How can I write a Linux bash script that tells me which computers are ON in my LAN?

• ip neighbor
• arp -a
• Arpwatch

Although an old question, it still seems to be important (at least important enough for me to deal with this). My script relies on nmap too, so nothing special here except that ou can define which interface you want to scan and the IP Range is created automagically (at least kind of).

This is what I came up with

#!/bin/bash
#Script for scanning the (local) network for other computers 

command -v nmap >/dev/null 2>&1 || { echo "I require nmap but it's not installed. Aborting." >&2; exit 1; }

if [ -n ""$@"" ];  then
    ip=$(/sbin/ifconfig $1 | grep 'inet '  | awk '{ print $2}' | cut -d"." -f1,2,3 )
    nmap -sP $ip.1-255
else
    echo -e "\nThis is a script for scanning the (local) network for other computers.\n"
    echo "Enter Interface as parameter like this:"
    echo -e "\t./scannetwork.sh $(ifconfig -lu | awk '{print $2}')\n"

    echo "Possible interfaces which are up are: "   
    for i in $(ifconfig -lu)
    do
        echo -e "\033[32m \t $i \033[39;49m"
    done

    echo "Interfaces which could be used but are down at the moment: "
    for i in $(ifconfig -ld)
    do
        echo -e "\033[31m \t $i \033[39;49m"
    done
    echo
fi

One remark: This script is created on OSX, so there might be some changes to linux environments.