Reading and Writing Configuration Files

Table of contents:

Problem

You want to store your applications configuration on disk, in a format parseable by Ruby but easily editable by someone with a text editor.

Solution

Put your configuration into a data structure, and write the data structure to disk as YAML. So long as you only use built-in Ruby data types (strings, numbers, arrays, hashes, and so on), the YAML file will be human-readable and -editable.

	require yaml
	configuration = { color => lue,
	 font => Septimus,
	 font-size => 7 }
	open(	ext.cfg, w) { |f| YAML.dump(configuration, f) }

	open(	ext.cfg) { |f| puts f.read }
	# --
	# font-size: 7
	# color: blue
	# font: Septimus

	open(	ext.cfg) { |f| YAML.load(f) }
	# => {"font-size"=>7, "color"=>"blue", "font"=>"Septimus"}

Its easy for a user to edit this: its just a colon-separated, line-delimited set of key names and values. Not a problem, even for a relatively unsophisticated user.

Discussion

YAML is a serialization format, designed to store data structures to disk and read them back later. But theres no reason why the data structures can be modified by other programs while they e on disk. Since simple YAML files are human-editable, they make good configuration files.

A YAML file typically contains a single data structure. The most common structures for configuration data are a hash (seen in the Solution) and an array of hashes.

	configuration = [ { 
ame => Alice, donation => 50 },
	 { 
ame => Bob, donation => 15, currency => "EUR" } ]
	open(donors.cfg, w) { |f| YAML.dump(configuration, f) }
	open(donors.cfg) { |f| puts f.read }
	# ---
	# - name: Alice
	# donation: 50
	# - name: Bob
	# donation: 15
	# currency: EUR

In Recipe 5.1 we advise saving memory by using symbols as hash keys instead of strings. If your hash is going to be converted into human-editable YAML, you should always use strings. Otherwise, people editing the YAML may become confused. Compare the following two bits of YAML:

	puts { measurements => metric }.to_yaml
	# ---
	# measurements: metric
	puts { :measurements => :metric }.to_yaml
	# ---
	# :measurements: :metric

Outside the context of a Ruby program, the symbol :measurements is too easy to confuse with the string ":measurements".

Problem

Solution

Discussion

See Also