Get IPv4/netmask network spec (e.g. 192.168.0.0/24) for current network, useful for nmap etc.

getnet.sh

#!/usr/bin/env bash
############
# run $ ./getnet.sh [iface]
# 
# defaults to first non-local interface, but you can specify an interface.
# prints IP/Network in form 0.0.0.0/0 to STDOUT, so $(./getnet.sh en0) works
#
# e.g. run $ nmap -sP $(./getnet.sh en0)
# ^ will pingscan your current wifi network on macOS
#
# Works with macOS and most Linux distros including Ubuntu
# REQUIRES `ipcalc` be installed! `brew install ipcalc` or `sudo apt install ipcalc`

iface=$(ifconfig | egrep "^\S+:" | grep -v lo | grep UP | head -n 1 | cut -d ':' -f 1)

if [ -n "$1" ]; then
    iface="$1"
    shift
fi

>&2 echo "Using interface '$iface'"

if ! (which ipcalc > /dev/null); then
    >&2 echo "ipcalc not installed"
    exit 1
fi

addrstr=$(ifconfig ${iface} | egrep "inet\s+[0-9]+\.")
# >&2 echo "INET: $addrstr"
ip=$(echo "$addrstr" | awk '{print $2}')
nm=$(echo "$addrstr" | awk '{print $4}')

# >&2 echo "$ip/$nm"
ipnet=$(ipcalc "$ip/$nm" | egrep "Network:\s" | awk '{print $2}')

echo $ipnet