foo.vim

let data1 = []
let data2 = []

let cmd1 = ['sh', '-c', 'ag --vimgrep foo']
let cmd2 = ['sh', '-c', 'rg --no-heading --vimgrep -i foo']

function! s:stdout1(channel, data)
  let g:data1 += [a:data]
endfunction
function! s:stdout2(channel, data)
  let g:data2 += [a:data]
endfunction

function! s:exit1(channel)
  echohl WarningMsg
  echomsg 'RESULT AG: '. string(g:data1[:10])
  echohl NONE
endfunction
function! s:exit2(channel)
  echohl ErrorMsg
  echomsg 'RESULT RG: '. string(g:data2[:10])
  echohl NONE
endfunction

echomsg job_start(cmd1, {
      \ 'err_io':   'out',
      \ 'out_cb':   function('s:stdout1'),
      \ 'close_cb': function('s:exit1'),
      \ })
echomsg job_start(cmd2, {
      \ 'err_io':   'out',
      \ 'out_cb':   function('s:stdout2'),
      \ 'close_cb': function('s:exit2'),
      \ })