Find unique lines

Question

How can I find the unique lines and remove all duplicates from a file? My input file is

1
1
2
3
5
5
7
7

I would like the result to be:

Answer 1

uniq has the option you need:

   -u, --unique
          only print unique lines

$ cat file.txt
1
1
2
3
5
5
7
7
$ uniq -u file.txt
2
3