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Description:

cli config

A library to take config values from configuration files, CLI arguments, and
environment variables.
Usage #
Configuration can be provided from commandline arguments, environment variables,
and configuration files. This library makes these accessible via a uniform API.
Configuration can be provided via the three sources as follows:

commandline argument defines as -Dsome_key=some_value,
environment variables as SOME_KEY=some_value, and
config files as JSON or YAML as {'some_key': 'some_value'}.

The default lookup behavior is that commandline argument defines take precedence
over environment variables, which take precedence over the configuration file.
If a single value is requested from this configuration, the first source that
can provide the value will provide it. For example
config.string('some_key') with {'some_key': 'file_value'} in the config file
and -Dsome_key=cli_value as commandline argument returns
'cli_value'. The implication is that you can not remove keys from the
configuration file, only overwrite or append them.
If a list value is requested from this configuration, the values provided by the
various sources can be combined or not. For example
config.optionalStringList('some_key', combineAllConfigs: true) returns
['cli_value', 'file_value'].
The config is hierarchical in nature, using . as the hierarchy separator for
lookup and commandline defines. The hierarchy should be materialized in the JSON
or YAML configuration file. For environment variables __ is used as hierarchy
separator.
Hierarchical configuration can be provided via the three sources as follows:

commandline argument defines as -Dsome_key.some_nested_key=some_value,
environment variables as SOME_KEY__SOME_NESTED_KEY=some_value, and
config files as JSON or YAML as
some_key:
some_nested_key:
some_value
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The config is opinionated on the format of the keys in the sources.

Command-line argument keys should be lower-cased alphanumeric
characters or underscores, with . for hierarchy.
Environment variables keys should be upper-cased alphanumeric
characters or underscores, with __ for hierarchy.
Config files keys should be lower-cased alphanumeric
characters or underscores.

In the API they are made available lower-cased and with underscores, and
. as hierarchy separator.
Example usage #
This example creates a configuration which first looks for command-line defines
in the arguments list then looks in Platform.environment, then looks in any
local configuration file.
final config = await Config.fromArguments(arguments: arguments);
final pathValue =
config.optionalPath('my_path', resolveUri: true, mustExist: false);
print(pathValue?.toFilePath());
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