mosquitto_pub and mosquitto_sub commands in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of mosquitto_pub with QoS 1

What is the output or result when running this command?

mosquitto_pub -h test.mosquitto.org -t "home/temperature" -m "22" -q 1

IOT Protocols

mosquitto_pub -h test.mosquitto.org -t "home/temperature" -m "22" -q 1

A The command fails with a syntax error due to missing -p port option

B The message "22" is published but with QoS 0, ignoring the -q flag

C The message "22" is published to topic "home/temperature" with QoS 1, and the command exits with status 0

D The command connects but does not send any message because -m is ignored

💡 Hint

The -q flag sets the Quality of Service level for message delivery.

The command publishes the message "22" to the topic "home/temperature" on the public broker with QoS 1, which means the message is delivered at least once. The command exits successfully.

💻 Command Output

intermediate

mosquitto_sub output for retained messages

If a retained message "ON" exists on topic "home/lights", what will this command output immediately after subscribing?

mosquitto_sub -h test.mosquitto.org -t "home/lights"

IOT Protocols

mosquitto_sub -h test.mosquitto.org -t "home/lights"

A It prints an error about missing -m flag

B It immediately prints "ON" and then waits for new messages

C It prints nothing until a new message is published

D It disconnects immediately without output

💡 Hint

Retained messages are sent to new subscribers immediately.

When subscribing to a topic with a retained message, mosquitto_sub receives and prints that retained message right away before waiting for new messages.

❓ Configuration

advanced

Correct mosquitto_pub command to use TLS with client certificate

Which mosquitto_pub command correctly publishes message "Hello" to topic "secure/topic" using TLS with client certificate and key files located at /certs/client.crt and /certs/client.key?

A mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cafile /certs/ca.crt --cert /certs/client.crt --key /certs/client.key -p 8883

B mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cert /certs/client.crt --key /certs/client.key

C mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --tls --cert /certs/client.crt --key /certs/client.key

D mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cafile /certs/ca.crt -p 1883

💡 Hint

TLS requires specifying the CA file and client cert/key along with the secure port.

Option A correctly specifies the CA certificate file, client certificate, client key, and uses the standard TLS port 8883. This enables secure TLS connection with client authentication.

❓ Troubleshoot

advanced

Troubleshooting mosquitto_sub connection failure

You run this command:

mosquitto_sub -h 192.168.1.100 -t "sensor/data" -p 1884

But it fails to connect. What is the most likely cause?

A The topic name "sensor/data" is invalid

B The mosquitto_sub command requires -m flag to connect

C The IP address 192.168.1.100 is unreachable due to firewall

D The broker is not listening on port 1884, default is 1883

💡 Hint

Check the broker's listening port and firewall settings.

By default, mosquitto brokers listen on port 1883. If the broker is not configured to listen on 1884, the connection will fail. The topic name is valid, and -m is not required for subscribing.

🔀 Workflow

expert

Sequence to test message delivery with mosquitto_pub and mosquitto_sub

Arrange the steps in correct order to test publishing and subscribing to a topic using mosquitto_pub and mosquitto_sub on the same machine.

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 2,3,1,4

💡 Hint

Subscribe first to receive messages published after subscription.

You must start the subscriber first to listen for messages. Then publish the message. The subscriber will output the message, and finally you can stop the subscription.

Practice

1. What is the main purpose of the mosquitto_pub command in MQTT?

easy

A. To send messages to a specific MQTT topic

B. To listen for messages on a topic

C. To create a new MQTT broker

D. To delete MQTT topics

Solution

1. Step 1: Understand the role of mosquitto_pub

   The mosquitto_pub command is used to publish or send messages to MQTT topics.

2. Step 2: Compare with other options

   mosquitto_sub listens for messages, not mosquitto_pub. Creating or deleting topics is not done by this command.

3. Final Answer:

   To send messages to a specific MQTT topic -> Option A

4. Quick Check:

   mosquitto_pub = send messages [OK]

Hint: Remember: pub means publish (send) messages [OK]

Common Mistakes:

• Confusing mosquitto_pub with mosquitto_sub
• Thinking it manages topics instead of messages
• Assuming it listens for messages

2. Which of the following is the correct syntax to subscribe to the topic home/temperature using mosquitto_sub?

easy

A. mosquitto_sub -m home/temperature

B. mosquitto_sub -t home/temperature

C. mosquitto_sub -p home/temperature

D. mosquitto_sub -s home/temperature

Solution

1. Step 1: Identify the correct option flag for topic

   The -t flag is used to specify the topic in mosquitto_sub.

2. Step 2: Verify other flags

   -p is for port, -m is for message in mosquitto_pub, and -s is not a valid flag here.

3. Final Answer:

   mosquitto_sub -t home/temperature -> Option B

4. Quick Check:

   -t means topic [OK]

Hint: Use -t to specify topic in mosquitto_sub [OK]

Common Mistakes:

• Using -p instead of -t for topic
• Confusing mosquitto_pub flags with mosquitto_sub
• Using invalid flags like -s

3. What will be the output when running mosquitto_sub -t sensors/humidity in one terminal followed by mosquitto_pub -t sensors/humidity -m 45 in another terminal connected to the same broker?

medium

A. The subscriber terminal will display nothing

B. The publisher terminal will display '45'

C. The subscriber terminal will display '45'

D. Both terminals will display '45'

Solution

1. Step 1: Understand the publish-subscribe flow

   mosquitto_pub sends the message '45' to topic sensors/humidity. The subscriber listens on the same topic.

2. Step 2: Identify where the message appears

   The subscriber terminal receives and displays the message. The publisher terminal does not display the message.

3. Final Answer:

   The subscriber terminal will display '45' -> Option C

4. Quick Check:

   Publisher sends, subscriber receives message [OK]

Hint: Subscriber shows messages sent to its topic [OK]

Common Mistakes:

• Expecting publisher to show message output
• Thinking subscriber won't receive messages
• Confusing topic names or broker connection

4. You run mosquitto_sub -t home/lights but do not see any messages even after publishing with mosquitto_pub -t home/lights -m ON. What is the most likely cause?

medium

A. The subscriber is not connected to the same MQTT broker as the publisher

B. The topic name is misspelled in the publisher command

C. The message payload is empty

D. The subscriber command is missing the -m flag

Solution

1. Step 1: Check connection to MQTT broker

   Both publisher and subscriber must connect to the same broker to exchange messages.

2. Step 2: Evaluate other options

   Topic names match, message payload is 'ON' (not empty), and subscriber does not use -m flag (which is for publishing).

3. Final Answer:

   The subscriber is not connected to the same MQTT broker as the publisher -> Option A

4. Quick Check:

   Same broker needed for pub-sub communication [OK]

Hint: Check broker connection if no messages received [OK]

Common Mistakes:

• Assuming subscriber needs -m flag
• Ignoring broker address mismatch
• Overlooking topic name typos

5. You want to publish the message "Alert: Temperature high!" to the topic alerts/temperature using mosquitto_pub with QoS level 1 and retain the message. Which command is correct?

hard

A. mosquitto_pub -r -q 1 -m "Alert: Temperature high!" alerts/temperature

B. mosquitto_pub -m "Alert: Temperature high!" -t alerts/temperature -q 0 -r

C. mosquitto_pub -t alerts/temperature -m "Alert: Temperature high!" -q 2

D. mosquitto_pub -t alerts/temperature -m "Alert: Temperature high!" -q 1 -r

Solution

1. Step 1: Identify flags for QoS and retain

   -q 1 sets QoS level 1, and -r sets the retain flag in mosquitto_pub.

2. Step 2: Check command syntax and order

   mosquitto_pub -t alerts/temperature -m "Alert: Temperature high!" -q 1 -r correctly uses -t for topic, -m for message, -q 1 for QoS, and -r for retain. Other options have wrong QoS, missing retain, or incorrect argument order.

3. Final Answer:

   mosquitto_pub -t alerts/temperature -m "Alert: Temperature high!" -q 1 -r -> Option D

4. Quick Check:

   Use -q 1 and -r to set QoS and retain [OK]

Hint: Use -q for QoS and -r to retain message [OK]

Common Mistakes:

• Using wrong QoS level
• Forgetting the -r flag for retain
• Incorrect argument order causing errors