[mrspuratic]

This feature has been part of bash since 5.1 (ca 2020), though it may not be enabled in all distros.

  cd src/bash-5.3/examples/loadables
  make accept
  enable -f ./accept accept
  (accept -r RHOST -v SOCKETFD -b 127.0.0.1 8000; 
    read -u $SOCKETFD SOCKETDATA; 
    printf "%s: %s\n" "$RHOST" "$SOCKETDATA";
    printf "goodbye, world\n" 1>&${SOCKETFD} ) &
  nc 127.0.0.1 8000 <<< "hello, world"

For real use you may need to add "exec {SOCKETFD}<&-" to close the FD.

Edit: https://news.ycombinator.com/item?id=39369749 (2024)