Why use 0d_ in DiffOrig in Vim?

Question

In the online manual of Vim, :help DiffOrig will show the recommended command sequence to get changes of current editing file.

Here it is:

command DiffOrig vert new | set bt=nofile | r # | 0d_ | diffthis
                   \ | wincmd p | diffthis

I wonder what the effect of 0d_ is. I tried 0d_ in normal mode, it works like dd, but I can't understand why it used here.

Answer 1

Let's explain it a bit: Suppose you have original foo.txt containing (with line numbers):

1 a
2 c
3 d
~

You have added a line containing “b” between lines 1 and 2:

• :vert new creates a new, empty, buffer in a vertical split (:help :new)

• :set bt=nofile makes it a scratch buffer (:help 'bt'). Note:

  1 a  | 1 ·<cursor here
  2 b  | ~
  3 c  | ~
  4 d  | ~
  ~    | ~
  

• :r # inserts after current line, contents of alternate file (#), as stored on the file system. You haven't saved the other buffer, so you get original content. (:help alternate-file).
  :help :r tells you that it always inserts after. Therefore:

  1 a  | 1
  2 b  | 2 a
  3 c  | 3 c
  4 d  | 4 d
  ~    | ~
  

• :0d_ removes the first line. Why 0, I don't know it really, I would rather write it :1d_. :help range tells:

  When using a 0 (zero) this is interpreted as a 1 by most commands

  The _ specifies that it goes to the black-hole register. See :help :d about the :d ex command, it works linewise.

The rest is obvious.