intersection.rb

#!/usr/bin/env ruby

# call with filenames as arguments
# outputs lines which appear in all files
# (stripping leading and trailing whitespace, in no predictable order)

# $ cat > foo
# apple  
# orange
# pear
# $ cat > bar
# pear 
# orange
# pineapple
# $ cat > baz
# pineapple
# apple
# orange
# $ ./intersection.rb foo bar baz
# orange


require 'set'

lines = {}

ARGF.each_line do |line|
  (lines[ARGF.filename] ||= Set.new) << line.strip
end

sets = lines.values

intersection = sets.inject(sets.pop) {|intersection,set| intersection & set}
intersection.each do |line|
  puts line
end