#!/bin/bash

# function launch {
#   exec 3>&1 1>&2 2>&3 ./useless.sh
#   exec 3>&1 1>&2 2>&3
# }

function on_exit {
  FAILURE=${1:-0}
  [[ $FAILURE -gt 0 ]] && >&2 echo EXIT "$FAILURE"
  trap - ERR TERM INT EXIT
}

export -f on_exit
{
  myvar=$(bash -c "
    set -TEue -o pipefail;
    trap 'on_exit \$?' ERR TERM INT EXIT
    . ./useless.sh
  " 3>&1 1>&2 2>&3 | tee /dev/fd/4 | 3>&5 1>&2 2>&3 tee /dev/fd/5)
} 5>&1 4>&2 # magic trick: fd4 grabs stderr, then show up again on stderr

echo ----------
echo -e "stderr is:\n$myvar"