mosquitto_pub and mosquitto_sub commands in IOT Protocols - Full Explanation

Introduction

Imagine you want to send messages between devices in a simple and organized way. The problem is how to publish messages and listen for them easily in an IoT system. The mosquitto_pub and mosquitto_sub commands help solve this by letting you send and receive messages using the MQTT protocol.

Explanation

mosquitto_pub command

This command is used to send or publish messages to a specific topic on an MQTT broker. You specify the topic and the message content, and the broker then distributes the message to all devices subscribed to that topic. It is a simple way to share information or commands with other devices.

mosquitto_pub lets you send messages to a topic on the MQTT broker.

mosquitto_sub command

This command subscribes to a topic on the MQTT broker to receive messages published to that topic. When a message arrives, it is displayed on your screen or can be processed by your program. This allows devices to listen for updates or commands sent by others.

mosquitto_sub lets you listen for messages on a topic from the MQTT broker.

MQTT broker role

The MQTT broker acts like a post office that receives messages from publishers and delivers them to subscribers. It manages topics and ensures messages reach the right devices. Both mosquitto_pub and mosquitto_sub communicate through this broker.

The MQTT broker routes messages between publishers and subscribers.

Basic usage and options

Both commands have options to specify the broker address, topic name, message content (for publishing), and quality of service levels. These options control how messages are sent and received, allowing flexibility for different network conditions and needs.

Options in mosquitto_pub and mosquitto_sub customize message sending and receiving.

Real World Analogy

Imagine a neighborhood bulletin board where people can post notes (publish) and others can check the board to read new notes (subscribe). The bulletin board keeper (broker) makes sure notes are posted and visible to everyone interested.

mosquitto_pub command → A person posting a note on the bulletin board

mosquitto_sub command → A person checking the bulletin board for new notes

MQTT broker role → The bulletin board keeper who manages posting and reading

Basic usage and options → Choosing which bulletin board to use and what kind of notes to post or read

Diagram

Diagram

┌───────────────┐       publishes       ┌───────────────┐
│ mosquitto_pub │ ───────────────────▶ │ MQTT Broker   │
└───────────────┘                      └───────────────┘
                                         │
                                         │ delivers messages
                                         ▼
                                ┌───────────────┐
                                │ mosquitto_sub │
                                └───────────────┘

This diagram shows mosquitto_pub sending messages to the MQTT broker, which then delivers them to mosquitto_sub subscribers.

Key Facts

mosquitto_pub → A command-line tool to publish messages to an MQTT topic.

mosquitto_sub → A command-line tool to subscribe and receive messages from an MQTT topic.

MQTT broker → A server that routes messages between publishers and subscribers.

Topic → A named channel where messages are published and subscribed to.

Quality of Service (QoS) → A setting that controls message delivery guarantees in MQTT.

Common Confusions

Thinking mosquitto_pub and mosquitto_sub communicate directly without a broker.

Thinking mosquitto_pub and mosquitto_sub communicate directly without a broker. Both commands always communicate through the MQTT broker, which manages message delivery between them.

Believing mosquitto_sub can send messages.

Believing mosquitto_sub can send messages. mosquitto_sub only receives messages; to send messages, use mosquitto_pub.

Summary

mosquitto_pub sends messages to an MQTT topic through a broker.

mosquitto_sub listens for messages on an MQTT topic from the broker.

The MQTT broker manages message delivery between publishers and subscribers.

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