1. intro

we will use some basic commands to ensure the MQTT broker is working well and receive published messages from your IOT devices.

Tip install on your workstation the package with the mqtt commands 
yum -y install mosquitto

the broker has a DNS entry with mq.d01.net and can be interrogated, first on a high level with:

mosquitto_sub -h mq.d01.net -u admin -P xx -t '$SYS/#' -d
Client null sending CONNECT
Client null received CONNACK (0)
Client null sending SUBSCRIBE (Mid: 1, Topic: $SYS/#, QoS: 0, Options: 0x00)
Client null received SUBACK
Subscribed (mid: 1): 0
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/version', ... (24 bytes))
mosquitto version 1.6.15
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/uptime', ... (12 bytes))
1510 seconds
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/clients/total', ... (1 bytes))
1
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/clients/active', ... (1 bytes))
1
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/clients/connected', ... (1 bytes))
1
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/received/1min', ... (5 bytes))
20.18
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/received/5min', ... (5 bytes))
15.10
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/received/15min', ... (5 bytes))
10.22
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/sent/1min', ... (6 bytes))
267.92
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/sent/5min', ... (5 bytes))
78.90
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/messages/sent/15min', ... (5 bytes))
33.75
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/publish/sent/1min', ... (6 bytes))
250.24
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/publish/sent/5min', ... (5 bytes))
64.60
Client null received PUBLISH (d0, q0, r1, m0, '$SYS/broker/load/publish/sent/15min', ... (5 bytes))
23.94
///

next, we will interrogate the broker again, specific on the tree level:

mosquitto_sub -h mq.d01.net -u admin -P xx -t '#' -d
Client null sending CONNECT
Client null received CONNACK (0)
Client null sending SUBSCRIBE (Mid: 1, Topic: #, QoS: 0, Options: 0x00)
Client null received SUBACK
Subscribed (mid: 1): 0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/relay/0', ... (3 bytes))
off
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input/0', ... (1 bytes))
0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input_event/0', ... (26 bytes))
{"event":"","event_cnt":0}
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/relay/0', ... (3 bytes))
off
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input/0', ... (1 bytes))
0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input_event/0', ... (26 bytes))
{"event":"","event_cnt":0}
Client null sending PINGREQ
Client null received PINGRESP
///
Tip learn from previous sample what topicname your IOT device is using

2. query

we will query a specific topic entry, based on wildcards:

mosquitto_sub -h mq.d01.net -u admin -P xx -t 'shellies/#' -d
Client null sending CONNECT
Client null received CONNACK (0)
Client null sending SUBSCRIBE (Mid: 1, Topic: shellies/#, QoS: 0, Options: 0x00)
Client null received SUBACK
Subscribed (mid: 1): 0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/relay/0', ... (3 bytes))
off
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input/0', ... (1 bytes))
0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input_event/0', ... (26 bytes))
{"event":"","event_cnt":0}
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/relay/0', ... (3 bytes))
off
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input/0', ... (1 bytes))
0
Client null received PUBLISH (d0, q0, r0, m0, 'shellies/shelly1-3083988C0311/input_event/0', ... (26 bytes))
{"event":"","event_cnt":0}
Client null sending PINGREQ
Client null received PINGRESP

3. cases

shellies/shellyplug-s-43F7D0/relay/0
shellies/shelly1-3083988C0311/relay/0
mosquitto_sub -h d01cid.ddns.net -u admin -P Passw0rd -t 'shellies/ssw041/relay/0'