how do I configure vim to set the executable bit on scripts?

[rm-rf]

I am trying to configure vim so that when I write a new script it will automatically set the executable bit on it when I write to it for the first time without the annoying

W16: Warning: Mode of file "b.sh" has changed since editing started
See ":help W16" for more info.
[O]K, (L)oad File:

No matter what I do though, it doesn't seem to work.

here is my .vimrc:

so ~/.vim/mod.vim

autocmd BufNewFile * let fname=expand("%:p")
autocmd BufNewFile *.pl execute "autocmd BufWritePost ".fname." call SetExecutableBit()" 
autocmd BufNewFile *.sh execute "autocmd BufWritePost ".fname." call SetExecutableBit()"
autocmd BufNewFile *.rb execute "autocmd BufWritePost ".fname." call SetExecutableBit()"
autocmd BufNewFile *.py execute "autocmd BufWritePost ".fname." call SetExecutableBit()"

and this is my .vim/mod.vim file:

function! SetExecutableBit()
	let fname=expand("%:p")
	execute "autocmd FileChangedShell ".fname 
	execute "!chmod a+x ".fname
	execute "autocmd! FileChangedShell ".fname
endfunction

[rm-rf]

nevermind I figured it out. it turns out I have to change it from "BufWritePost" to "BufWrite"