Is there a way to tell what autocmds have run?

Question

If the autocmd has an obvious side effect, this is not too hard to tell; but sometimes the result is more subtle and it would be nice to just see "Hey, here's all the autocmds that ran on startup." I checked :history but autocmds don't seem to show up there.

Answer 1

Vim's 'verbose' option will give you this information. Set 'verbose' to a large enough number to get the debug information you need.

'verbose' 'vbs'     number  (default 0)
            global
            {not in Vi, although some versions have a boolean
            verbose option}
    When bigger than zero, Vim will give messages about what it is doing.
    Currently, these messages are given:
    >= 1    When the viminfo file is read or written.
    >= 2    When a file is ":source"'ed.
    >= 5    Every searched tags file and include file.
    >= 8    Files for which a group of autocommands is executed.
    >= 9    Every executed autocommand.
    >= 12   Every executed function.
    >= 13   When an exception is thrown, caught, finished, or discarded.
    >= 14   Anything pending in a ":finally" clause.
    >= 15   Every executed Ex command (truncated at 200 characters).

    This option can also be set with the "-V" argument.  See |-V|.
    This option is also set by the |:verbose| command.

    When the 'verbosefile' option is set then the verbose messages are not
    displayed.

Since you looking into autocommand's at startup, I would suggest you launch vim with the -V ('verbose') option and supply a 'verbosefile' to write this debug data to a file (debug.log). Then promptly close vim via +q

vim -V10debug.log +q

Now debug.log will have lot's of Vim's startup information including autocommand events.

For more information see:

:h -V
:h 'verbose'
:h 'verbosefile'