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A highly customisable application configuration interface for building terminal tools.

TTY::Config provides application configuration component for TTY toolkit.


This is a one-stop shop for all your configuration needs:

  • Read and write config files in YAML, JSON, TOML, INI, XML, HCL and Java Properties formats
  • Add custom marshallers or override the built-in ones
  • Set and read settings for deeply nested keys
  • Set defaults for undefined settings
  • Read settings with indifferent access
  • Merge configuration settings from other hash objects
  • Read values from environment variables


Add this line to your application's Gemfile:

gem "tty-config"

And then execute:

$ bundle

Or install it yourself as:

$ gem install tty-config


1. Usage

Initialize the configuration and provide the name:

config =
config.filename = "investments"

Then configure values for different nested keys with set and append:

config.set(:settings, :base, value: "USD")
config.set(:settings, :color, value: true)
config.set(:coins, value: ["BTC"])

config.append("ETH", "TRX", "DASH", to: :coins)

You can get any value by using fetch:

config.fetch(:settings, :base)
# => "USD"

# => ["BTC", "ETH", "TRX", "DASH"]

And call write to persist the configuration to investments.yml file:

# =>
# ---
# settings:
#   base: USD
#   color: true
# coins:
#  - BTC
#  - ETH
#  - TRX
#  - DASH

To read an investments.yml file, you need to provide the locations to search in:

config.append_path Dir.pwd
config.append_path Dir.home

Finally, call read to convert configuration file back into an object again:

1.1 app

An example of an application configuration:

class App
  attr_reader :config

  def initialize
    @config =
    @config.filename = "investments"
    @config.extname = ".toml"
    @config.append_path Dir.pwd
    @config.append_path Dir.home

  def self.config
    @config ||=

2. Interface

2.1 set

To set configuration setting use set method. It accepts any number of keys and value by either using :value keyword argument or passing a block:

config.set(:base, value: "USD")
config.set(:base) { "USD" }

The block version of specifying a value will mean that the value is evaluated every time it's being read.

You can also specify deeply nested configuration settings by passing sequence of keys:

config.set(:settings, :base, value: "USD")

Which is equivalent to:

config.set("settings.base", value: "USD")

Internally all configuration settings are stored as string keys for ease of working with configuration files and command line application's inputs.

2.2 set_if_empty

To set a configuration setting only if it hasn't been set before use set_if_empty:

config.set_if_empty(:base, value: "USD")

Similar to set it allows you to specify arbitrary sequence of keys followed by a key value or block:

config.set_if_empty(:settings, :base, value: "USD")

2.3 set_from_env

To read configuration options from environment variables use set_from_env. At minimum it requires a single argument which will match the name of ENV variable. The name of this parameter is case insensitive.

Given the following environment variables:

ENV["HOST"] = ""
ENV["PORT"] = "7727"

You can make the config aware of the above env variables:


Then you can retrieve values like any other configuration option:

# => ""
# => "7727"

If you want the configuration key name to be different from ENV variable name use a block:

config.set_from_env(:host) { "HOSTNAME" }
config.set_from_env(:host) { :hostname }

You can also configure settings for deeply nested keys:

config.set_from_env(:settings, :base) { "CURRENCY" }
config.set_from_env(:settings, :base) { :currency }
config.set_from_env("settings.base") { "CURRENCY" }
config.set_from_env("settings.base") { :currency }

And assuming ENV["CURRENCY"]=USD:

config.fetch(:settings, :base)
# => USD

You can also prefix your environment variables with env_prefix= or use a different separator with env_separator.

It's important to recognise that set_from_env doesn't record the value for the environment variables. They are read each time from the ENV when fetch is called.

2.4 fetch

To get a configuration setting use fetch, which can accept default value either with a :default keyword or a block that will be lazy evaluated:

config.fetch(:base, default: "USD")
config.fetch(:base) { "USD" }

Similar to set operation, fetch allows you to retrieve deeply nested values:

config.fetch(:settings, :base) # => USD

Which is equivalent to:


fetch has indifferent access so you can mix string and symbol keys, all the following examples retrieve the value:

config.fetch(:settings, :base)
config.fetch("settings", "base")
config.fetch(:settings, "base")
config.fetch("settings", :base)

2.5 merge

To merge in other configuration settings as hash use merge:

config.set(:a, :b, value: 1)
config.set(:a, :c, value: 2)

config.merge({"a" => {"c" => 3, "d" => 4}})

config.fetch(:a, :c) # => 3
config.fetch(:a, :d) # => 4

Internally all configuration settings are stored as string keys for ease of working with file values and command line applications inputs.

2.6 coerce

You can initialize configuration based on a hash, with all the keys converted to symbols:

hash = {"settings" => {"base" => "USD", "exchange" => "CCCAGG"}}
config = TTY::Config.coerce(hash)
# =>
# {settings: {base: "USD", exchange: "CCCAGG"}}

2.7 append

To append arbitrary number of values to a value under a given key use append:

config.set(:coins) { ["BTC"] }

config.append("ETH", "TRX", to: :coins)
# =>
# {coins: ["BTC", "ETH", "TRX"]}

You can also append values to deeply nested keys:

config.set(:settings, :bases, value: ["USD"])

config.append("EUR", "GBP", to: [:settings, :bases])
# =>
# {settings: {bases: ["USD", "EUR", "GBP"]}}

2.8 remove

Use remove to remove a set of values from a key.

config.set(:coins, value: ["BTC", "TRX", "ETH", "DASH"])

config.remove("TRX", "DASH", from: :coins)
# =>
# ["BTC", "ETH"]

If the key is nested the :from accepts an array:

config.set(:holdings, :coins, value: ["BTC", "TRX", "ETH", "DASH"])

config.remove("TRX", "DASH", from: [:holdings, :coins])
# =>
# ["BTC", "ETH"]

2.9 delete

To completely delete a value and corresponding key use delete:

config.set(:base, value: "USD")
# =>
# "USD"

You can also delete deeply nested keys and their values:

config.set(:settings, :base, value: "USD")
config.delete(:settings, :base)
# =>
# "USD"

You can provide an optional default value in a block that will be returned when a key is not set:

config.delete(:settings, :unknown) { |key| "#{key} isn't set" }
# =>
# "unknown isn't set"

2.10 alias_setting

In order to alias a configuration setting to another name use alias_setting.

For example, given an already existing setting:

config.set(:base, value: "baz")

You can alias it to another name:

config.alias_setting(:base, to: :currency)

And then access like any other configuration setting:

# => "USD"

Deep nested configuration options are also supported:

config.set(:settings, :base, value: "USD")

And then can be aliased like so:

config.alias_setting(:settings, :base, to: [:settings, :currency])
config.alias_setting("settings.base", to [:settings, :currency])

You can then access the deep nested settings:

config.fetch(:settings, :currency)
# => "USD"
# => "USD"

2.11 validate

To ensure consistency of the data, you can validate values being set at arbitrarily deep keys using validate method, that takes an arbitrarily nested key as its argument and a validation block.

config.validate(:settings, :base) do |key, value|
  if value.length != 3
    raise TTY::Config::ValidationError, "Currency code needs to be 3 chars long."

You can assign multiple validations for a given key and each of them will be run in the order they were registered when checking a value.

When setting value all the validations will be run:

config.set(:settings, :base, value: "PL")
# raises TTY::Config::ValidationError, "Currency code needs to be 3 chars long."

If the value is provided as a proc or a block then the validation will be delayed until the value is actually read:

config.set(:settings, :base) { "PL" }
config.fetch(:settings, :base)
# raises TTY::Config::ValidationError, "Currency code needs to be 3 chars long."

2.12 filename=

By default, TTY::Config searches for config named configuration file. To change this use filename= method without the extension name:

config.filename = "investments"

Then any supported extensions will be searched for such as .yml, .json and .toml.

2.13 extname=

By default ".yml" extension is used to write configuration out to a file but you can change that with extname=:

config.extname = ".toml"

2.14 append_path

You need to tell the TTY::Config where to search for configuration files. To search multiple paths for a configuration file use append_path or prepend_path methods.

For example, if you want to search through /etc directory first, then user home directory and then current directory do:

config.append_path("/etc/")   # look in /etc directory
config.append_path(Dir.home)  # look in user's home directory
config.append_path(Dir.pwd)   # look in current working directory

None of these paths are required, but you should provide at least one path if you wish to read a configuration file.

2.15 prepend_path

The prepend_path allows you to add configuration search paths that should be searched first.

config.append_path(Dir.pwd)   # look in current working directory second
config.prepend_path(Dir.home) # look in user's home directory first

2.16 read

There are two ways for reading configuration files and both use the read method. One attempts to guess extension and format of your data, the other allows you to request specific extension and format.

Currently the supported file formats are:

  • yaml for .yaml, .yml extensions
  • json for .json extension
  • toml for .toml extension
  • ini for .ini, .cnf, .conf, .cfg, .cf extensions
  • hcl for .hcl extensions
  • xml for .xml extension
  • jprops for .properties, .props, .prop extensions

Calling read without any arguments searches through provided locations to find configuration file and reads it. Therefore, you need to specify at least one search path that contains the configuration file together with actual filename. When filename is specified then all known extensions will be tried.

For example, to find file called investments in the current directory do:

config.append_path(Dir.pwd)       # look in current working directory
config.filename = "investments"   # file to search for

Find and read the configuration file:

You can also specify directly the file to read without setting up any search paths or filenames. If you specify a configuration with a known file extension, an appropriate format will be guessed, in this instance TOML:"./investments.toml")

In cases where you wish to specify a custom file extension, you will need to also specify the file format to use.

For example, if you have a configuration file formatted using YAML notation with extension called .config, to read it do:"investments.config", format: :yaml)

2.17 write

By default TTY::Config, persists configuration file in the current working directory with a config.yml name. However, you can change the default file name by specifying the filename and extension type:

config.filename = "investments"
config.extname = ".toml"

Now, by invoking write you will persist the current configuration to investments.toml file.

config.write   # writes "investments.toml" in the current directory

To write the current configuration to a file in a custom location, you can specify a direct location path and filename as an argument:

# may raise an error if any of the path directories are missing

Alternatively, if the filename doesn't need to change you can specify only a custom path using the :path keyword:

config.write(path: "/custom/path/to")
# may raise an error if any of the path directories are missing

If the /custom/path/to doesn't exist an error will be raised. You can set the :create option to make any missing directories in the path:

config.write("/custom/path/to/investments.toml", create: true)
config.write(path: "/custom/path/to", create: true)

When the investments.toml file already exists the TTY::Config::WriteError error will be raised.

To create a configuration file regardless of whether it exists or not, use :force flag:

config.write(force: true)
config.write("/custom/path/to/investments.toml", force: true)
config.write(path: "/custom/path/to", force: true)

By default, only the current directory is searched. You can specify additional location paths to be searched for already existing configuration to overwrite:

config.append_path("/custom/path/to")  # search in "/custom/path/to" for config file

By setting the :create option to true, you can ensure that even when no path is found that has a configuration file, the first location will be used and all missing directories created.

To ensure that a configuration file is written no matter what, use both :create and :force:

config.write(create: true, force: true)

2.18 exist?

To check if a configuration file exists within the configured search paths use exist? method:

config.exist? # => true

2.19 env_prefix=

Given the following variables:

ENV["MYTOOL_PORT"] = "7727"

You can inform configuration about common prefix using env_prefix:

config.env_prefix = "mytool"

Then set configuration key name to environment variable name:


And retrieve the value:

# => ""
# => "7727"

2.20 env_separator=

By default, the _ character is used to separate parts in the environment variable name and it can be changed using the env_separator= like so:

config.env_separator = "___"

Given the following environment variable:

ENV["SERVER__PORT"] = "123"

Then we can make configuration aware of the above variable name in one of these ways:

config.set_from_env(:server, :port)

And retrieve the value:

config.fetch(:server, :port)
# => "123"

2.21 autoload_env

The autoload_env method allows you to automatically read environment variables. In most cases you would combine it with env_prefix= to only read a subset of variables. When using autoload_env, anytime the fetch is called a corresponding environment variable will be checked.

For example, given an environment variable MYTOOL_HOST set to localhost:


And loading environment variables with a prefix of MYTOOL:

config.env_prefix = "mytool"

You can retrieve value with:

# => "localhost"

2.22 register_marshaller

There are number of built-in marshallers that handle the process of serializing internal configuration from and back into a desired format, for example, a JSON string.

Currently supported formats out-of-the-box are: YAML, JSON, TOML, INI, XML, HCL & Java Properties.

To create your own marshaller use the TTY::Config::Marshaller interface. You need to provide the implementation for the following marshalling methods:

  • marshal
  • unmarshal

In addition, you will need to specify the extension types this marshaller will handle using the extension method. The method accepts a list of names preceded by a dot:

extension ".ext1", ".ext2", ".ext3"

Optionally, you can provide a dependency or dependencies that will be lazy loaded if the extension is used. For this use the dependency method.

You can either specify dependencies as a list of names:

dependency "toml"
dependency "toml", "tomlrb"

Or provide dependencies in a block:

dependency do
  require "toml"
  require "tomlrb"

Putting it all together, you can create your own marshaller like so:

class MyCustomMarshaller
  include TTY::Config::Marshaller

  dependency "my_dep"

  extension ".ext1", ".ext2"

  def marshal(object)

  def unmarshal(content)

And then let the configuration know about your marshaller by calling the register_marshaller:

config.register_marshaller(:my_custom, MyCustomMarshaller)

Bear in mind that you can also override the built-in implementation of a marshaller. For example, if you find a better performing Ruby gem for TOML parsing, register your custom marshaller under the :toml name like so:

config.register_marshaller(:toml, MyTOMLMarshaller)

2.23 unregister_marshaller

By default, the TTY::Config is ready to recognize various extensions. See 2.16 read section for more details. But, you're free to remove the default marshallers from the internal registry with unregister_marshaller method.

For example, to remove all the built-in marshallers do:

config.unregister_marshaller :yaml, :json, :toml, :ini, :xml, :hcl, :jprops

3. Examples

3.1 Working with env vars

TTY::Config fully supports working with environment variables. For example, there are couple of environment variables that your configuration is interested in, which normally would be set in terminal but for the sake of this example we assign them:

ENV["MYTOOL_PORT"] = "7727"

Then in order to make your configuration aware of the above, you would use env_prefix= and set_from_env:

config.env_prefix = "mytool"

Or automatically load all prefixed environment variables with autoload_env:

config.env_prefix = "mytool"

And then retrieve values with fetch:

#=> ""
# => "7727"

3.2 Working with optparse

This is an example of combining tty-config with optparse stdlib.

Let's assume you want to create a command line tool that among many options accepts --host|-h and --port|-p flags. In addition, these flags will take precedence over the options specified in the configuration file.

First, you need to parse the flags and store results away in options hash:

require "optparse"

options = {}

option_parser = do |opts|
  opts.on("-h", "--host HOSTNAME_OR_IP", "Hostname or IP Adress") do |h|
    options[:host] = h
  opts.on("-p", "--port PORT", "Port of application", Integer) do |p|
    options[:port] = p
  opts.on("-c", "--config FILE",
         "Read config values from file (defaults: ./config.yml, ~/.config.yml") do |c|
    options[:config_file_path] = c


Then, you create a configuration instance:

config =

And setup config filename:

config_filename = options[:config_file_path] || "config.yml"

As well as add configuration file locations to search in:

config.append_path Dir.pwd
config.append_path Dir.home

Once config is initialized, you can read the configuration from a config file:

begin  # by default the "config.yml" is read
rescue TTY::Config::ReadError => read_error
  STDERR.puts "\nNo configuration file found:"
  STDERR.puts read_error

Then merge options passed as arguments with those stored in a configuration file:


Provide optional validation to ensure both host and port are configured:

if !config.fetch(:host) || !config.fetch(:port)
  STDERR.puts "Host and port have to be specified (call with --help for help)."
  exit 1


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the TTY::Config project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.

Copyright (c) 2018 Piotr Murach. See LICENSE for further details.