Moving STDERR and STDOUT around for great justice.

gistfile1.sh

# A program which outputs "error" to STDERR and "output" to STDOUT.
a_program() {
  echo error >&2
  echo output
}

# Run a_program.
# - Redirect a_program's STDERR (file descriptor 2) to the STDIN of a tee process.
#   - tee echoes its STDIN to the given filename and to its STDOUT.
#   - Redirect tee's STDOUT to its STDERR.
a_program 2> >(tee errors.txt >&2)

# The upshot: All output on a_program's STDOUT will end up in errors.txt and on the whole pipeline's STDERR.

error_processor() {
  (echo "Got errors: "; cat) | say
}

# Do the above, but send errors to error_processor
a_program 2> >(tee >(error_processor) >&2)