MQTT related example code of Arduino

Get_IP_Ethernet.ino

#include <SPI.h>
#include <Ethernet.h>

// Any MAC that won't conflice with any device in your network will do
byte mac[] = { 0x00, 0xAA, 0xBB, 0xCC, 0xDe, 0x02 };

EthernetClient client;

void setup() {
  
  Serial.begin(9600);
  // Wait until console is opened
  while(!Serial);

  if (Ethernet.begin(mac) == 0) {
    Serial.println("Can not get IP address");
    for(;;)
      ;
  }

  Serial.print("IP address:");
  // How you retrieve local IP
  Serial.println(Ethernet.localIP());
  
}

void loop() {
}

MQTT_basic_example.ino

/*
 Basic MQTT example

 This sketch demonstrates the basic capabilities of the library.
 It connects to an MQTT server then:
  - publishes "hello world" to the topic "outTopic"
  - subscribes to the topic "inTopic", printing out any messages
    it receives. NB - it assumes the received payloads are strings not binary

 It will reconnect to the server if the connection is lost using a blocking
 reconnect function. See the 'mqtt_reconnect_nonblocking' example for how to
 achieve the same result without blocking the main loop.
 
*/

#include <SPI.h>
#include <Ethernet.h>
#include <PubSubClient.h>

// Update these three with values suitable for your network.
byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
// IP of mqtt client (e.g. Arduino)
IPAddress ip(192, 168, 0, 111);
// IP of mqtt broker (e.g. Raspberry Pi)
IPAddress server(192, 168, 0, 100);

void callback(char* topic, byte* payload, unsigned int length) {
  Serial.print("Message arrived [");
  Serial.print(topic);
  Serial.print("] ");
  for (int i=0;i<length;i++) {
    Serial.print((char)payload[i]);
  }
  Serial.println();
}

EthernetClient ethClient;
PubSubClient client(ethClient);

void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Attempt to connect
    if (client.connect("arduinoClient")) {
      Serial.println("connected");
      // Once connected, publish an announcement...
      client.publish("outTopic","hello world");
      // ... and resubscribe
      client.subscribe("inTopic");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void setup()
{
  Serial.begin(57600);
  while(!Serial);
  client.setServer(server, 1883);
  client.setCallback(callback);

  Ethernet.begin(mac, ip);
  // Allow the hardware to sort itself out
  delay(1500);
}

void loop()
{
  if (!client.connected()) {
    reconnect();
  }
  client.loop();
}

MQTT_htu21d_led.ino

/*
 Basic MQTT example

 This sketch demonstrates the basic capabilities of the library.
 It connects to an MQTT server then:
  - publishes "hello world" to the topic "outTopic"
  - subscribes to the topic "inTopic", printing out any messages
    it receives. NB - it assumes the received payloads are strings not binary

 It will reconnect to the server if the connection is lost using a blocking
 reconnect function. See the 'mqtt_reconnect_nonblocking' example for how to
 achieve the same result without blocking the main loop.
 
*/

#include <SPI.h>
#include <Ethernet.h>
#include <PubSubClient.h>
#include <Wire.h>
#include "HTU21D.h"
HTU21D humidity;

// Update these with values suitable for your network.
byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
IPAddress ip(192, 168, 1, 109);
IPAddress server(192, 168, 1, 108);

int LED_PIN = 2;

void callback(char* topic, byte* payload, unsigned int length) {
  char message_buff[100];
  Serial.println("Message arrived:  topic: " + String(topic));
  Serial.println("Length: " + String(length,DEC));
  // create character buffer with ending null terminator (string)
  for (int i = 0; i < length; i++) {
    message_buff[i] = payload[i];
  }
  message_buff[length] = '\0';
  String msgString = String(message_buff);
  Serial.println("Payload: " + msgString);
  if(msgString == "on") {
    digitalWrite(LED_PIN, HIGH);
    Serial.println("LED on");
  } else if(msgString == "off") {
    digitalWrite(LED_PIN, LOW);
    Serial.println("LED off");
  } else {
    Serial.println("Do nothing");
  }
}

EthernetClient ethClient;
PubSubClient client(ethClient);

void reconnect() {
  // Loop until we're reconnected
  while (!client.connected()) {
    Serial.print("Attempting MQTT connection...");
    // Attempt to connect
    if (client.connect("arduinoClient")) {
      Serial.println("connected");
      // Once connected, publish an announcement...
      client.publish("outTopic","hello world");
      // ... and resubscribe
      client.subscribe("inTopic");
    } else {
      Serial.print("failed, rc=");
      Serial.print(client.state());
      Serial.println(" try again in 5 seconds");
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
}

void setup()
{
  Serial.begin(57600);
  while(!Serial);
  // start HTU21D
  humidity.begin();
  // start mqtt client
  client.setServer(server, 1883);
  client.setCallback(callback);

  Ethernet.begin(mac, ip);

  // start LED
  pinMode(2, OUTPUT);
  // Allow the hardware to sort itself out
  delay(1500);
}

void loop()
{
  if (!client.connected()) {
    reconnect();
  }
  client.loop();

  // read temperature
  float temp = humidity.readTemperature();
  // read humidity
  float humd = humidity.readHumidity();

  String result = "Temperature: " + String(temp, 1) + "; Humidity: " + String(humd, 1) + "%";

  // uncomment to try
//  if(temp > 26) {
//    client.publish("inTopic", "on");
//  } else {
//    client.publish("inTopic", "off");
//  }
  Serial.println(result);
  client.publish("outTopic",string2char(result));
  delay(1000);

}

char* string2char(String command){
    if(command.length()!=0){
        char *p = const_cast<char*>(command.c_str());
        return p;
    }
}