Netcat in Bash

nc.sh

#!/bin/bash

set -eu

PROXY=false

usage() {
    echo "Usage: $0 [-X connect -x <proxy:port>] <host> <port>"
}

while getopts ":X:x:" OPT; do
    case "$OPT" in
        X)
            if [[ "$OPTARG" != "connect" ]]; then
                echo "Unsupported argument to -X: ${OPTARG}" >&2
                usage
                exit 1
            fi
            PROXY=true
            ;;
        x)
            PROXY_HOST="${OPTARG%:*}"
            PROXY_PORT="${OPTARG#*:}"
            ;;
        ?)
            usage
            exit 0
            ;;
    esac
done

shift $((OPTIND-1))

if [[ "$#" != 2 ]]; then
    echo "No host/port given" >&2
    usage
    exit 1
fi

HOST="$1"
PORT="$2"

if $PROXY; then
    if [[ -z "$PROXY_HOST" || -z "$PROXY_PORT" ]]; then
        echo '"-X connect" given without -x proxy:port' >&2
        usage
        exit 1
    fi
    printf -v PROXIED "%s:%d" "$HOST" "$PORT"
    HOST="$PROXY_HOST"
    PORT="$PROXY_PORT"
fi

exec 3<> "/dev/tcp/${HOST}/${PORT}"

if $PROXY; then
    printf "CONNECT %s HTTP/1.1\r\nHost: %s\r\n\r\n" "$PROXIED" "$PROXIED" >&3
    read -u 3 -r LINE
    if [[ "$LINE" != "HTTP/1.1 200 Connection established"$'\r' ]]; then
        echo "Connecting to proxy failed" >&2
        exit 1
    fi
fi

cat <&3 &
READER=$!
cat >&3

kill $READER 2>/dev/null