# CloudAMQP with MQTT from the command line

The [Mosquitto](https://mosquitto.org) clients, `mosquitto_pub` and `mosquitto_sub`, publish and subscribe from a terminal: a quick way to try MQTT, check your connection details or watch a topic.

```bash
sudo apt-get install mosquitto-clients   # Debian and Ubuntu
brew install mosquitto                   # macOS
```

## Publish and subscribe

Set `MQTT_HOST`, `MQTT_PORT`, `MQTT_USERNAME` and `MQTT_PASSWORD` to the MQTT details on your instance’s details page in the CloudAMQP console. Port 8883 uses TLS. Without them, the scripts connect to a local broker.

Save the publisher as `publisher.sh`:

```bash
#!/bin/sh
set -eu

port=${MQTT_PORT:-1883}
# Port 8883 uses TLS, verified against the system's certificates
[ "$port" = 8883 ] && set -- --tls-use-os-certs

# QoS 1 waits for the broker's PUBACK
mosquitto_pub "$@" -h "${MQTT_HOST:-localhost}" -p "$port" \
  -u "${MQTT_USERNAME:-guest}" -P "${MQTT_PASSWORD:-guest}" \
  -q 1 -t hello -m "Hello world!"
echo " [x] Sent 'Hello world!'"
```

Save the subscriber as `worker.sh`:

```bash
#!/bin/sh
set -eu

port=${MQTT_PORT:-1883}
# Port 8883 uses TLS, verified against the system's certificates
[ "$port" = 8883 ] && set -- --tls-use-os-certs

# Subscribes with QoS 1, and prints each message until you press CTRL+C
exec mosquitto_sub "$@" -h "${MQTT_HOST:-localhost}" -p "$port" \
  -u "${MQTT_USERNAME:-guest}" -P "${MQTT_PASSWORD:-guest}" \
  -q 1 -t hello -F " [x] Received '%p'"
```

Make them executable with `chmod +x publisher.sh worker.sh`, then run `./worker.sh` in one terminal and `./publisher.sh` in another.

A runnable example is in [shell/mosquitto in cloudamqp/examples](https://github.com/cloudamqp/examples/tree/main/shell/mosquitto).

## Good to know

- **Put the vhost in the username**, as `vhost:user`. MQTT has no vhost of its own, and the console shows the username that way.
- **Use `--tls-use-os-certs` for TLS** on port 8883. It verifies the server certificate against the system’s certificates; don’t add `--insecure`.
- **Use QoS 1** with `-q 1`, so `mosquitto_pub` waits for the broker’s PUBACK.
- **Add `-d` to see each packet**, which helps when a connection fails.