Simplify a Command Line with YAML

It is easy to manage command line input for one or two parameters by relying on the order. You can remember the order, document the order, or add some simple help message to your program. This works fine for a few items. Dealing with the ordering becomes difficult as the number of parameters grows. I decided to try using YAML to avoid the complexity of parsing the command line.

The following is a simple example of using YAML in Ruby instead of command line arguments.

The data file:

key: data
project:
    name: MyFirst
    repo: svn://localhost/svn/myfirst

Rather simple data, but data nonetheless. Assuming the data file is named data.yml, I’ll experiment with it in Ruby’s interactive shell:

>> require 'yaml'
=> true
>> input = YAML.load_file('data.yml')
=> {"project"=>{"name"=>"MyFirst", "location"=>"svn://localhost/svn/myfirst"}, "key"=>"data"}
>> puts "Received key value: #{input['key']}"
Received key value: data
=> nil

The example loads the file in to the ‘input’ hash. Next we use the hash key ‘key’ to get its value ‘data’. The ‘project’ collection from our input is another hash. We can access it:

> project = input["project"]
=> {"name"=>"MyFirst", "location"=>"svn://localhost/svn/myfirst"}
>> project.each { | k, y| puts "#{k} : #{y}" }
name : MyFirst
location : svn://localhost/svn/myfirst
=> {"name"=>"MyFirst", "location"=>"svn://localhost/svn/myfirst"}

Putting the example together as a script that reads the data file from the command line:

#!/usr/bin/env ruby
require 'yaml'

filename = ARGV[0]
input = YAML.load_file(filename)

puts "Key value: #{input['key']}"
# grab the 'project' hash
project = input["project"]
# output the key : value pairs from the hash
project.each { | key, value| puts "#{key} : #{value}" }

The simple script outputs:

$ ./simpleyaml.rb data.yml
Key value: data
name : MyFirst
location : svn://localhost/svn/myfirst

Here is the same script in Python:

#!/usr/bin/env python
import yaml
import sys

f = open(sys.argv[1])
input = yaml.load(f)
print "Key value: " + input["key"]
project = input["project"]
for key, value in project.items():
  print key + " : " + value