adding new command-line app tutorial

src/site/docs/tutorials/cmdline/index.markdown

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+---
+layout: default
+title: "Write Command-Line Apps"
+description: "Basics for command-line apps"
+has-permalinks: true
+tutorial:
+  id: dart-io
+next: /
+next-title: "Home"
+prev: indexeddb/
+prev-title: "Use IndexedDB"
+---
+
+{% capture whats_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.
+* Most input and output requires the use of Streams.
+* Streams provide a series of asynchronous data events.
+* To handle asynchronous data, you need to use Futures.
+
+{% endcapture %}
+
+{% capture sample_links %}
+
+<p> This tutorial features these examples:</p>
+* helloworld
+* dcat
+
+<p>
+Don't have the source code?
+<a href="https://github.com/dart-lang/dart-tutorials-samples/archive/master.zip">
+  Download it.
+</a>
+
+{% endcapture %}
+
+{% capture content %}
+
+<div class="tute-target-title">
+<h1>{{page.title}}</h1>
+<h3>An introduction to standalone apps</h3>
+</div>
+
+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.
+
+* [Running an app with the standalone Dart VM](#run-the-first-app)
+* [Review briefly the dcat example code](#dcat-code)
+* [Parsing command-line arguments](#cmd-line-args)
+* [Reading and writing with stdin, stdout, and stderr](#std-in-out-err)
+* [Getting info about a file](#filesystementity)
+* [Reading a file](#reading-a-file)
+* [Writing a file](#writing-a-file)
+* [Getting environment information](#env-var)
+* [Setting exit codes](#exit-codes)
+* [Summary](#summary)
+* [Other resources](#other-resources)
+* [What next?](#what-next)
+
+## Running an app with the standalone Dart VM {#run-the-first-app}
+
+To run a command-line app, you need the Dart VM (`dart`),
+which comes in the [Dart SDK download](/tools/sdk/).
+(If you downloaded Dart Editor, you already have the Dart VM.)
+
+If you installed the Dart download in a directory called `~/myDartDownload`,
+you can find `dart`
+in `~/myDartDownload/dart-sdk/bin`.
+
+<div markdown="1">
+
+![The path to the Dart VM](images/filestructure.png)
+
+</div>
+
+By putting this directory in your PATH
+you can refer to the `dart` command and other commands,
+such as the dart analyzer, by name.
+
+Let's run a small program.
+
+<ol>
+<li markdown="1">
+Create a file called `helloworld.dart` that contains this code:
+
+{% prettify dart %}
+void main() {
+  print('Hello, World!');
+}
+{% endprettify %}
+</li>
+
+<li markdown="1">
+In the directory that contains the file you just created,
+run the program with the command as shown by the highlighted text.
+
+{% prettify bash %}
+% [[highlight]]dart helloworld.dart[[/highlight]]
+Hello, World!
+%
+{% endprettify %}
+
+</li>
+</ol>
+
+### Running in checked mode
+
+You can run the Dart VM in checked mode by using the `--checked` flag.

[sethladd, 2014/01/03 17:54:54]

Can we lead with why you might want to do this? Or, we can punt on this and just
say "there's lots of options for dart, go here to learn more about them"

[mem, 2014/01/03 19:22:14]

Done.

+
+{% prettify bash %}
+% [[highlight]]dart --checked helloworld.dart[[/highlight]]
+Hello, World!
+%
+{% endprettify %}
+
+The `--checked` option enables assertion and type checks.
+Checked mode is useful during development for finding errors
+related to types and for testing `assert` statements.
+We recommend running your programs in checked mode during prototyping
+and development, and turning it off for production.
+
+Note that the flags and options passed to the Dart VM go before the name
+of the Dart file to run.
+
+## Review briefly the dcat example code {#dcat-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.
+
+<pre class="prettyprint lang-dart">
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:args/args.dart';
+
+const LINE_NUMBER = 'line-number';
+var NEWLINE = '\n';
+
+ArgResults argResults;
+
+void main(<a href="#" class="dart-popover" data-toggle="popover" title="Command-line arguments" data-html="true" data-trigger="hover focus" data-content="Command-line arguments are passed in by the system when the program starts.">List&lt;String&gt; arguments</a>) {
+  final parser = new ArgParser()
+      ..addFlag(LINE_NUMBER, negatable: false, abbr: 'n');
+
+  <a href="#" class="dart-popover" data-toggle="popover" title="Arguments parser" data-html="true" data-trigger="hover focus" data-content="The ArgParser class provides help with parsing command-line arguments.">argResults = parser.parse(arguments);</a>
+  List&lt;String&gt; paths = argResults.rest;
+
+  dcat(paths, argResults[LINE_NUMBER]);
+}
+
+Future dcat(List&lt;String&gt; paths, bool showLineNumbers) {
+  if (paths.isEmpty) {
+    // No files provided as arguments. Read from stdin and print each line.
+    return <a href="#" class="dart-popover" data-toggle="popover" title="Standard I/O streams" data-html="true" data-trigger="hover focus" data-content="This code reads from the standard input stream and pipes the data to the standard output stream.">stdin.pipe(stdout);</a>
+  } else {
+    return Future.forEach(paths, (path) {
+      int lineNumber = 1;
+      <a href="#" class="dart-popover" data-toggle="popover" title="Open a file for reading" data-html="true" data-trigger="hover focus" data-content="Use the File class to represent a file on the native file system.">Stream&lt;List&lt;int&gt;&gt; stream = new File(path).openRead();</a>
+      return stream
+          <a href="#" class="dart-popover" data-toggle="popover" title="Data converters" data-html="true" data-trigger="hover focus" data-content="Convert data as it becomes available on a stream.">.transform(UTF8.decoder)</a>
+          .transform(const LineSplitter())
+          <a href="#" class="dart-popover" data-toggle="popover" title="Get data from a stream" data-html="true" data-trigger="hover focus" data-content="Listen to a stream to get data when it becomes available.">.listen((line)</a> {
+            if (showLineNumbers) {
+              stdout.write('${lineNumber++} ');
+            }
+            stdout.writeln(line);
+          }).asFuture().catchError((_) =&gt; _handleError(path));
+    });
+  }
+}
+
+_handleError(String path) {
+  <a href="#" class="dart-popover" data-toggle="popover" title="Test a path" data-html="true" data-trigger="hover focus" data-content="You can get information about the file system on which your program is running.">FileSystemEntity.isDirectory(path)</a>.then((isDir) {
+    if (isDir) {
+      print('error: $path is a directory');
+    } else {
+      print('error: $path not found');
+    }
+  });
+  <a href="#" class="dart-popover" data-toggle="popover" title="Exit code" data-html="true" data-trigger="hover focus" data-content="A well-behaved command-line app sets an exit code to indicate whether the program was successful.">exitCode = 2;</a>
+}
+
+</pre>
+
+## Parsing command-line arguments {#cmd-line-args}
+
+The
+<a href="https://api.dartlang.org/args.html" target="_blank">args</a>
+package, a software bundle that contains a library of Dart code, provides
+parser support for transforming raw command-line arguments
+into a set of options, flags, and additional values.
+Import the library as follows:
+
+{% prettify dart %}
+import 'package:args/args.dart';
+{% endprettify %}
+
+The args library contains two classes:
+
+| Library | Description |
+|---|---|
+| <a href="https://api.dartlang.org/args/ArgParser" target="_blank">ArgParser</a> | A class that parses command-line arguments |
+| <a href="https://api.dartlang.org/args/ArgResults" target="_blank">ArgResults</a> | The result of parsing command-line arguments using ArgParser. |
+{: .table }
+
+Let's take a look at the `dcat` sample,
+which uses ArgParser and ArgResults to parse and store its command-line arguments.
+
+<ol>
+<li markdown="1">
+Copy the sample file from the github repo:
+<a href="https://github.com/dart-lang/dart-tutorials-samples/blob/master/bin/dcat.dart">dcat.dart</a>.
+</li>
+
+<li markdown="1">
+Run the program from the command line as shown by the highlighted text.
+
+{% prettify bash %}
+$ [[highlight]]dart dcat.dart -n quotes.txt[[/highlight]]
+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
+...
+{% endprettify %}
+
+The program displays the contents of the source code file and
+preceeds each line with a line number.
+</li>
+
+</ol>
+
+The following diagram shows how the `dcat` command line used above
+is parsed into the `ArgResults` object.
+
+![ArgsParser parses command-line arguments](images/commandlineargs.png)
+
+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:
+
+<pre class="prettyprint lang-dart">
+
+...
+<a href="#" class="dart-popover" data-toggle="popover" title="Parsed arguments" data-html="true" data-trigger="hover focus" data-content="This object contains parsed options and flags.">ArgResults argResults;</a>
+
+void main(<a href="#" class="dart-popover" data-toggle="popover" title="Command-line arguments" data-html="true" data-trigger="hover focus" data-content="The system passes command-line arguments into the program in a list of strings.">List&lt;String&gt; arguments</a>) {
+  final parser = new ArgParser()
+      <a href="#" class="dart-popover" data-toggle="popover" title="Define a valid flag" data-html="true" data-trigger="hover focus" data-content="Add a flag definition to the command-line argument parser. This code defines the flag -n, which when used displays line numbers.">..addFlag(LINE_NUMBER, negatable: false, abbr: 'n')</a>;
+
+  argResults = parser.<a href="#" class="dart-popover" data-toggle="popover" title="Parse the arguments" data-html="true" data-trigger="hover focus" data-content="Parse the arguments that were passed into the main() function. The parser stops parsing if it finds an undefined option or flag.">parse(arguments)</a>;
+  List&lt;String&gt; paths = <a href="#" class="dart-popover" data-toggle="popover" title="Remaining arguments" data-html="true" data-trigger="hover focus" data-content="To get the arguments that remain after parsing all of the valid options and flags, use the rest property.">argResults.rest</a>;
+
+  dcat(paths, <a href="#" class="dart-popover" data-toggle="popover" title="Refer to options and flags by name" data-html="true" data-trigger="hover focus" data-content="You can refer to an option or flag by name treating the ArgResults object like a Map.">argResults[LINE_NUMBER])</a>;
+}
+...
+</pre>
+
+The 
+<a href="https://api.dartlang.org/args.html" target="_blank">API docs</a>
+for the args library
+provide detailed information
+to help you use ArgsParser and ArgResults classes.
+
+## Reading and writing with stdin, stdout, and stderr {#std-in-out-err}
+
+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 |
+|---|---|
+| <a href="https://api.dartlang.org/dart_io.html#stdout" target="_blank">stdout</a> | The standard output |
+| <a href="https://api.dartlang.org/dart_io.html#stderr" target="_blank">stderr</a> | The standard error |
+| <a href="https://api.dartlang.org/dart_io.html#stdin" target="_blank">stdin</a> | The standard input |
+{: .table }
+
+Import the dart:io library as follows:
+
+{% prettify dart %}
+import 'dart:io';
+{% endprettify %}
+
+Only command-line applications, not web applications,
+can use the dart:io library.
+
+### 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.
+
+{% prettify dart %}
+if (showLineNumbers) {
+  [[highlight]]stdout.write('${lineNumber++} ');[[/highlight]]
+}
+
+[[highlight]]stdout.writeln(line);[[/highlight]]
+{% endprettify %}
+
+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 or if the file is not found.
+
+{% prettify dart %}
+if (isDir) {
+  [[highlight]]stderr.writeln('error: $path is a directory');[[/highlight]]
+} else {
+  [[highlight]]stderr.writeln('error: $path not found');[[/highlight]]
+}
+{% endprettify %}
+
+### 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`:
+
+{% prettify dart %}
+import 'dart:io';
+
+void main() {
+  stdout.writeln('Type something');
+  String input = stdin.readLineSync();
+  stdout.writeln('You typed: $input');
+}
+{% endprettify %}
+
+The `readLineSync()` method reads text from the standard input stream,
+blocking until the user types in text and presses return.
+This little program prints out the typed text.
+
+In the `dcat` program,
+if the user does not provide a filename on the command line,
+the program instead reads synchronously from stdin
+using the `pipe()` method.
+
+{% prettify dart %}
+return [[highlight]]stdin[[/highlight]].pipe(stdout);
+{% endprettify %}
+
+In this case,
+the user types in lines of text and the program copies them to stdout.
+The user signals the end of input by typing &lt;ctl-d&gt;.
+
+{% prettify bash %}
+$ [[highlight]]dart dcat.dart[[/highlight]]
+[[highlight]]The quick brown fox jumped over the lazy dog.[[/highlight]]
+The quick brown fox jumped over the lazy dog.
+...
+{% endprettify %}
+
+## Getting info about a file {#filesystementity}
+
+The 
+<a href="https://api.dartlang.org/dart_io/FileSystemEntity.html" target="_blank">FileSystemEntity</a>
+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 within a Future.
+
+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.
+
+{% prettify dart %}
+[[highlight]]FileSystemEntity.isDirectory(path)[[/highlight]].then((isDir) {
+  if (isDir) {
+    stderr.writeln('error: $path is a directory');
+  } else {
+    stderr.writeln('error: $path not found');
+  }
+  exit(2);
+});
+{% endprettify %}
+
+Other interesting methods in the `FileSystemEntity` class
+include `isFile()`, `exists()`, `stat()`, `delete()`,
+and `rename()`,
+all of which also use a Future to return a value.
+
+FileSystemEntity is the superclass for the File, Directory, and Link classes.
+
+## Reading a file {#reading-a-file}
+
+`dcat` opens each file listed on the command line
+with the `openRead()` method,
+which returns a stream.
+The `listen()` method registers a callback function that runs
+when data becomes available on the stream.
+The callback function writes that data to stdout.
+
+{% prettify dart %}
+return Future.forEach(paths, (path) {
+  int lineNumber = 1;
+  Stream<List<int>> stream = new File(path).openRead();
+
+  return stream
+      ...
+      [[highlight]].listen((line) {
+        if (showLineNumbers) {
+          stdout.write('${lineNumber++} ');
+        }
+        stdout.writeln(line);
+      })[[/highlight]].asFuture()
+          .catchError((_) => _handleError(path));
+});
+{% endprettify %}
+
+The following shows the rest of the
+code, which uses two decoders that transform the data before the
+`listen()` callback function runs.
+The UTF8 decoder converts the data into Dart strings.
+`LineSplitter` splits the data at newlines.
+
+{% prettify dart %}
+return Future.forEach(paths, (path) {
+  int lineNumber = 1;
+  Stream<List<int>> stream = new File(path).openRead();
+
+  return stream
+      [[highlight]].transform(UTF8.decoder)
+      .transform(const LineSplitter())[[/highlight]]
+      .listen((line) {
+        if (showLineNumbers) {
+          stdout.write('${lineNumber++} ');
+        }
+        stdout.writeln(line);
+      }).asFuture()
+          .catchError((_) => _handleError(path));
+});
+{% endprettify %}
+
+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:
+
+{% prettify dart %}
+import 'dart:convert';
+{% endprettify %}
+
+## Writing a file {#writing-a-file}
+
+The easiest way to write text to a file is to
+create a 
+<a href="https://api.dartlang.org/dart_io/File.html" target="_blank">File</a>
+object and use the `writeAsString()` method:
+
+{% prettify dart %}
+File quotesFile = new File('quotes.txt');
+String stronger = 'That which does not kill us makes us stronger. -Nietzsche';
+
+quotesFile.writeAsString(stronger, mode: FileMode.APPEND)
+    .then((_) { print('Data written.'); });
+{% endprettify %}
+
+The `writeAsString()` method writes the data asynchronously via a Future.
+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.
+
+{% prettify dart %}
+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");
+quotes.close().then((_) { print('done'); } );
+{% endprettify %}
+
+## Getting environment information {#env-var}
+
+Use the
+<a href="https://api.dartlang.org/dart_io/Platform.html" target="_blank">Platform</a>
+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 a mutable map.
+
+{% prettify dart %}
+Map environmentVars = Platform.environment;
+
+print('PWD = ${environmentVars["PWD"]}');
+print('LOGNAME = ${environmentVars["LOGNAME"]}');
+print('PATH = ${environmentVars["PATH"]}');
+{% endprettify %}
+
+`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 {#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.
+
+{% prettify dart %}
+_handleError(String path) {
+  FileSystemEntity.isDirectory(path).then((isDir) {
+    if (isDir) {
+      stderr.writeln('error: $path is a directory');
+    } else {
+      stderr.writeln('error: $path not found');
+    }
+    [[highlight]]exitCode = 2;[[/highlight]]
+  });
+}
+{% endprettify %}
+
+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.
+
+<aside class="alert alert-info" markdown="1">
+Generally speaking, you are better off using the `exitCode` property,
+which sets the exit code but allows the program to continue through to its
+natural completion.
+</aside>
+
+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 |
+{: .table }
+
+## Summary {#summary}
+
+This tutorial described some basic API found in these classes from the dart:io library:
+
+| API | Description |
+|---|---|
+| <a href="https://api.dartlang.org/dart_io/IOSink.html" target="_blank">IOSink</a> | Helper class for objects that consume data from streams. |
+| <a href="https://api.dartlang.org/dart_io/File.html" target="_blank">File</a> | Represents a file on the native file system |
+| <a href="https://api.dartlang.org/dart_io/Directory.html" target="_blank">Directory</a> | Represents a directory on the native file system |
+| <a href="https://api.dartlang.org/dart_io/FileSystemEntity.html" target="_blank">FileSystemEntity</a> | Superclass for File and Directory |
+| <a href="https://api.dartlang.org/dart_io/Platform.html" target="_blank">Platform</a> | Provides information about the machine and operating system |
+| <a href="https://api.dartlang.org/dart_io.html#stdout" target="_blank">stdout</a> | The standard output |
+| <a href="https://api.dartlang.org/dart_io.html#stderr" target="_blank">stderr</a> | The standard error |
+| <a href="https://api.dartlang.org/dart_io.html#stdin" target="_blank">stdin</a> | The standard input |
+| <a href="https://api.dartlang.org/dart_io.html#exitCode" target="_blank">exitCode</a> | Sets the exit code |
+| <a href="https://api.dartlang.org/dart_io.html#exit" target="_blank">exit()</a> | Sets the exit code and quits |
+{: .table }
+
+In addition, this tutorial covers two classes that help with command-line arguments:
+
+| Class | Description |
+|---|---|
+| <a href="https://api.dartlang.org/args/ArgParser" target="_blank">ArgParser</a> | A class that transforms a list of raw arguments and into a set of options, flags, and remaining values. |
+| <a href="https://api.dartlang.org/args/ArgResults" target="_blank">ArgResults</a> | The result of parsing raw command line arguments using ArgParser. |
+{: .table }
+
+## Other resources {#other-resources}
+
+Check out the [Command-line Apps Programmers' Guide](/docs/serverguide.html)
+to find more resources related to writing command-line apps.
+
+See the
+[Dartiverse Search walkthrough](/docs/dart-up-and-running/contents/ch05.html)
+for an example of another kind of command-line app: an HTTP server.
+
+Refer to the API docs for <a href="https://api.dartlang.org/dart_io.html" target="_blank">dart:io</a>,
+<a href="https://api.dartlang.org/dart_async.html" target="_blank">dart:async</a>,
+<a href="https://api.dartlang.org/dart_convert.html" target="_blank">dart:convert</a>,
+and the
+<a href="https://api.dartlang.org/dart_args.html" target="_blank">args</a>
+package for more classes, functions, and properties.
+
+## What next? {#what-next}
+
+The [Get Input from a Form](/docs/tutorials/forms/) tutorial
+features a client-server.
+The code for the server, which uses CORS headers and handles
+POST requests, is explained in detail.
+
+{% endcapture %}
+
+{% include tutorial.html %}