How can I pipe stderr, and not stdout?

Question

I have a program that writes information to stdout and stderr, and I need to process the stderr with grep, leaving

Answer 1

I just came up with a solution for sending stdout to one command and stderr to another, using named pipes.

Here goes.

mkfifo stdout-target
mkfifo stderr-target
cat < stdout-target | command-for-stdout &
cat < stderr-target | command-for-stderr &
main-command 1>stdout-target 2>stderr-target

It's probably a good idea to remove the named pipes afterward.