How can I redirect matching lines to a file, and non-matching lines to a different file?

Question

Is there a script/program/utility already available for the following requirement in a optimised way?

someCommand |
   tee >(grep "pattern" > LinesWhichMatch) |
   grep -v "pattern" > LinesWhichDoesNotMatch

Answer 1

You could use awk for that.

command | awk '{ if (/pattern/) { print > "match" } else { print > "nomatch" } }'

Answer 2

Here is a sed example:
Note: sed's w command will overwrite an existing file each time the script is run, but only if that particular write command is triggered; hence the rm

rm -f file-{yes,not}  
sed -ne '/pattern/bY; w file-not' -e 'b; :Y; w file-yes' file

Answer 3

You can append files in awk:

awk '{if (/pattern/) print >>"matched"; else print >>"unmatched"; }

or shorter:

awk '{print >>(/pattern/?"matched":"unmatched")}'