I used the code below to save output from an Arduino Uno to a text file.

The problem is whenever I run the code the old data in the text file is deleted.

I don't want the old data to be deleted, what should I change?

import processing.serial.*;
Serial mySerial;
PrintWriter output;
void setup() {
   mySerial = new Serial( this, Serial.list()[0], 9600 );
   output = createWriter( "data.txt" );
void draw() {
    if (mySerial.available() > 0 ) {
         String value = mySerial.readString();
         if ( value != null ) {
              output.println( value );

void keyPressed() {
    output.flush();  // Writes the remaining data to the file
    output.close();  // Finishes the file
    exit();  // Stops the program
  • 1
    See this SO answer for an example. Mar 18, 2015 at 7:40
  • I think the problem is with your file name it's the same every time as the same-named file with the same extension can not be stored. To solve you can add a variable before your file name that increments every time so your file name gets changed every time and you can store that variable in EEPROM for further use.
    – Dharmik
    Mar 16, 2021 at 11:38

2 Answers 2


There are a couple of ways to append to a text file.

  1. You can either read the entire file in a temporary variable, then append the new data to it and then write to file.

  2. you can implement using printWriter instead like mentioned here: https://processing.org/discourse/beta/num_1267767630.html

Let me know if it works.


Load configuration data from the SPIFF file, return 0 if OK

  • SPIFFS open function needs path and mode:
  • mode = "r", "w", "a", "r+", "w+", "a+" (text file) or "rb", "wb", "ab", "rb+", "wb+", "ab+" (binary)
  • where r = read, w = write, `a = append
    • means file is open for update (read and write)
  • b means the file os open for binary operations
  • Returns 0 if OK else:
  • -1 = No SPIFFS file system
  • -2 = File does not exist
  • -3 = File too short
  • -4 = Checksum does not compare
  • 1
    This probably wasn't clear to you, but they're asking about on the host side, using Processing, which looks a Arduino code (for good reasons) but isn't actually Arduino code.
    – timemage
    Mar 17, 2021 at 21:05

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