Dart Stdout

last modified April 4, 2025

The Stdout class in Dart provides methods for writing output to the console. It's part of Dart's dart:io library and is essential for command-line applications.

Stdout offers more control than basic print statements, including byte-level writing, line control, and synchronous operations. It's the standard way to interact with console output in Dart.

Basic Definition

Stdout is an abstract class representing standard output. It's typically accessed via stdout from the dart:io library.

Key features include text and byte writing, line control, and terminal interaction. It supports both synchronous and asynchronous operations for different use cases.

Basic Stdout Usage

This example shows basic console output using Stdout.

import 'dart:io';

void main() {
  stdout.write('Hello, ');
  stdout.write('Dart!');
  stdout.writeln();
  stdout.writeln('This is a new line');
  
  print('Using print() for comparison');
}

We use write() for continuous output and writeln() for line-terminated output. Unlike print(), Stdout methods don't automatically add newlines.

$ dart main.dart
Hello, Dart!
This is a new line
Using print() for comparison

Writing Bytes to Stdout

This example demonstrates writing raw bytes to standard output.

import 'dart:io';

void main() {
  // Write bytes directly
  stdout.add([72, 101, 108, 108, 111]); // 'Hello' in ASCII
  
  // Write a single byte
  stdout.writeByte(10); // Newline character
  
  // Write from a stream
  var stream = Stream.fromIterable([66, 121, 101]);
  stdout.addStream(stream).then((_) {
    stdout.writeln();
  });
}

We use add() for byte lists, writeByte() for single bytes, and addStream() for asynchronous byte streams. This is useful for binary data.

$ dart main.dart
Hello
Bye

Controlling Line Behavior

This example shows different line-writing techniques with Stdout.

import 'dart:io';

void main() {
  // Write without newline
  stdout.write('First line');
  
  // Force immediate output
  stdout.flush();
  
  // Write with newline
  stdout.writeln('Second line');
  
  // Write multiple lines
  stdout.writeAll(['Third', 'Fourth', 'Fifth'], '\n');
  stdout.writeln();
}

flush() ensures output is written immediately. writeAll() writes multiple items with a separator. These methods provide precise control over line endings.

$ dart main.dart
First lineSecond line
Third
Fourth
Fifth

Checking Terminal Capabilities

This example demonstrates checking terminal properties before output.

import 'dart:io';

void main() {
  if (stdout.hasTerminal) {
    print('Terminal width: ${stdout.terminalColumns}');
    print('Terminal height: ${stdout.terminalLines}');
    
    if (stdout.supportsAnsiEscapes) {
      stdout.writeln('\x1B[31mRed Text\x1B[0m');
    }
  } else {
    stdout.writeln('No terminal detected');
  }
}

We check for terminal presence, get dimensions, and test for ANSI escape support. This helps create adaptive console applications that work in different environments.

$ dart main.dart
Terminal width: 80
Terminal height: 24
Red Text

Synchronous vs Asynchronous Writing

This example compares synchronous and asynchronous output methods.

import 'dart:io';

void main() async {
  // Synchronous writing
  stdout.write('Sync 1 ');
  stdout.write('Sync 2 ');
  
  // Asynchronous writing
  await stdout.addStream(Stream.fromIterable([
    'Async 1 '.codeUnits,
    'Async 2 '.codeUnits
  ].expand((x) => x)));
  
  stdout.writeln();
  print('Done');
}

Synchronous writes execute immediately while asynchronous writes using addStream() complete later. Mixing them requires care to maintain output order.

$ dart main.dart
Sync 1 Sync 2 Async 1 Async 2 
Done

Best Practices

• Buffering: Use flush() when immediate output is needed
• Error handling: Check hasTerminal for CLI apps
• Performance: Prefer bulk writes for large outputs
• ANSI codes: Verify supportsAnsiEscapes first

Source

Dart Stdout Documentation

This tutorial covered Dart's Stdout class with practical examples showing basic output, byte writing, terminal control, and asynchronous operations.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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