Dart Lesson 79 of 102 3 min read

Checking If a File Exists and Deleting Files in Dart

Learn to check whether a file or directory exists in Dart, and to delete, rename, move and copy files safely with dart:io.

Before reading a file you may want to know whether it is there. When you are done with one, you may want to remove it, rename it or copy it.

Does it exist? #

import 'dart:io';

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

  if (await file.exists()) {
    print('Found ${file.path}');
  } else {
    print('${file.path} is missing');
  }

  // Synchronous version
  print(File('pubspec.yaml').existsSync());
}

File.exists() is true only if the path exists and is a file. For a folder, use Directory(path).exists().

What kind of thing is at this path? #

When you do not know whether a path is a file, a folder or a link, ask FileSystemEntity.

import 'dart:io';

Future<void> describe(String path) async {
  final type = await FileSystemEntity.type(path);

  final label = switch (type) {
    FileSystemEntityType.file => 'a file',
    FileSystemEntityType.directory => 'a directory',
    FileSystemEntityType.link => 'a link',
    _ => 'not there',
  };
  print('$path is $label');
}

Future<void> main() async {
  await describe('pubspec.yaml');
  await describe('lib');
  await describe('nothing-here');
}

pubspec.yaml is a file
lib is a directory
nothing-here is not there

There are also the shortcuts FileSystemEntity.isFile(path) and FileSystemEntity.isDirectory(path).

Check, or just try? #

Checking and then acting leaves a small gap in which another program could delete the file. For that reason, the most robust code simply attempts the operation and handles the failure.

import 'dart:io';

Future<String?> readIfPresent(String path) async {
  try {
    return await File(path).readAsString();
  } on PathNotFoundException {
    return null;
  }
}

Future<void> main() async {
  final text = await readIfPresent('maybe.txt');
  print(text ?? 'No such file, using defaults');
}

No such file, using defaults

Use exists() when you need to decide something, such as whether to create a default config file. Use try/catch when you are about to use the file.

Delete a file #

import 'dart:io';

Future<void> main() async {
  final file = File('temp.txt');
  await file.writeAsString('temporary');

  await file.delete();
  print('Still there? ${await file.exists()}');
}

Still there? false

Deleting is permanent. The file does not go to the recycle bin. Deleting a file that does not exist throws a PathNotFoundException.

Delete only if it exists #

import 'dart:io';

Future<bool> deleteIfExists(String path) async {
  try {
    await File(path).delete();
    return true;
  } on PathNotFoundException {
    return false;
  }
}

Future<void> main() async {
  print(await deleteIfExists('old-cache.txt'));
}

Rename or move #

rename returns a File for the new location. Renaming to a path in a different folder moves the file.

import 'dart:io';

Future<void> main() async {
  final draft = File('draft.txt');
  await draft.writeAsString('My essay');

  final finalFile = await draft.rename('essay.txt');

  print(finalFile.path);
  print(await draft.exists());
}

essay.txt
false

If the target already exists it is replaced. A rename across different drives may fail; in that case copy the file and delete the original.

Copy #

import 'dart:io';

Future<void> main() async {
  final original = File('essay.txt');
  final backup = await original.copy('essay_backup.txt');

  print('${await original.length()} bytes copied to ${backup.path}');
}

Delete safely #

Before deleting anything based on user input or a calculated path:

• Print or log the full path first while you are developing.
• Check that the path is inside the folder you expect.
• Never build a delete path from unchecked user input.
• For folders, delete(recursive: true) removes everything inside. Double-check the path. See directories.

Quick reference #

Try it yourself #

Write a “safe save” function: if data.txt already exists, rename it to data.bak (replacing any old backup), then write the new contents to data.txt. Test it by saving three times and checking both files.