json.rb

#!/usr/bin/ruby

require 'json'

DESC_HEAD = /^\.([^\.\[]+)/
DEREF_HEAD = /^\[(\d+)\]/

def usage
  $stderr.puts <<USAGE
usage: json descriptor

A descriptor consists of the following potential components in series:

  .field  Descends into the "field" key of an object
  [num]   Descends into the "num" index of an array
  .       The root descriptor (cannot be chained)

JSON is parsed from stdin and the output is printed to stdout.  If no elements
of the input JSON match the specified descriptor, the output will be empty.

EXAMPLE

$ cat > /tmp/test.json
{
  "user": {
    "firstname": "Daniel",
    "lastname": "Spiewak"
  },
  "magic": {
    "keys": [
      {
        "id": 42,
        "hash": "abcdef09876"
      },
      {
        "id": 11,
        "hash": "1234cafebabe"
      }
    ]
  }
}
^D
$ json .user.firstname < /tmp/test.json
Daniel
$ json .magic.keys[1].id < /tmp/test.json
11
USAGE
end

def descend(field)
  lambda { |input| input[field] }
end

def parse_descriptor(descriptor)
  if descriptor.strip.empty?
    []
  else
    if DESC_HEAD =~ descriptor
      [descend($1)] + parse_descriptor($')
    elsif DEREF_HEAD =~ descriptor
      [descend($1.to_i)] + parse_descriptor($')
    elsif '.' == descriptor
      [lambda { |x| x }]
    else
      $stderr.puts "Unrecognized desctiptor '#{descriptor}'"
      exit -1
    end
  end
end

if ARGV.length != 1
  usage
  exit -1
end

actions = parse_descriptor ARGV.first

input = JSON.load $stdin
result = actions.inject input do |acc, f|
  f.call acc
end

$stdout.puts result.to_s