Dart Lesson 78 of 102 3 min read

Writing Files in Dart

Learn how to write files in Dart: create and overwrite with writeAsString, append, write bytes, and write large files with IOSink.

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Writing is as direct as reading. One method call creates the file if needed and puts your text in it.

Write a string #

import 'dart:io';

Future<void> main() async {
  final file = File('greeting.txt');

  await file.writeAsString('Hello, file!\n');
  print(await file.readAsString());
}

Hello, file!

If the file does not exist it is created. If it does exist, its contents are replaced.

Append instead of overwrite #

Pass mode: FileMode.append to add to the end.

import 'dart:io';

Future<void> main() async {
  final log = File('app.log');

  await log.writeAsString('App started\n');
  await log.writeAsString('User logged in\n', mode: FileMode.append);
  await log.writeAsString('User logged out\n', mode: FileMode.append);

  print(await log.readAsString());
}

App started
User logged in
User logged out

Mode	Effect
`FileMode.write` (default)	Replace everything
`FileMode.append`	Add to the end
`FileMode.writeOnly`	Replace, without read access
`FileMode.writeOnlyAppend`	Append, without read access

Write several lines #

Join the lines with a newline, or build the text with a StringBuffer.

import 'dart:io';

Future<void> main() async {
  final tasks = ['Buy milk', 'Call Hari', 'Finish the Dart tutorial'];

  await File('tasks.txt').writeAsString('${tasks.join('\n')}\n');
  print('Saved ${tasks.length} tasks');
}

Create missing folders first #

Writing to reports/2026/summary.txt fails if the reports/2026 folder does not exist. create(recursive: true) makes the file and any missing parent folders.

import 'dart:io';

Future<void> main() async {
  final file = File('reports/2026/summary.txt');

  await file.create(recursive: true);
  await file.writeAsString('Total sales: 1200\n');

  print('Wrote ${file.path}');
}

Write bytes #

import 'dart:io';

Future<void> main() async {
  final source = File('photo.jpg');
  final bytes = await source.readAsBytes();

  await File('photo_backup.jpg').writeAsBytes(bytes);
  print('Copied ${bytes.length} bytes');
}

For a plain copy, await source.copy('photo_backup.jpg') does the same without loading the file into memory.

Write a lot, piece by piece, with IOSink #

Calling writeAsString in a loop reopens the file each time. For many writes, open the file once with openWrite, which gives you an IOSink.

import 'dart:io';

Future<void> main() async {
  final sink = File('numbers.csv').openWrite();

  sink.writeln('number,square');
  for (var i = 1; i <= 5; i++) {
    sink.writeln('$i,${i * i}');
  }

  await sink.flush(); // push buffered data to disk
  await sink.close(); // release the file

  print(await File('numbers.csv').readAsString());
}

number,square
1,1
2,4
3,9
4,16
5,25

Always close a sink. Until you do, some data may still be sitting in memory. openWrite(mode: FileMode.append) appends.

Writing structured data #

To save a list or a map, convert it to JSON text first. See working with JSON.

import 'dart:convert';
import 'dart:io';

Future<void> main() async {
  final settings = {'theme': 'dark', 'fontSize': 16, 'recent': ['a.txt', 'b.txt']};

  await File('settings.json')
      .writeAsString(const JsonEncoder.withIndent('  ').convert(settings));

  final loaded = jsonDecode(await File('settings.json').readAsString());
  print(loaded['theme']);
}

dark

Writing safely #

If the program crashes halfway through a write, the file can be left half written. For data that matters, write to a temporary file and then rename it over the original. A rename either happens fully or not at all.

import 'dart:io';

Future<void> saveSafely(String path, String contents) async {
  final temp = File('$path.tmp');
  await temp.writeAsString(contents, flush: true);
  await temp.rename(path);
}

Future<void> main() async {
  await saveSafely('important.txt', 'Do not lose this\n');
  print(await File('important.txt').readAsString());
}

Try it yourself #

Write a tiny diary program. Each time it runs, it reads a line from the keyboard and appends it to diary.txt with the current date and time in front. Then it prints how many entries the diary holds.