Content Based Filetype Detection in Vim

Yesterday I spent a little over an hour finally figuring out how to detect a file based on its contents in vim. It’s pretty easy!

I have become convinced that, for programs that you run, extensions declaring a filetype are wrong. If you were to think of a program as a black box, including .pl, .py, or .sh at the end to say what language they are is pointless extra information.

Vim almost always can detect a file automatically based on the contents of the file, but sometimes there are rarely used files that need the detection added.


I have been using dash, the Debian Almquist Shell, since about a week after Shellshock was announced. I like dash; part of the reason is because it’s simpler than bash (or zsh) and thus has a simpler surface attack area. More though, is because it is very fast to start up, and forces me to write almost strictly POSIX shell. The manpage is concise and readable, too.

Unfortunately, vim tends to think that dash scripts are conf files. Here’s how I fixed it:

  • In ~/.vim/ftdetect/dash.vim:
function! DetectDash()
   if getline(1) =~ '#!\/bin\/dash'
     setfiletype sh

augroup filetypedetect
  au BufRead,BufNewFile * call DetectDash()
augroup END

And that’s all you have to do, if it’s as simple as looking at the shebang. I suspect I’ll end up making this fancier by improving the regex, but this works for everything I write. It is so refreshing to leave off the # vim: ft=sh!


I’ve posted about upstart twice before this, and despite the fact that it is a sinking ship, it continues to do the heavy init lifting at work and likely will for at least another year. Frustratingly, Upstart detection in vim is pretty bad. As far as I can tell it only works if the file is in the /etc/init directory.

I did a lot of Upstart work yesterday (which I’ll hopefully post about soon) and was annoyed that syntax highlighting was broken most of the time. I then came up with the following, in the obviously named ~/.vim/ftdetect/upstart.vim:

function! DetectUpstart()
   let likely=0
   for i in getline(1, 1000)
      if i =~ '\v^(start|stop) on'
         setfiletype upstart
      elseif i =~ '\v^(respawn limit|(end|(pre|post)-start) script)'
         let likely += 2
      elseif i =~ '\v^(set(uid|gid)|env|description|author|respawn)'
         let likely += 1

      if likely > 4
         setfiletype upstart


augroup filetypedetect
  au BufRead,BufNewFile * call DetectUpstart()
augroup END

It’s weird and heuristic based, but it seems to work pretty well. Also shows some vimscript that is not actually that hard to read.

I am glad to have finally dug into this. The official docs are somewhat hard to follow, so I hope the above example can lead people in the right direction.

If you’d like to learn more about vim, I can recommend two excellent books. I first learned how to use vi from Learning the vi and Vim Editors. The new edition has a lot more information and spends more time on Vim specific features. It was helpful for me at the time, and the fundamental model of vi is still well supported in Vim and this book explores that well.

Second, if you really want to up your editing game, check out Practical Vim. It’s a very approachable book that unpacks some of the lesser used features in ways that will be clearly and immediately useful. I periodically review this book because it’s such a treasure trove of clear hints and tips.

Posted Wed, Sep 20, 2017