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

This will redirect command1 stderr to command2 stdin, while leaving command1 stdout as is.

exec 3>&1
command1 2>&1 >&3 3>&- | command2 3>&-
exec 3>&-

Taken from LDP