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A Ruby library for working with the CFG configuration format.


Add this line to your application's Gemfile:

gem 'cfg-config'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install cfg-config


The CFG configuration format is a text format for configuration files which is similar to, and a superset of, the JSON format. It dates from before its first announcement in 2008 and has the following aims:

  • Allow a hierarchical configuration scheme with support for key-value mappings and lists.
  • Support cross-references between one part of the configuration and another.
  • Provide a string interpolation facility to easily build up configuration values from other configuration values.
  • Provide the ability to compose configurations (using include and merge facilities).
  • Provide the ability to access real application objects safely, where supported by the platform.
  • Be completely declarative.

It overcomes a number of drawbacks of JSON when used as a configuration format:

  • JSON is more verbose than necessary.
  • JSON doesn’t allow comments.
  • JSON doesn’t provide first-class support for dates and multi-line strings.
  • JSON doesn’t allow trailing commas in lists and mappings.
  • JSON doesn’t provide easy cross-referencing, interpolation, or composition.

A simple example

With the following configuration file, test0.cfg:

a: 'Hello, '
b: 'world!'
c: {
  d: 'e'
'f.g': 'h'
christmas_morning: `2019-12-25 08:39:49`
home: `$HOME`
foo: `$FOO|bar`

You can load and query the above configuration using, for example, irb:

Loading a configuration

The configuration above can be loaded as shown below. In the REPL shell:

2.7.1 :001 > require 'CFG/config'
 => true
2.7.1 :002 > include CFG
 => Object
2.7.1 :003 > cfg = CFG::Config::new("test0.cfg")

The successful new() call returns a Config instance which can be used to query the configuration.

Access elements with keys

Accessing elements of the configuration with a simple key is not much harder than using a Hash:

2.7.1 :004 > cfg['a']
 => "Hello, "
2.7.1 :005 > cfg['b']
 => "world!"

Access elements with paths

As well as simple keys, elements can also be accessed using path strings:

2.7.1 :006 > cfg['c.d']
 => "e"

Here, the desired value is obtained in a single step, by (under the hood) walking the path c.d – first getting the mapping at key c, and then the value at d in the resulting mapping.

Note that you can have simple keys which look like paths:

2.7.1 :007 > cfg['f.g']
 => "h"

If a key is given that exists in the configuration, it is used as such, and if it is not present in the configuration, an attempt is made to interpret it as a path. Thus, f.g is present and accessed via key, whereas c.d is not an existing key, so is interpreted as a path.

Access to date/time objects

You can also get native Ruby date/time objects from a configuration, by using an ISO date/time pattern in a backtick-string:

2.7.1 :008 > cfg['christmas_morning']
 => #<DateTime: 2019-12-25T08:39:49+00:00 ((2458843j,31189s,0n),+0s,2299161j)>

Access to other Ruby objects

Access to other Ruby objects is also possible using the backtick-string syntax, provided that they are one of:

  • Environment variables
  • Public fields of public classes
  • Public static methods without parameters of public classes
2.7.1 :009 > require 'date'
 => false
2.7.1 :010 > DateTime::now - cfg['now']
 => (-148657/86400000000000)

Access to environment variables

To access an environment variable, use a backtick-string of the form $VARNAME:

2.7.1 :011 > cfg['home'] == ENV['HOME']
 => true

You can specify a default value to be used if an environment variable isn’t present using the $VARNAME|default-value form. Whatever string follows the pipe character (including the empty string) is returned if the VARNAME is not a variable in the environment.

2.7.1 :012 > cfg['foo']
 => "bar"

For more information, see the CFG documentation.