This doesn't work, because:

  $ printf linkedintrouble | openssl sha1
  (stdin)= 3ac85868a20c977661a12f770f0d116f87c74831

The leading '(stdin)=' messes the pattern being fed to 'grep'.

Yes, I've read http://partmaps.org/era/unix/award.html#cat . The output of sha1sum already contains a trailing '-' which is something I wanted to feed into 'grep' using command substitution, so that 'grep' can now just accept the input stream from 'stdin'. Now, how do you feed the input to grep via 'stdin' if you don't want to use 'cat'?

BTW, the commands involving 'openssl' can be fixed in this manner.

  $ printf linkedintrouble | openssl sha1 | cut -c10- | grep -f - combo_not.txt 
  3ac85868a20c977661a12f770f0d116f87c74831

does your grep have an -f option?

   printf linkedintrouble |sha1sum |sed 's/ .*//' |grep -f - combo_not.txt

If you look where we started ( http://news.ycombinator.com/item?id=4076559 ), I'm not trying to feed the regex pattern to grep via stdin, but I'm trying to feed the input stream to be searched for the pattern to grep via stdin.