#!/bin/bash

function exit {
  code=${1:-0}
  trap - ERR TERM INT EXIT

  # a bit of magic: read content of file still on write!
  count=$(wc -c stderr | cut -d' ' -f1)
  content=$(head -qc$count stderr)
  >&4 echo "DETECTED0: [$content]"

  >&4 echo EXIT ${code}
}

set -TEue -o pipefail
trap 'exit $?' ERR TERM INT EXIT
[[ -f stderr ]] && rm stderr
export -f exit

# magic FD
exec 3>stderr 4>/dev/stderr
source ./useless.sh 2>&3
exec 3>&- 4>&-

trap - ERR TERM INT EXIT
[[ -s stderr ]] && >&2 cat stderr