How to remove duplicates from a file and write to the same file?

Question

I know my title is not much self-explanatory but let me try to explain it here.

I have a file name test.txt which has some duplicate lines. Now, what I want

Answer 1

You can use vim for editing file in-place:

$ ex -s +'%!sort' -cxa test.txt

Multiple files:

$ ex -s +'bufdo!%!sort' -cxa *.*