tcpecho.sh

#!/bin/ksh

if [[ -z "$1" ]] || (( $1 <= 1024 )); then
    echo 'usage: tcpserver port'
    exit 1
fi

typeset port=$1
typeset pipe=`mktemp -u`
mkfifo $pipe
trap "rm -f $pipe" INT

function server {
    echo "server listening on $port" >&2
    cat $pipe | nc -l localhost "$port" | while read line; do
        printf "%s\n" "$line"
    done
}

function loopback {
    cat - > $pipe
}

server | loopback