diff <(cut -d, -f2 file1) <(cut -d, -f3 file2)

def identical_files?(file1, field1, file2, field2)
  %x{diff <(cut -d, -f#{field1} #{file1}) <(cut -d, -f#{field2} #{file2})}.blank?
end

`bash -c 'cat <(echo foo)'`  #=> "foo"

require 'csv'

csv_file1 = CSV.open('file1')
csv_file2 = CSV.open('file2')

until (csv_file1.eof? || csv_file2.eof?) do
  row1 = csv_file1.shift
  row2 = csv_file2.shift

  # do something to diff the fields
  puts "#{ csv_file1.lineno }: #{ row1[1] } == #{ row2[2] } --> #{ row1[1] == row2[2] }"
end

[
  [csv_file1, 'file1'],
  [csv_file2, 'file2']
].each do |f, fn|
  puts "Hit EOF for #{ fn }" if f.eof?
  f.close
end