Rdocs are here:

These are some recipes for useful things you might want to do with ruby-marc. If you need a file of MARC records to work with, you can download one from here:

Recipe: read in a large file of marc records, pull out one of them and write it to a separate file

require 'rubygems'
require 'marc'

reader ='/usr/local/projects/bl-demo/data/lc_records.utf8.mrc')
record = reader.first
writer ='/tmp/file2')