I use Alt-! (Alt-Bang) a lot in Emacs. One of the big things I use it for is
Alt-! cat $logfile | grep 'this' # show me one kind of event
or sometimes
Alt-! cat $logfile | grep 'this' | wc -l # count that one event's occurrences
Two things:
1) No tab-completion from this prompt: why not?
2) What if instead of $logfile, I want to scan one of the Emacs buffers?
To scan an Emacs buffer, use M-| instead of M-!: it passes the region as input to the command. Use M-1 M-| if you want the output of the command to replace the region.
For the particular command you mention, use M-x grep if you want to see all matches. Or you can open it and see the matches with M-x occur.
Alt-| does is shell-command-on-region
with a(ny) numeric prefix (e.g. C-u 1 Alt-|) the region is replaced by the result, otherwise that appears in new buffer
C-u provides a default value (4^n when you press C-u n times), so when the number is irrelevant, or just needs to be some positive integer, you needn't use C-u 1 when you can just use C-u on its own (or M-1 or C-1).
shell-commandprompt. (tab is bound tominibuffer-complete-shell-command)