mosquitto_pub and mosquitto_sub commands in IOT Protocols - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the mosquitto_pub command?

The mosquitto_pub command is used to send (publish) messages to a topic on an MQTT broker.

beginner

What does the mosquitto_sub command do?

The mosquitto_sub command subscribes to a topic on an MQTT broker to receive messages published to that topic.

beginner

How do you specify the topic when using mosquitto_pub?

Use the -t option followed by the topic name. For example: mosquitto_pub -t 'home/temperature'.

beginner

How can you receive messages from a topic using mosquitto_sub?

Use the -t option with the topic name. For example: mosquitto_sub -t 'home/temperature' will listen for messages on that topic.

beginner

What option do you use with mosquitto_pub to specify the message content?

Use the -m option followed by the message text. For example: mosquitto_pub -t 'home/temperature' -m '22.5' sends the message '22.5'.

Which command is used to subscribe to MQTT messages?

A mosquitto_sub

B mosquitto_pub

C mqtt_subscribe

D mqtt_publish

The mosquitto_sub command subscribes to topics to receive messages.

How do you specify the topic in mosquitto_pub?

A -m

B -t

C -p

D -q

The -t option sets the topic name.

What option sends the message content in mosquitto_pub?

A -m

B -t

C -s

D -c

The -m option specifies the message to publish.

Which command would you use to listen for messages on topic 'sensor/data'?

A mosquitto_pub -t sensor/data

B mosquitto_sub -t sensor/data

C mosquitto_pub -m sensor/data

D mosquitto_sub -m sensor/data

Use mosquitto_sub -t sensor/data to subscribe and listen to that topic.

What happens if you run mosquitto_pub -t 'home/light' -m 'ON'?

A It subscribes to the 'home/light' topic.

B It publishes the message 'ON' to the 'home/light' topic.

C It starts the MQTT broker.

D It deletes the 'home/light' topic.

This command sends the message 'ON' to the 'home/light' topic.

Explain how to publish a message to an MQTT topic using mosquitto_pub.

Think about how you tell the command where and what to send.

Describe how mosquitto_sub works to receive messages.

Focus on how you listen to messages from a topic.

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