network={
        key_mgmt=NONE
}

rpi ~$ cat get_ip_addr.sh
#!/bin/bash
# wait 30 sec if interface gets online
/lib/systemd/systemd-networkd-wait-online --interface=$1 --timeout=30 --quiet
if [ $? -ne 0 ]; then
    exit 1
else
    /sbin/ip -4 -br addr show $1 | /bin/grep -Po "d+.d+.d+.d+"
fi

rpi ~$ chmod +x get_ip_addr.sh
rpi ~$ ./get_ip_addr.sh wlan0
192.168.4.37

rpi ~$ ./get_ip_addr.sh xxx
Event loop failed: Connection timed out
rpi ~$