gistfile1.txt

#!/usr/bin/ruby
if ARGV.size < 1
  puts "#{$0} folder1 [folder2 [folder3 ...]]"
  exit 1
end

ARGV.each do |folder|
  `find #{folder} -name '*.txt'`.each do |f|
    file = f.chomp
    content = `grep critical #{file}`
    open(file, "w") { |io| io.print content }
  end
end