Diffsplit with output of external command in Vim?

Question

I know I can enter diff mode in vim by calling diffsplit <filename>.

I also know I can call an external command and replace the file contents with its output (e.g. in this answer someone uses a json formatting tool by calling :%!python -m json.tool).

Say I wanted to do something similar, but wanted to see the diff of the current file and the output of the json formatter, and choose which changes to keep. How can I sent the output of the tool to diffsplit?

Notes:

1. I could send the output of the formatter to a file in /tmp or something and then do vimdiff on the file from there. But it would be nice to avoid having to read and write an extra file to/from disk.
2. I mainly want to do this for the whole file. If there's a way to send a specific range to the formatter and then diff that range (or diff the result of the command glued into the original file) that would be a nice bonus.
3. I am not actually using a json formatter but am looking for a generic solution that would work for this situation (and similar).

Answer 1

I have this one-liner and I hope this helps:

:vnew | setlocal buftype=nofile | exec "read !ls -l" | windo diffthis

You may (and probably will) replace ls -l with any other commands, and you may need shellescape() and expand() to compose your shell command for exec.

Regarding sending selected lines to the command line, you may better do a function, as after vnew it may be hard to get the selected lines (unless you have yanked it).

Hope these helps: :h buftype :h exec :h read! :h windo

Answer 2

Here is what I came up with for:

augroup FormatDiff
  autocmd!
  autocmd TextChanged * if &diff && get(g:, 'diff_format_prg', '') != '' | silent execute '%!'.g:diff_format_prg | let g:ignore = 1 | diffupdate | unlet g:ignore | | endif
  autocmd FilterWritePre if &diff && get(g:, 'diff_format_prg', '') != '' && get(g:, 'ingore', 0) == 0 | silent execute '%!'.g:diff_format_prg | endif
augroup END

I then did let g:diff_format_prg = 'jq -S .'. Followed by by :diffsplit anotherfile.json

The overview:

Try and run the formatter on the very first diff e.g. :diffpslit. And then for subsequent file changes (TextChange) run formatter and then do :diffupdate without triggering an extra formatting.

Some thoughts:

• I am not sure how I feel about auto-fromatting on TextChange
• Probably need to check for buffer-local variables instead of just global, g:diff_format_prg, if this was to be generalized
• Need a better way of displaying formatter errors or handling them at all
• Will fire on all filter commands in a diff'ed buffer (unexpected)
• I am not certain how this could be applied to just a specific buffer range

For more help see:

:h TextChange
:h FilterWritePre
:h get()
:h b:
:h g:
:h :range!
:h diff
:h diffupdate