localchat.sh

#!/bin/bash

port=
mode=
interface=
host_ip=
error_code_fail=2
min_port=1024
max_port=65535
declare -a output_text

function print_help_text() {
    echo "
    This local chat script accepts the follow flags:

    Modes of operation (required)
    -s|--server-mode
    -c|--client-mode
    ...
    "
}

function validate_interface() {
    iface_array=$(get_available_interfaces)

    if ! containsElement "$1" "${iface_array[@]}"; then
        echo "Error: Invalid interface"
        echo "Avalaible interfaces: ${iface_array[@]}"
        exit
    fi

    return $? # exit status of last command
}

containsElement() {
    if [[ $2 =~ $1 ]]; then
        return 0
    fi
    return 1
}

function get_available_interfaces() {
    array=()
    for iface in $(ifconfig | cut -d ' ' -f1 | tr ':' '\n' | awk NF); do
        array+=("$iface")
    done
    echo ${array[@]}
}

function determine_ip() {
    host_ip=$(ip -o -4 addr show "${1}" | awk '{ split($4, ip_addr, "/"); print ip_addr[1] }')
    if [[ $host_ip =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
        output_text+=("Host Private Address: ${host_ip}.")
        return 0
    else
        output_text+=("Error! Failed to obtain private ip for the specified interface: ${1}\n
        ")
        return $error_code_fail
    fi
}

function can_bind_port() {
    # status code of this command indicates whether
    # there was output (bad, means port is in use)
    # or not (good, no processes using the port)
    ! lsof -i udp:"$1" >/dev/null
}

function find_available_port() {
    echo "Checking for an available port..."

    open_port_net_yet_found=true
    while [ open_port_net_yet_found ]; do
        temp_port=$(shuf -i${min_port}-${max_port} -n1) # generate random port number in range
        echo "...Trying port ${temp_port}"
        can_bind_port "$temp_port"
        port_status=$? # get return status of previous command
        if [ "$port_status" -eq "0" ]; then # 0 means port is safe to use
            port=$temp_port
            open_port_net_yet_found=false
            break
        fi
    done
    echo "Port ${port} Available! Binding..."
    return 0
}

function start_server() {
    echo "+-----------------------------------+"
    echo " Starting Local Chat in SERVER mode"
    echo "+-----------------------------------+"
    echo
    while IFS= read -p "Me: " -r input; do
        send_message "$input"
        # -l: listen mode -u use UDP protocol
    done | nc "$host_ip" -p "$port" -l -u
}

function start_client() {
    echo "+-----------------------------------+"
    echo " Starting Local Chat in CLIENT mode"
    echo "+-----------------------------------+"
    echo
    while IFS= read -p "Me: " -r input; do
        send_message "$input"
        # connect to listening server using UDP protocol
    done | nc "$host_ip" "$port" -u
}

function send_message() {
    payload="$*"
    # Transmit message ONLY if input is not empty
    if [ "$1" ]; then
        if [ "$mode" = "SERVER" ]; then
            printf "\nServer: %s \nMe: " "${payload}"
        else # client mode
            printf "\nClient: %s \nMe: " "${payload}"
        fi
        return 0 # payload transmitted
    else
        return $error_code_fail # blank input
    fi
}

function init() {
    POSITIONAL=()
    while [[ $# -gt 0 ]]; do
        key="$1"

        case $key in
        -c | --client-mode)
            mode="CLIENT"
            shift # past argument
            ;;
        -a | --server-address)
            host_ip="$2"
            shift # past argument
            shift # past value
            ;;
        -p | --server-port)
            port="$2"
            shift # past argument
            shift # past value
            ;;
        -i | --interface)
            interface="$2"
            shift # past argument
            shift # past value
            ;;
        -s | --server-mode)
            mode="SERVER"
            shift # past argument
            ;;
        *) # unknown option
            POSITIONAL+=("$1") # save it in an array for later
            print_help_text
            shift # past argument
            exit
            ;;
        esac
    done
    # set -- "${POSITIONAL[@]}" # restore positional parameters

    case $mode in

    SERVER)
        validate_interface "$interface"
        determine_ip "$interface"
        find_available_port
        for i in "${output_text[@]}"; do echo "$i"; done
        start_server
        ;;

    CLIENT)
        for i in "${output_text[@]}"; do echo "$i"; done
        start_client
        ;;

    "")
        echo "Error: Operation mode must be specified"
        exit 1
        ;;

    *)
        echo "Internal configuration failure"
        exit 1
        ;;

    esac

}

init "$@"

# meat without all the extra fat
# while IFS= read -p "Me: " -r input; do printf "\nServer: %s \nMe: " "$input"; done | nc localhost 55555 -l
# while IFS= read -p "Me: " -r input; do printf "\nClient: %s \nMe: " "$input"; done | nc localhost 55555