NodeMCU_ESP8266_SPIFF_file_system_basics

// NodeMCU SPI Flash File System (#SPIFFS) module example
// Writing 10 strings to SPIFFS, and reading back afterwards.

#include "FS.h"

// The Amount of data to write and read from SPIFFS
int writenumber = 25;

void setup() {
  Serial.begin(9600);
  Serial.println("\nNodeMCU SPI Flash File System (#SPIFFS) module example. Writing 10 strings to SPIFFS, and reading back afterwards.");
  SPIFFS.begin();

  // Next lines have to be done ONLY ONCE!!!!!
  // Unquote them for the 1st run !!!!
  // When SPIFFS is formatted ONCE you can comment these lines out!!
  // START unquoting below this line.....

  // Serial.println("Please wait 30 secs for SPIFFS to be formatted");
  // SPIFFS.format();
  // Serial.println("Spiffs formatted");

  // END unquoting below this line.....


  Serial.println("Started...");
  Serial.println();
}

void loop() {

  // open file for writing
  File f = SPIFFS.open("/f.txt", "w");
  if (!f) {
      Serial.println("file open failed");
  }
  Serial.println("====== Writing to SPIFFS file =========");
  // write 10 strings to file
  for (int i=1; i<=writenumber; i++){
    f.print("Millis() : ");
    f.println(millis());
    Serial.println(millis());
  }

  f.close();

  // open file for reading
  f = SPIFFS.open("/f.txt", "r");
  if (!f) {
      Serial.println("file open failed");
  }  Serial.println("====== Reading from SPIFFS file =======");
  // read 10 strings from file
  for (int i=1; i<=writenumber; i++){
    String s=f.readStringUntil('\n');
    Serial.print(i);
    Serial.print(":");
    Serial.println(s);
  }

  // wait a few seconds before doing it all over again
  delay(10000);

}