How do I append text to a file with vim script?

Question

I want a function to append text to a file (not a buffer) in vim. As far as I can see, there is no appendfile(). But the desired functionality can be emulated with readfile() and writefile():

fu! TQ84_log (S)

   let l:f = readfile('my.log')
   call add(l:f, a:S)
   call writefile(l:f, 'my.log')

endfu

Since my.log can grow quite large, I'd rather not read and write the entire file when I want to add a line. So, I came up with another "solution":

fu! TQ84_log (S)
   silent execute "!echo " . a:S . ">> my.log"    
endfu

This works (on windows, that is) as expected. Yet, when I invoke TQ84_log(), that cmd.exe window pops up for a short time. This is a bit distracting.

Is there a better solution for my problem?

Answer 1

If you have a range of lines in your current buffer that you want to append to the log file, then

:[range]w >> my.log

does exactly what you want. (Well, maybe not exactly. For example, it has the side effect of making my.log the alternate file, and if you plan to use :e# or something, it may mess you up.)

If you already have the log message in a variable, then you could open up a scratch buffer, use append() or :put to add the line(s) to your scratch buffer, then :w >> my.log and close the scratch buffer.

:help special-buffers
:help :put
:help :w

Here is a complete log function. There is room for improvement: the combination of :new and :q may not restore the layout if you have split windows, and :put on an empty buffer leaves you with a blank line that you probably do not want.

function! Mylog(message, file)
  new
  setlocal buftype=nofile bufhidden=hide noswapfile nobuflisted
  put=a:message
  execute 'w >>' a:file
  q
endfun