Deleting lines from one file which are in another file

Question

I have a file f1:

line1
line2
line3
line4
..
..

I want to delete all the lines which are in another file f2:

Answer 1

if you have Ruby (1.9+)

#!/usr/bin/env ruby 
b=File.read("file2").split
open("file1").each do |x|
  x.chomp!
  puts x if !b.include?(x)
end

Which has O(N^2) complexity. If you want to care about performance, here's another version

b=File.read("file2").split
a=File.read("file1").split
(a-b).each {|x| puts x}

which uses a hash to effect the subtraction, so is complexity O(n) (size of a) + O(n) (size of b)

here's a little benchmark, courtesy of user576875, but with 100K lines, of the above:

$ for i in $(seq 1 100000); do echo "$i"; done|sort --random-sort > file1
$ for i in $(seq 1 2 100000); do echo "$i"; done|sort --random-sort > file2
$ time ruby test.rb > ruby.test

real    0m0.639s
user    0m0.554s
sys     0m0.021s

$time sort file1 file2|uniq -u  > sort.test

real    0m2.311s
user    0m1.959s
sys     0m0.040s

$ diff <(sort -n ruby.test) <(sort -n sort.test)
$

diff was used to show there are no differences between the 2 files generated.