emacs can invoke shell and execute commands-- can they act on Emacs buffers?

Question

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?

Answer 1

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

Answer 2

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.