M-! in dired-mode

[Paul Pogonyshev]

Hi,

M-! runs `shell-command' in `dired-mode'.  However, I cannot figure
when it runs on file under point and when without a file at all, it
seems to be completely random.  I mean, the buffer prompt does
unambigously say whether it will be run on a file or not, but that
prompt itself varies for no apparent reason and I couldn't find
anything in help...

Can anyone enlighten me (or just point to correct info node)?

Paul