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2. Testing Mosquitto from the terminal

Testing

If you have already completed Part 1, now you can test mosquitto broker in your local machine. Open one terminal (Win + cmd) and go to the route where mosquitto was installed

cd C:\Program Files\mosquitto

Info

If you have already configured the variable path in Part 1, the previous step should not be necessary.

Creating a subscriber

This terminal will act as a subscriber and will receive the messages published in the topic /test. To do so, run the following command

mosquitto_sub -h localhost -t /test

  1. mosquitto_sub

    • This is the Mosquitto client tool used to subscribe to an MQTT topic.
    • It listens for messages published to specific topics.

  2. -h localhost

    • -h specifies the host or broker address to connect to.
    • localhost means the broker is running on the same machine where the command is being executed. If the broker is on a remote server, you'd need to replace localhost with the server's IP address or domain name.

  3. -t /test

    • -t specifies the topic to subscribe to.
    • /test is the topic name in this example. Topics are hierarchical strings used by MQTT brokers to route messages. In this case, the client will subscribe to the /test topic and receive any messages published to it.

Explanation:

The command subscribes to the MQTT topic /test on a broker running locally (at localhost). Messages sent to the /test topic by other MQTT clients or publishers will be received and displayed by this subscriber.

Creating a publisher

Without closing that terminal, open another one (Win + cmd). This second terminal will act as a publisher that will publish the message This is a testing message: Hello, MQTT! in the topic /test. This message will be received by the subscriber and will display it in the terminal

mosquitto_pub -h localhost -t /test -m "This is a testing message: Hello, MQTT!"

  1. mosquitto_pub

    • This is the Mosquitto client tool used to publish messages to an MQTT topic.
    • It sends messages to the specified topic on the broker.

  2. -h localhost

    • -h specifies the host or broker address to connect to.
    • localhost means the broker is running on the same machine where the command is being executed. If the broker is on a remote server, you'd replace localhost with the server's IP address or domain name.

  3. -t test

    • -t specifies the topic to which the message will be published.
    • test is the topic name in this example. Other MQTT clients subscribed to this topic will receive the message.

  4. -m "This is a testing message: Hello, MQTT!"

    • -m specifies the message to publish.
    • "This is a testing message: Hello, MQTT!" is the message content being sent to the test topic. Clients subscribed to the topic will see this exact message.

Explanation:

The command publishes the message "This is a testing message: Hello, MQTT!" to the MQTT topic test on a broker running locally (at localhost). Any MQTT clients subscribed to the test topic will receive this message.

Expected result

If you have correctly done all the steps, you should have seen the following:

Note

The subscriber will keep running indefinitely, waiting for messages. On the other hand, the publisher runs only once to send a single message. If you want to send more messages, you will need to re-run the entire command each time