In this post you’re going to test the Mosquitto Broker and Client on a Raspberry Pi by subscribing to an MQTT topic and publishing sample messages.
Recommended resources:
- You need a Raspberry Pi board – read Best Raspberry Pi Starter Kits
- How to Install Mosquitto Broker on Raspberry Pi
- What is MQTT and How It Works
- Getting Started with Node-RED on Raspberry Pi
If you like home automation and you want to build a complete home automation system, I recommend downloading my home automation course.
Testing MQTT Broker Installation
After installing MQTT Broker, I recommend installing an MQTT Cllient to test the Broker installation and publish sample messages.
The next command shows how to install MQTT Mosquitto Client:
[email protected]:~ $ sudo apt-get install mosquitto-clients
You’ll have to type Y and press Enter to confirm the installation.
Run Mosquitto on background as a daemon:
[email protected]:~ $ mosquitto -d
Subscribing to testTopic Topic
To subscribe to an MQTT topic with Mosquitto Client open a terminal Window #1 and enter the command:
[email protected]:~ $ mosquitto_sub -d -t testTopic
You’re now subscribed to a topic called testTopic.
Publishing “Hello World!” Message to testTopic Topic
To publish a sample message to testTopic, open a terminal Window #2 and run this command:
[email protected]:~ $ mosquitto_pub -d -t testTopic -m "Hello world!"
The message “Hello World!” is received in Window #1 as illustrated in the figure above.
Publishing a Message to Multiple Clients
Having Window #1 still subscribed to topic testTopic, open a new terminal Window #3 and run this command to subscribe to testTopic topic:
[email protected]:~ $ mosquitto_sub -d -t testTopic
On Window #2 publish the “Hello World!” message:
[email protected]:~ $ mosquitto_pub -d -t testTopic -m "Hello world!"
Since two clients are subscribed to testTopic topic, they will both receive “Hello world!” message.
This simple example shows how MQTT works and how your devices (for example: ESP8266) could be subscribed to the same topic to receive messages or a device could publish messages to multiple devices. We’ll explore this concept further in future blog posts.
Wrapping up
In the next blog post, we will experiment with MQTT and ESP8266 to see how everything works with practical examples.
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Hi Rui, Odd things happening for me on this one. I am using a Rpi2b, Raspian Pixel, Mosquitto ver 1.4.14. When I type in the second terminal window “mosquitto_pub -d -t testTopic -m “Hello World” the following occurs:
1 I receive an Error: Unknown option ‘world’
2 I noticed when I typed the command again on the first depression of the quote (shift + ‘) the quote did not show. (it does show in text editor).
AND if I type “mosquitto_pub -d -t testTopic -m Hello World (without the quotes) I get the same error, but if I type mosquitto_pub -d -t testTopic -m Hello, the message “Hello” is transmitted. Any thoughts on this?
Put double quotes around “Hello World” and it should work.
I have tried using my PC with a virtual machine and linux installed and it works, Thanks!
awesome tutorial, thanks. So many questions answered in my head, going through it and seeing it work really helps the understanding.
Hi Rui,
This is a very helpful tutorial series for someone starting with MQTT. May I suggest that the flow of the process would be easier to follow if the order of the Terminal window images was changed. The window where the command is entered should appear first, followed by the windows where the message is received.
Also, in the paragraph before Wrapping Up, you mention “a device could publish messages to multiple devices”. This contradicts your earlier explanation that messages are published to TOPICS, and devices must subscribe to topics to receive messages.
I’ve been following RNT for a few years and learned a lot. Keep up the great work.
Hello Michael. Thanks for the helpful feedback!
Both statements are true. Multiple boards can be subscribed to the same topic. So, one board can publish readings to all devices subscribed to that topic.
I’m glad you’ve found our content helpful.
Regards,
Rui
This was helpful, but not real clear how to get mosquitto to respond to a different machine.
To clarify to myself, I did this between two RaspberryPi’s. Then tried to Node-RED to help me understand.
explanation methodology is well:)