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

For those who want to redirect stdout and stderr permanently to files, grep on stderr, but keep the stdout to write messages to a tty:

# save tty-stdout to fd 3
exec 3>&1
# switch stdout and stderr, grep (-v) stderr for nasty messages and append to files
exec 2> >(grep -v "nasty_msg" >> std.err) >> std.out
# goes to the std.out
echo "my first message" >&1
# goes to the std.err
echo "a error message" >&2
# goes nowhere
echo "this nasty_msg won't appear anywhere" >&2
# goes to the tty
echo "a message on the terminal" >&3