dart:io library Null safety

File, socket, HTTP, and other I/O support for non-web applications.

Important: Browser-based apps can't use this library. Only the following can import and use the dart:io library:

• Servers
• Command-line scripts
• Flutter mobile apps
• Flutter desktop apps

This library allows you to work with files, directories, sockets, processes, HTTP servers and clients, and more. Many operations related to input and output are asynchronous and are handled using Futures or Streams, both of which are defined in the dart:async library.

To use the dart:io library in your code:

import 'dart:io';

For an introduction to I/O in Dart, see the dart:io library tour.

File, Directory, and Link

An instance of File, Directory, or Link represents a file, directory, or link, respectively, in the native file system.

You can manipulate the file system through objects of these types. For example, you can rename a file or directory:

File myFile = File('myFile.txt');
myFile.rename('yourFile.txt').then((_) => print('file renamed'));

Many methods provided by the File, Directory, and Link classes run asynchronously and return a Future.

FileSystemEntity

File, Directory, and Link all extend FileSystemEntity. In addition to being the superclass for these classes, FileSystemEntity has a number of static methods for working with paths.

To get information about a path, you can use the FileSystemEntity static methods such as FileSystemEntity.isDirectory, FileSystemEntity.isFile, and FileSystemEntity.exists. Because file system access involves I/O, these methods are asynchronous and return a Future.

FileSystemEntity.isDirectory(myPath).then((isDir) {
  if (isDir) {
    print('$myPath is a directory');
  } else {
    print('$myPath is not a directory');
  }
});

HttpServer and HttpClient

The classes HttpClient and HttpServer provide low-level HTTP functionality.

Instead of using these classes directly, consider using more developer-friendly and composable APIs found in packages.

For HTTP clients, look at package:http.

For HTTP servers, look at Write HTTP servers on dart.dev.

Process

The Process class provides a way to run a process on the native machine. For example, the following code spawns a process that recursively lists the files under web.

Process.start('ls', ['-R', 'web']).then((process) {
  stdout.addStream(process.stdout);
  stderr.addStream(process.stderr);
  process.exitCode.then(print);
});

Using Process.start returns a Future, which completes with a Process object when the process has started. This Process object allows you to interact with the process while it is running. Using Process.run returns a Future, which completes with a ProcessResult object when the spawned process has terminated. This ProcessResult object collects the output and exit code from the process.

When using Process.start, you need to read all data coming on the Process.stdout and Process.stderr streams, otherwise the system resources will not be freed.

WebSocket

The WebSocket class provides support for the web socket protocol. This allows full-duplex communications between client and server applications.

A web socket server uses a normal HTTP server for accepting web socket connections. The initial handshake is a HTTP request which is then upgraded to a web socket connection. The server upgrades the request using WebSocketTransformer and listens for the data on the returned web socket. For example, here's a mini server that listens for 'ws' data on a WebSocket:

runZoned(() async {
  var server = await HttpServer.bind('127.0.0.1', 4040);
  server.listen((HttpRequest req) async {
    if (req.uri.path == '/ws') {
      var socket = await WebSocketTransformer.upgrade(req);
      socket.listen(handleMsg);
    }
  });
}, onError: (e) => print("An error occurred."));

The client connects to the WebSocket using the WebSocket.connect method and a URI that uses the Web Socket protocol. The client can write to the WebSocket with the WebSocket.add method. For example,

var socket = await WebSocket.connect('ws://127.0.0.1:4040/ws');
socket.add('Hello, World!');

Check out the websocket_sample app, which uses WebSockets to communicate with a server.

Socket and ServerSocket

Clients and servers use Sockets to communicate using the TCP protocol. Use ServerSocket on the server side and Socket on the client. The server creates a listening socket using the bind() method and then listens for incoming connections on the socket. For example:

ServerSocket.bind('127.0.0.1', 4041)
  .then((serverSocket) {
    serverSocket.listen((socket) {
      socket.transform(utf8.decoder).listen(print);
    });
  });

A client connects a Socket using the connect() method, which returns a Future. Using write(), writeln(), or writeAll() are the easiest ways to send data over the socket. For example:

Socket.connect('127.0.0.1', 4041).then((socket) {
  socket.write('Hello, World!');
});

Besides Socket and ServerSocket, the RawSocket and RawServerSocket classes are available for lower-level access to async socket IO.

Standard output, error, and input streams

This library provides the standard output, error, and input streams, named stdout, stderr, and stdin, respectively.

The stdout and stderr streams are both IOSinks and have the same set of methods and properties.

To write a string to stdout:

stdout.writeln('Hello, World!');

To write a list of objects to stderr:

stderr.writeAll([ 'That ', 'is ', 'an ', 'error.', '\n']);

The standard input stream is a true Stream, so it inherits properties and methods from the Stream class.

To read text synchronously from the command line (the program blocks waiting for user to type information):

String? inputText = stdin.readLineSync();