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.