mosquitto_pub and mosquitto_sub commands in IOT Protocols - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to publish a message 'Hello' to topic 'test/topic' using mosquitto_pub.

IOT Protocols

mosquitto_pub -t [1] -m "Hello"

Drag options to blanks, or click blank then click option'

Amqtt/topic

Bhello/world

Ctest/topic

Dtopic/test

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong topic name.

Confusing the topic with the message.

The -t option specifies the topic to publish to. Here, 'test/topic' is the correct topic.

2fill in blank

medium

Complete the command to subscribe to the topic 'sensor/data' using mosquitto_sub.

IOT Protocols

mosquitto_sub -t [1]

Drag options to blanks, or click blank then click option'

Adata/sensor

Bsensor/data

Csensor/info

Ddata/sensors

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping words in the topic name.

Using a similar but incorrect topic.

The -t option specifies the topic to subscribe to. 'sensor/data' is the correct topic here.

3fill in blank

hard

Fix the error in the command to publish message 'Data' to topic 'home/room' with QoS level 1.

IOT Protocols

mosquitto_pub -t home/room -m Data -q [1]

Drag options to blanks, or click blank then click option'

A1

B2

C0

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid QoS levels like 3.

Confusing QoS with other options.

The -q option sets the QoS level. Valid values are 0, 1, or 2. Here, QoS 1 is required.

4fill in blank

hard

Fill both blanks to subscribe to topic 'alerts' with verbose output and clean session disabled.

IOT Protocols

mosquitto_sub -t [1] [2]

Drag options to blanks, or click blank then click option'

Aalerts

B-v

C-c

D--clean-session

Attempts:

3 left

💡 Hint

Common Mistakes

Using --clean-session which enables clean session.

Not using verbose mode when needed.

The topic is 'alerts'. The -v option enables verbose output. Clean session is disabled by default, so no extra option is needed here.

5fill in blank

hard

Fill all three blanks to publish message 'Update' to topic 'device/status' with retain flag and QoS 2.

IOT Protocols

mosquitto_pub -t [1] -m [2] [3] -q 2

Drag options to blanks, or click blank then click option'

Adevice/status

B"Update"

C-r

D-d

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the message.

Using -d which enables debug mode instead of retain.

The topic is 'device/status'. The message must be quoted as "Update". The -r option sets the retain flag.

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