Executing vim command based on filetype?

Question

I'm trying to have the command

    let b:match_words='<:>,<\@<=\([^/][^ \t>]*\)[^>]*\%(>\|$\):<\@<=/\1>'

run every time I open an html file. I tried putting the line

    autocmd FileType html let b:match_words='<:>,<\@<=\([^/][^ \t>]*\)[^>]*\%(>\|$\):<\@<=/\1>'

in a file named html.vim in both my ftdetect and ftplugin folders and nothing happened. How do I have the command run everytime I'mm in an html file?

The command is to change the matching behavior of matchit btw.

Answer 1

In general, your autocmd is alright; the problem is that you're trying to redefine the b:match_words definition done in $VIMRUNTIME/ftplugin/html.vim, so the execution order becomes important.

The place for these customizations is in the after directory, i.e. ~/.vim/after/ftplugin/html.vim; just create a new file and put the :let command in there.

You can observe the sequence of sourced scripts via :scriptnames. In other cases, when you're not overriding default behavior, the :autocmd FileType is alright, but I prefer putting these (e.g. custom mappings) into ~/.vim/ftplugin/html_mymappings.vim, as it provides better separation and helps keeping your .vimrc short and understandable.

The ftdetect subdirectory is for filetype detection, i.e. inspecting file path / name / contents to determine the correct filetype. It doesn't apply here, as the filetype is html.