How to connect both stdin and stdout of some process to netcat in Bash 3, or, how to emulate netcat traditional's -e switch

check-redis.sh

#! /bin/bash

if (( $# ))
then
    echo "usage: mkfifo r.fifo ; nc -q 1 localhost 6379 < r.fifo | $0 > r.fifo" >&2
    exit 2
fi

KEY="test-key"
VAL="val"
IFS=$'\r'

while :
do
    echo "GET $KEY"
    read -r LINE
    if [[ "$LINE" =~ ^\$[[:digit:]]+$ ]]
    then
        read -r LINE
        echo "$KEY exists: $LINE" >&2
        break
    elif [[ "$LINE" =~ ^\$- ]]
    then
        echo "$KEY does not exist" >&2
        echo "SET $KEY $VAL"
        read -r LINE
        if [[ "$LINE" != '+OK' ]]
        then
            echo "SET error: $LINE" >&2
            exit 1
        fi
    else
        echo "GET error: $LINE" | hd >&2
        exit 1
    fi
done

echo "DEL $KEY"
read -r LINE
echo "$KEY deleted: $LINE" >&2
exit 0