What's the difference between autocmd {cmd} and autocmd! {cmd}

Question

For example, what's the difference between calling this command this way:

autocmd BufWritePre * call StripTrailingWhitespace()

vs. this way (with the bang after autocmd):

autocmd! BufWritePre * call StripTrailingWhitespace()

Answer 1

The command

autocmd! BufWritePre * call StripTrailingWhitespace()

Removes all autocmds for the event BufWritePre and the file pattern * from the default autocmd-group and sets a new autocmd for this event and pattern to call StripTrailingWhitespace().

Example:

autocmd BufWritePre * echomsg "First"
autocmd BufWritePre * echomsg "Second"

If you then enter :autocmd BufWritePre you will get the following (plus maybe other autocmds defined for this event):

--- Autocommands ---
BufWrite
    *         echomsg "First"
              echomsg "Second"

Then you execute

autocmd! BufWritePre * echomsg "Third"

followed by :autocmd BufWritePre and you get:

--- Autocommands ---
BufWrite
    *         echomsg "Third"

So the autocmds echoing "First" and "Second" were removed and the new autocmd echoing "Third" was defined.

See :help autocmd-remove.