#!/bin/bash

#
# An IP address looks like this:
#       192.168.0.5
# Four groups of 1-3 numbers in the range 0..255 separated by dots.
#
re='^([0-9]{1,3}\.){3}[0-9]{1,3}$'

#
# The address is expected as the only argument
#
if [[ $# -ne 1 ]]; then
    echo "Usage: $0 IP_address"
    exit 1
fi

#
# Validate against the regex
#
if [[ $1 =~ $re ]]; then
    #
    # Look at the components and check they are all in range
    #
    for d in ${1//./ }; do
        if [[ $d -lt 0 || $d -gt 255 ]]; then
            echo "$1 is not a valid IP address (contains $d)"
            exit 1
        fi
    done

    echo "$1 is a valid IP address"
else
    echo "$1 is not a valid IP address"
fi