Reading and Writing Configuration Files

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".

See Also

  • Recipe 13.1, "Serializing Data with YAML"


Strings

Numbers

Date and Time

Arrays

Hashes

Files and Directories

Code Blocks and Iteration

Objects and Classes8

Modules and Namespaces

Reflection and Metaprogramming

XML and HTML

Graphics and Other File Formats

Databases and Persistence

Internet Services

Web Development Ruby on Rails

Web Services and Distributed Programming

Testing, Debugging, Optimizing, and Documenting

Packaging and Distributing Software

Automating Tasks with Rake

Multitasking and Multithreading

User Interface

Extending Ruby with Other Languages

System Administration



Ruby Cookbook
Ruby Cookbook (Cookbooks (OReilly))
ISBN: 0596523696
EAN: 2147483647
Year: N/A
Pages: 399

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