vim/vi/sed: Act on a certain number of lines from the end of the file

Question

Just as we can delete (or substitute, or yank, etc.) the 4th to 6th lines from the beginning of a file in vim:

:4,6d 

I'd like to delete (or substitute, or yank, etc.) the 4th last to the 6th lines from the end of a file. It means, if the file has 15 lines, I'd do:

:10,12d

But one can't do this when they don't know how many lines are in the files -- and I'm going to use it on a batch of many files. How do I do this in vim and sed?

I did in fact look at this post, but have not found it useful.

Answer 1

Well, using vim you can try the following -- which goes quite intuitive, anyway:

:$-4,$-5d

Now, using sed I couldn't find an exact way to do it, but if you can use something other than sed, here goes a solution with head and tail:

head -n -4 file.txt && tail -2 file.txt

Answer 2

In Vim, you can subtract the line numbers from $, which stands for the last line, e.g. this will work on the last 3 lines:

:$-2,$substitute/...

---

In sed, this is not so easy, because it works on the stream of characters, and cannot simply go back. You would have to store a number of last seen lines in the hold space, and at the end of the stream work on the hold space. Here are some recipes from sed1line.txt:

# print the last 10 lines of a file (emulates "tail")
sed -e :a -e '$q;N;11,$D;ba'

# print the last 2 lines of a file (emulates "tail -2")
sed '$!N;$!D'

# delete the last 2 lines of a file
sed 'N;$!P;$!D;$d'

# delete the last 10 lines of a file
sed -e :a -e '$d;N;2,10ba' -e 'P;D'   # method 1
sed -n -e :a -e '1,10!{P;N;D;};N;ba'  # method 2