Mach can read settings in from a set of configuration files. These configuration files are either named machrc or .machrc and are specified by the bootstrap script. In mozilla-central, these files can live in ~/.mozbuild and/or topsrcdir.

Settings can be specified anywhere, and used both by mach core or individual commands.

Core Settings

These settings are implemented by mach core.

  • alias - Create a command alias. This is useful if you want to alias a command to something else, optionally including some defaults. It can either be used to create an entire new command, or provide defaults for an existing one. For example:
mochitest = mochitest -f browser
browser-test = mochitest -f browser

Defining Settings

Settings need to be explicitly defined, along with their type, otherwise mach will throw when trying to access them.

To define settings, use the SettingsProvider() decorator in an existing mach command module. E.g:

from mach.decorators import SettingsProvider

class ArbitraryClassName(object):
    config_settings = [
        ('', 'string', "A helpful description"),
        ('foo.baz', 'int', "Another description", 0, {'choices': set([0,1,2])}),

@SettingsProvider’s must specify a variable called config_settings that returns a list of tuples. Alternatively, it can specify a function called config_settings that returns a list of tuples.

Each tuple is of the form:

('<section>.<option>', '<type>', '<description>', default, extra)

type is a string and can be one of: string, boolean, int, pos_int, path

description is a string explaining how to define the settings and where they get used. Descriptions should ideally be multi-line paragraphs where the first line acts as a short description.

default is optional, and provides a default value in case none was specified by any of the configuration files.

extra is also optional and is a dict containing additional key/value pairs to add to the setting’s metadata. The following keys may be specified in the extra dict:

  • choices - A set of allowed values for the setting.


Sometimes a section should allow arbitrarily defined options from the user, such as the alias section mentioned above. To define a section like this, use * as the option name. For example:

('foo.*', 'string', 'desc')

This allows configuration files like this:

arbitrary1 = some string
arbitrary2 = some other string

Finding Settings

You can see which settings are available as well as their description and expected values by running:

./mach settings  # or
./mach settings --list

Accessing Settings

Now that the settings are defined and documented, they’re accessible from individual mach commands if the command receives a context in its constructor. For example:

from mach.decorators import (

class ExampleSettings(object):
    config_settings = [
        ('a.b', 'string', 'desc', 'default'),
        ('', 'string', 'desc',),
        ('foo.baz', 'int', 'desc', 0, {'choices': set([0,1,2])}),

class Commands(object):
    def __init__(self, context):
        self.settings = context.settings

    @Command('command', category='misc',
             description='Prints a setting')
    def command(self):
        for option in