Write Command-Line Apps

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An introduction to standalone apps

What’s the point?

  • Command-line applications need to do input and output.
  • The dart:io library provides I/O functionality.
  • The args package helps define and parse command-line arguments.
  • The dart:async library supports asynchronous programming with Future and Stream classes.
  • A Future represents a value that will be available at some time in the future.
  • Streams provide a series of asynchronous data events.
  • Most input and output requires the use of Streams.

This tutorial teaches you how to build command-line apps and shows you a few small command-line applications. These programs use resources that most command-line applications need, including the standard output, error, and input streams, command-line arguments, files and directories, and more.

This tutorial uses the dcat example. You can get it by downloading the tutorial examples from GitHub.

Running an app with the standalone Dart VM

To run a command-line app, you need the Dart VM (dart), which comes when you install the Dart SDK.

The location of the SDK installation directory (we’ll call it <sdk-install-dir>) depends on your platform and how you installed the SDK. You can find dart in <sdk-install-dir>/bin. By putting this directory in your PATH you can refer to the dart command and other commands, such as dartanalyzer, by name.

Let’s run a small program.

  1. Create a file called helloworld.dart that contains this code:

    void main() {
      print('Hello, World!');
    }
    
  2. In the directory that contains the file you just created, run the program with the command as shown by the highlighted text.

    % dart helloworld.dart
    Hello, World!
    %
    

The Dart VM supports many options. Use dart --help to see commonly used options. Use dart --verbose to see all options.

Review briefly the dcat example code

Take a quick look at the code for a small sample called dcat, which displays the contents of any files listed on the command line. This program uses various classes, functions, and properties available to command-line apps. This tutorial goes into detail about this app in the following sections. For a brief look now, hover over the highlighted code below for explanations.

import 'dart:io';
import 'dart:convert';
import 'dart:async';

import 'package:args/args.dart';

const lineNumber = 'line-number';

ArgResults argResults;

void main(List<String> arguments) {
  exitCode = 0; //presume success
  final parser = new ArgParser()
      ..addFlag(lineNumber, negatable: false, abbr: 'n');

  argResults = parser.parse(arguments);
  List<String> paths = argResults.rest;

  dcat(paths, argResults[lineNumber]);
}

Future dcat(List<String> paths, bool showLineNumbers) async {
  if (paths.isEmpty) {
    // No files provided as arguments. Read from stdin and print each line.
    stdin.pipe(stdout);
  } else {
    for (var path in paths) {
      int lineNumber = 1;
      Stream lines = new File(path)
          .openRead()
          .transform(UTF8.decoder)
          .transform(const LineSplitter());
      try {
        await for (var line in lines) {
          if (showLineNumbers) {
            stdout.write('${lineNumber++} ');
          }
          stdout.writeln(line);
        }
      } catch (_) {
        _handleError(path);
      }
    }
  }
}

Future _handleError(String path) async {
  if (await FileSystemEntity.isDirectory(path)) {
      stderr.writeln('error: $path is a directory');
    } else {
      exitCode = 2;
    }
  });
}

Parsing command-line arguments

The args package, a software bundle that contains a library of Dart code, provides parser support for transforming raw command-line arguments into a set of options, flags, and additional values. Import the library as follows:

import 'package:args/args.dart';

The args library contains two classes:

Library Description
ArgParser A class that parses command-line arguments
ArgResults The result of parsing command-line arguments using ArgParser.

Let’s take a look at the dcat sample, which uses ArgParser and ArgResults to parse and store its command-line arguments.

  1. Copy the sample file from the github repo: dcat.dart.

  2. Run the program from the command line as shown by the boldface text.

    $ dart dcat.dart -n quotes.txt
    1 Be yourself. Everyone else is taken. -Oscar Wilde
    2 Don't cry because it's over, smile because it happened. -Dr. Seuss
    3 You only live once, but if you do it right, once is enough. -Mae West
    ...
    

    The program displays the contents of the source code file and preceeds each line with a line number.

The following diagram shows how the dcat command line used above is parsed into the ArgResults object.

ArgsParser parses command-line arguments

You can access flags and options by name, treating the ArgResults object like a Map. You can access other values with properties such as rest.

Here’s the code from dcat that deals with command-line arguments:

...
ArgResults argResults;

void main(List<String> arguments) {
  exitCode = 0; //presume success
  final parser = new ArgParser()
    ..addFlag(lineNumber, negatable: false, abbr: 'n');

  argResults = parser.parse(arguments);
  List<String> paths = argResults.rest;

  dcat(paths, argResults[lineNumber]);
}
...

The API docs for the args library provide detailed information to help you use ArgsParser and ArgResults classes.

Reading and writing with stdin, stdout, and stderr

Like other languages, Dart has standard output, standard error, and standard input streams. The standard I/O streams are defined at the top level of the dart:io library,

Stream Description
stdout The standard output
stderr The standard error
stdin The standard input

Import the dart:io and dart:async libraries as follows:

import 'dart:io';
import 'dart:async';

Only command-line applications, not web applications, can use the dart:io library. The async, await, and await for keywords are built into the Dart language, but you must import dart:async to refer to the Future or Stream class.

stdout

Here’s the code from the dcat program that writes the line number to the stdout (if the -n flag is set) followed by the line from the file.

if (showLineNumbers) {
  stdout.write('${lineNumber++} ');
}

stdout.writeln(line);

The write() and writeln() methods take an object of any type, convert it to a string, and print it. The writeln() method also prints a newline character. dcat uses the write() method to print the line number so the line number and the text appear on the same line.

You can also use the writeAll() method to print a list of objects, or use addStream() to asynchronously print all of the elements from a stream.

stdout provides more functionality than the print() function. For example, you can display the contents of a stream with stdout. However, you must use print() instead of stdout for programs that are converted to and run in JavaScript.

stderr

Use stderr to write error messages to the console. The standard error stream has the same methods as stdout, and you use it in the same way. Although both stdout and stderr print to the console, their output is separate and can be redirected or piped at the command line or programmatically to different destinations.

This code from dcat prints an error message if the user tries to list a directory.

if (await FileSystemEntity.isDirectory(path)) {
  stderr.writeln('error: $path is a directory');
} else {
  exitCode = 2;
}

stdin

The standard input stream typically reads data synchronously from the keyboard, although it can read asynchronously and it can get input piped in from the standard output of another program.

Here’s a small program that reads a single line from stdin:

import 'dart:io';

void main() {
  stdout.writeln('Type something');
  String input = stdin.readLineSync();
  stdout.writeln('You typed: $input');
}

The readLineSync() method reads text from the standard input stream, blocking until the user types in text and presses return. This little program prints out the typed text.

In the dcat program, if the user does not provide a filename on the command line, the program instead reads synchronously from stdin using the pipe() method.

stdin.pipe(stdout);

In this case, the user types in lines of text and the program copies them to stdout. The user signals the end of input by typing <ctl-d>.

$ dart dcat.dart
The quick brown fox jumped over the lazy dog.
The quick brown fox jumped over the lazy dog.
...

Getting info about a file

The FileSystemEntity class in the dart:io library provides properties and static methods that help you inspect and manipulate the file system.

For example, if you have a path, you can determine whether the path is a file, a directory, a link, or not found by using the type() method from the FileSystemEntity class. Because the type() method accesses the file system, it performs the check asynchronously.

The following code from the dcat example uses FileSystemEntity to determine if the path provided on the command line is a directory. The Future returns a boolean that indicates if the path is a directory or not. Because the check is asynchronous, the code calls isDirectory() using await.

if (await FileSystemEntity.isDirectory(path)) {
  stderr.writeln('error: $path is a directory');
} else {
  exitCode = 2;
}

Other interesting methods in the FileSystemEntity class include isFile(), exists(), stat(), delete(), and rename(), all of which also use a Future to return a value.

FileSystemEntity is the superclass for the File, Directory, and Link classes.

Reading a file

dcat opens each file listed on the command line with the openRead() method, which returns a stream. The await for block waits for the file to be read asynchronously. The data prints to stdout when it becomes available on the stream.

for (var path in paths) {
  int lineNumber = 1;
  Stream lines = new File(path)
      .openRead()
      .transform(UTF8.decoder)
      .transform(const LineSplitter());
  try {
    await for (var line in lines) {
      if (showLineNumbers) {
        stdout.write('${lineNumber++} ');
      }
      stdout.writeln(line);
    }
  } catch (_) {
    _handleError(path);
  }
}

The following shows the rest of the code, which uses two decoders that transform the data before making it available in the await for block. The UTF8 decoder converts the data into Dart strings. LineSplitter splits the data at newlines.

for (var path in paths) {
  int lineNumber = 1;
  Stream lines = new File(path)
      .openRead()
      .transform(UTF8.decoder)
      .transform(const LineSplitter());
  try {
    await for (var line in lines) {
      if (showLineNumbers) {
        stdout.write('${lineNumber++} ');
      }
      stdout.writeln(line);
    }
  } catch (_) {
    _handleError(path);
  }
}

The dart:convert library contains these and other data converters, including one for JSON. To use these converters you need to import the dart:convert library:

import 'dart:convert';

Writing a file

The easiest way to write text to a file is to create a File object and use the writeAsString() method:

File quotesFile = new File('quotes.txt');
String stronger = 'That which does not kill us makes us stronger. -Nietzsche';

try {
  await quotesFile.writeAsString(stronger, mode: FileMode.APPEND);
  print('Data written.');
} catch (e) {
  print('Oops!');
}

The writeAsString() method writes the data asynchronously. It opens the file before writing and closes the file when done. To append data to an existing file, you can use the optional parameter mode and set its value to FileMode.APPEND. Otherwise, the mode is FileMode.WRITE and the previous contents of the file, if any, are overwritten.

If you want to write more data, you can open the file for writing. The openWrite() method returns an IOSink (the same type as stdin and stderr). You can continue to write to the file until done, at which time, you must close the file. The close() method is asynchronous and returns a Future.

IOSink quotes = new File('quotes.txt').openWrite(mode: FileMode.APPEND);

quotes.write('A woman is like a tea bag; ');
quotes.write('you never know how strong it is until it\'s in hot water.');
quotes.writeln(' -Eleanor Roosevelt');
await quotes.close();
print('Done!');

Getting environment information

Use the Platform class to get information about the machine and OS that the program is running on. Note: Use the Platform class from the dart:io library, not from the dart:html library.

Platform.environment provides a copy of the environment variables in an immutable map. If you need a mutable map (modifiable copy) you can use Map.from(Platform.environment).

Map environmentVars = Platform.environment;

print('PWD = ${environmentVars["PWD"]}');
print('LOGNAME = ${environmentVars["LOGNAME"]}');
print('PATH = ${environmentVars["PATH"]}');

Platform provides other useful properties that give information about the machine, OS, and currently running program. For example:

  • Platform.isMacOS()
  • Platform.numberOfProcessors
  • Platform.script.path

Setting exit codes

The dart:io library defines a top-level property, exitCode, that you can change to set the exit code for the current invocation of the Dart VM. An exit code is a number passed from the Dart program to the parent process to indicate the success, failure, or other state of the execution of the program.

The dcat program sets the exit code in the _handleError() function to indicate that an error occcurred during execution.

Future _handleError(String path) async {
  if (await FileSystemEntity.isDirectory(path)) {
    stderr.writeln('error: $path is a directory');
  } else {
    exitCode = 2;
  }
}

An exit code of 2 indicates that the program encountered an error.

An alternative to using exitCode is to use the top-level exit() function, which sets the exit code and quits the program immediately. For example, the _handleError() function could call exit(2) instead of setting exitCode to 2, but exit() would quit the program and it might not process all of the files on the command line.

Although you can use any number for an exit code, by convention, the codes in the table below have the following meanings:

Code Meaning
0 Success
1 Warnings
2 Errors

Summary

This tutorial described some basic API found in these classes from the dart:io library:

API Description
IOSink Helper class for objects that consume data from streams.
File Represents a file on the native file system
Directory Represents a directory on the native file system
FileSystemEntity Superclass for File and Directory
Platform Provides information about the machine and operating system
stdout The standard output
stderr The standard error
stdin The standard input
exitCode Sets the exit code
exit() Sets the exit code and quits

In addition, this tutorial covers two classes that help with command-line arguments:

Class Description
ArgParser A class that transforms a list of raw arguments and into a set of options, flags, and remaining values.
ArgResults The result of parsing raw command line arguments using ArgParser.

Other resources

Check out the Servers with Dart to find more resources related to writing command-line apps.

Refer to the API docs for dart:io, dart:async, dart:convert, and the args package for more classes, functions, and properties.

What next?