Read the first line of the current file to process it in a command (Emulating %TeX root)

Question

Currently I have the line

:nnoremap <leader>c :w<CR>:!arara %<CR><CR>

in my $MYVIMRC. If I press <leader>c, it will save the current file and run the command arara on it. However I would like its behavior to be the following:

1. Save the current file
2. IF the first line has the form "%!TeX root = <something>"
3. THEN run the command "arara <something>" instead
4. ELSE run "arara <current file>"

For this it is required that vim reads the first line of the current file to process it in a command. I don't know how or if this is possible since I am rather new to vimscript.

The above behavior is rather standard in TeX editors and I would like to emulate it in vim.

In case it is important, I am using neovim rather than plain vim.

Answer 1

Following command assumes that %!TeX root = <mainfile with path> is in the first line
(and it does not contain a second = in <mainfile with path>)

nnoremap <leader>c :w<CR>:!arara <C-r>=trim(split(getline(1), '=')[1])<CR><CR><CR>

Without extensive testing, following matches the first line against '\ctexroot' (\c means ignore case) and chooses accordingly the argument for arara:

function! Myarara()
    if match(getline(1), '\ctex root') >= 0
        echo system("arara ".trim(split(getline(1), '=')[1]))
    else
        echo system("arara ".@%)
    endif
endfunction
nnoremap <leader>c :w<CR>:call Myarara()<CR>

I personally would not be satisfied with this for following reasons:

• no use of quickfix list and error parsing (if you are not interested in this, you could use something like fswatch or entr in the shell which means you are not blocked in vim, no need for a normal mode mapping, a simple :w is enough)
• it blocks vim while running (compiling latex documents can take a while)

I would consider using one of vimtex, vim-dispatch, or asyncrun.vim.