stdin/stdout

Learn I/O with stdin and stdout.

stdin and stdout

stdin and stdout are the input/output objects in Dart's dart:io library. stdin.readLineSync() reads a line from the user; stdout.writeln('msg') prints a line to the screen.

Example: stdout.write('Name: '); final name = stdin.readLineSync(); stdout.writeln('Hello $name');. This asks the user for a name and prints a greeting. It is used for interactive input in CLI applications. readLineSync is synchronous; it waits until a line arrives.

Interactive input

A CLI tool talks to the user: it asks a question, waits for an answer, then acts. You print the question with stdout.write('Question: ') and read the answer with stdin.readLineSync(). The answer comes as a String? so you can give a default with ??.

Example: final name = stdin.readLineSync() ?? 'Guest';. If the user enters nothing or input ends, 'Guest' is used. This makes CLI tools resilient; unexpected input becomes a default instead of a crash.

Exercise Print with stdout

Add import 'dart:io';. Define a function readName that calls stdout.write('Name: ') and returns stdin.readLineSync() ?? 'Guest'. In main set final name = 'Ayşe'; and print with stdout.writeln('Hello $name').

Show example solution

import 'dart:io';

String readName() {
  stdout.write('Name: ');
  return stdin.readLineSync() ?? 'Guest';
}

void main() {
  final name = 'Ayşe';
  stdout.writeln('Hello $name');
}

Test yourself

Match the stdin/stdout parts

stdin

Input
stdout

Output
readLineSync

Reads a line
writeln

Writes a line
What does stdin.readLineSync return?
- double
- bool
- String?
- int
Show answer

String? — stdin.readLineSync returns String?.

The readLineSync call

String readName() {
  stdout.write('Name: ');
  return stdin.____() ?? 'Guest';
}

Show answer

String readName() {
  stdout.write('Name: ');
  return stdin.readLineSync() ?? 'Guest';
}

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