ALL. To disable this mechanism, set this parameter to None. In community.general 9.1.0 a special value auto was introduced for this parameter, with the effect that CmdRunner then tries to determine the best parseable locale for the runtime. It should become the default value in the future, but for the time being the default value is C.

When creating a context, the additional settings that can be passed to the call are:

  • args_order: str

    Establishes the order in which the arguments are rendered in the command line. This parameter is mandatory unless default_args_order was provided to the runner instance.

  • output_process: func

    Function to transform the output of the executable into different values or formats. See examples in section below.

  • check_mode_skip: bool

    Whether to skip the actual execution of the command when the module is in check mode. Defaults to False.

  • check_mode_return: any

    If check_mode_skip=True, then return this value instead.

Additionally, any other valid parameters for AnsibleModule.run_command() may be passed, but unexpected behavior might occur if redefining options already present in the runner or its context creation. Use with caution.

Processing results

As mentioned, CmdRunner uses AnsibleModule.run_command() to execute the external command, and it passes the return value from that method back to caller. That means that, by default, the result is going to be a tuple (rc, stdout, stderr).

If you need to transform or process that output, you can pass a function to the context, as the output_process parameter. It must be a function like:

def process(rc, stdout, stderr):
    # do some magic
    return processed_value    # whatever that is

In that case, the return of run() is the processed_value returned by the function.

PythonRunner

The PythonRunner class is a specialized version of CmdRunner, geared towards the execution of Python scripts. It features two extra and mutually exclusive parameters python and venv in its constructor:

from ansible_collections.community.general.plugins.module_utils.python_runner import PythonRunner
from ansible_collections.community.general.plugins.module_utils.cmd_runner import cmd_runner_fmt

runner = PythonRunner(
    module,
    command=["-m", "django"],
    arg_formats=dict(...),
    python="python",
    venv="/path/to/some/venv",
)

The default value for python is the string python, and the for venv it is None.

The command line produced by such a command with python="python3.12" is something like:

/usr/bin/python3.12 -m django <arg1> <arg2> ...

And the command line for venv="/work/venv" is like:

/work/venv/bin/python -m django <arg1> <arg2> ...

You may provide the value of the command argument as a string (in that case the string is used as a script name) or as a list, in which case the elements of the list must be valid arguments for the Python interpreter, as in the example above. See Command line and environment for more details.

If the parameter python is an absolute path, or contains directory separators, such as /, then it is used as-is, otherwise the runtime PATH is searched for that command name.

Other than that, everything else works as in CmdRunner.

Added in version 4.8.0.