Check if return value is a valid ipv4 address

check-ipv4-syntax.sh

#!/bin/bash
myAddrUrl="REPLACE.myaddr.io"

# max 6 tries every 5 seconds (if connection is currently offline)
if ! currentDynDns=$(dig +time=5 +tries=6 +short $myAddrUrl @86.54.11.100); then
    echo "Problem: dig network error or timeout."
    exit 1
fi

# check if ip syntax is correct
case "$currentDynDns" in
    ''|*[!.0-9]*) echo "Keine IPv4-Adresse" ; exit ;;
esac

# check if ipv4 has 4 blocks and only digits between 1-255
set -- $(echo "$currentDynDns" | tr '.' ' ')
[ $# -ne 4 ] && echo "Keine IPv4-Adresse" && exit

for octet in "$@"; do
    [ "$octet" -ge 0 ] 2>/dev/null && [ "$octet" -le 255 ] || {
        echo "Keine gültige IPv4-Adresse"
        exit
    }
done

echo "Gültige IPv4-Adresse: $currentDynDns"