Check if the arguments are IPv4 addresses.

is-ip

#! /bin/bash

set -eu
#set -x

# Check if the arguments are IPv4 addresses.
is-ip ()
{
  local -a ip    # Array for IP address
  local x        # Octet input. No -i, because read converts non
		 # numeric input to 0!
  local -i i     # Octet index
  local a        # Function argument
  for a; do
    # Read an IP address, which consists of four octets, three of
    # which are dot delimited.
    ip=()
    { i=3
      while ((--i >= 0)); do
	read -d. x; ip+=($x)
      done
      read x; ip+=($x)
    } <<<"$a"
    # Check the range 0..255 for every octet.
    i=4
    while ((--i >= 0)); do
      # The arithmetic expansion fails also for non numeric octets.
      if ! ((ip[i] >= 0 && ip[i] <= 255)) 2>/dev/null; then
	return 1
      fi
    done
  done
}

is-ip "$@"