mosquitto_pub and mosquitto_sub commands in IOT Protocols - Step-by-Step Execution

Process Flow - mosquitto_pub and mosquitto_sub commands

Start mosquitto_sub

↓

Subscribe to topic

↓

Wait for messages

↓

Start mosquitto_pub

↓

Publish message to topic

↓

Message received by mosquitto_sub

↓

Display message

↓

End

The subscriber listens on a topic waiting for messages. The publisher sends a message to that topic. The subscriber receives and shows the message.

Execution Sample

IOT Protocols

mosquitto_sub -t 'home/temperature'
mosquitto_pub -t 'home/temperature' -m '22C'

Subscribe to 'home/temperature' topic and publish '22C' message to it.

Process Table

Step	Command	Action	Result/Output
1	mosquitto_sub -t 'home/temperature'	Start subscriber listening on topic 'home/temperature'	Waiting for messages...
2	mosquitto_pub -t 'home/temperature' -m '22C'	Publish message '22C' to topic 'home/temperature'	Message sent
3	mosquitto_sub receives message	Subscriber gets message from topic	Received message: 22C
4	End	No more messages sent	Subscriber continues waiting or exits if stopped

💡 Execution stops when subscriber is manually stopped or no more messages are published.

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
topic	none	'home/temperature'	'home/temperature'	'home/temperature'
message	none	'22C'	'22C'	'22C'
subscriber_state	waiting	waiting	received message	waiting or stopped

Key Moments - 3 Insights

Why does mosquitto_sub show no output immediately after starting?

Because it waits silently for messages on the subscribed topic until a message is published, as shown in step 1 of the execution_table.

What happens if mosquitto_pub publishes to a different topic than mosquitto_sub is subscribed to?

The subscriber will not receive the message because it listens only to the specified topic, so no output appears in step 3.

Can mosquitto_sub receive multiple messages without restarting?

Yes, it keeps listening and displays each message received on the topic until manually stopped, as indicated in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what does mosquitto_sub display immediately after starting?

A "Waiting for messages..."

B "Message sent"

C "Received message: 22C"

D "No output"

💡 Hint

Check Step 1 in the execution_table under Result/Output.

At which step does the subscriber receive the published message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for when 'Received message: 22C' appears in the execution_table.

If the topic in mosquitto_pub is changed to 'home/humidity', what happens in the subscriber output?

A Subscriber receives '22C' message

B Subscriber shows 'Waiting for messages...' indefinitely

C Subscriber shows 'Message sent'

D Subscriber crashes

💡 Hint

Refer to key_moments about topic matching and step 3 in execution_table.

Concept Snapshot

mosquitto_sub -t <topic>  # Subscribe and wait for messages on <topic>
mosquitto_pub -t <topic> -m <message>  # Publish <message> to <topic>
Subscriber waits silently until message arrives
Publisher sends message once
Subscriber shows message when received
Topics must match for message delivery

Full Transcript

This visual execution shows how mosquitto_sub and mosquitto_pub commands work together. First, mosquitto_sub starts and listens on a topic, waiting silently for messages. Then mosquitto_pub sends a message to that topic. The subscriber receives the message and displays it. If the topics do not match, the subscriber does not receive the message. The subscriber keeps listening until stopped. This helps beginners see the flow of publishing and subscribing in MQTT using these commands.

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