Dart Lesson 82 of 102 3 min read

File Attributes and Metadata in Dart

Read file attributes in Dart: size, last modified and accessed times, type and permissions, using length, lastModified and stat.

On this page

Besides its contents, every file carries information about itself: how big it is, when it was last changed, and who may read it. This is the file’s metadata.

Size #

import 'dart:io';

String readableSize(int bytes) {
  const units = ['B', 'KB', 'MB', 'GB'];
  var size = bytes.toDouble();
  var unit = 0;
  while (size >= 1024 && unit < units.length - 1) {
    size /= 1024;
    unit++;
  }
  return '${size.toStringAsFixed(unit == 0 ? 0 : 1)} ${units[unit]}';
}

Future<void> main() async {
  final file = File('pubspec.yaml');
  final bytes = await file.length();

  print('$bytes bytes');
  print(readableSize(bytes));
  print(readableSize(5368709));
}

312 bytes
312 B
5.1 MB

Last modified #

import 'dart:io';

Future<void> main() async {
  final file = File('pubspec.yaml');
  final modified = await file.lastModified();
  final age = DateTime.now().difference(modified);

  print('Last modified: $modified');
  print('That was ${age.inDays} day(s) ago');
}

You can also change it with await file.setLastModified(DateTime.now()).

Everything at once with stat #

stat() returns a FileStat holding all the metadata from a single call. It works on files and directories.

import 'dart:io';

Future<void> main() async {
  final stat = await File('pubspec.yaml').stat();

  print('Type:     ${stat.type}');
  print('Size:     ${stat.size} bytes');
  print('Modified: ${stat.modified}');
  print('Accessed: ${stat.accessed}');
  print('Changed:  ${stat.changed}');
  print('Mode:     ${stat.modeString()}');
}

Type:     file
Size:     312 bytes
Modified: 2026-10-08 14:22:05.000
Accessed: 2026-10-10 09:01:44.000
Changed:  2026-10-08 14:22:05.000
Mode:     rw-r--r--

Property	Meaning
`type`	File, directory, link or not found
`size`	Size in bytes
`modified`	When the contents last changed
`accessed`	When it was last read
`changed`	When its metadata last changed. On Windows, the creation time
`mode` / `modeString()`	Permissions

stat does not throw for a missing path. It returns a result whose type is FileSystemEntityType.notFound.

Reading permissions #

modeString() shows permissions in the Unix style: three groups of rwx for the owner, the group and everyone else.

rw-r--r--
│  │  └── others: read
│  └───── group:  read
└──────── owner:  read and write

On Windows the value is less meaningful, since Windows uses a different permission system.

Dart cannot change permissions directly. Run the system command:

import 'dart:io';

Future<void> main() async {
  if (!Platform.isWindows) {
    final result = await Process.run('chmod', ['+x', 'tool.sh']);
    print(result.exitCode == 0 ? 'Now executable' : result.stderr);
  }
}

Practical example: find files changed recently #

import 'dart:io';

Future<void> main() async {
  final cutoff = DateTime.now().subtract(Duration(days: 7));

  await for (final entity in Directory('lib').list(recursive: true)) {
    if (entity is! File) continue;
    final stat = await entity.stat();
    if (stat.modified.isAfter(cutoff)) {
      print('${entity.path}  (${stat.size} bytes)');
    }
  }
}

Practical example: is the cache still fresh? #

import 'dart:io';

Future<bool> isFresh(File file, Duration maxAge) async {
  final stat = await file.stat();
  if (stat.type == FileSystemEntityType.notFound) return false;
  return DateTime.now().difference(stat.modified) < maxAge;
}

Future<void> main() async {
  final cache = File('weather_cache.json');

  if (await isFresh(cache, Duration(minutes: 30))) {
    print('Using cached data');
  } else {
    print('Cache is missing or old, downloading again');
  }
}

Comparing two files #

FileSystemEntity.identical(pathA, pathB) tells you whether two paths refer to the very same file on disk, for instance through a link. To check whether two different files have the same contents, compare their sizes first, which is quick, and their bytes only if the sizes match.

Try it yourself #

Write a program that lists every file in a folder, sorted from most to least recently modified, showing the date and a human-readable size for each.