# Writing your first CLI app

Let's start by going over the basics of writing the classic Hello World program.

Create a directory to work in:

{% tabs %}
{% tab title="Linux" %}

```bash
mkdir dcli_scripts
cd dcli_scripts
```

{% endtab %}

{% tab title="Windows" %}

```
mkdir dcli_scripts
cd dcli_scripts
```

{% endtab %}

{% tab title="OSx" %}

```
mkdir dcli_scripts
cd dcli_scripts
```

{% endtab %}
{% endtabs %}

Using your preferred editor create a file called 'hello.dart' with following content:

```dart
void main() {
    print('Hello World.');
}
```

{% hint style="info" %}
If you are using vscode you can simply type `code hello.dart` to edit the file.
{% endhint %}

## Running

Now let's run your script:

```dart
dart hello.dart
> Hello World.
```

When we run our script using the `dart` command, Dart performs JIT compilation of our script which slows down the startup time a little but makes for fast test iteration.

{% hint style="info" %}
In vscode you should see 'Run | Debug' just above main(). Click Debug to start your app.
{% endhint %}

## Compiling

You can compile your script to a native executable so that it launches and runs much faster.

{% hint style="info" %}
The [DCli tools](https://dcli.onepub.dev/dcli-tools-1/dcli-tools) allow you to run your app without compiling and without prefixing it with dart.
{% endhint %}

```bash
dart compile exe hello.dart
Generated: hello.exe
./hello.exe
> Hello World

ll hello.exe
-rwxrwxr-x 1 5,875,400 Sep  5 14:30 hello.exe*
```

You now have a completely self-contained executable which you can copy to any binary compatible machine.

The exe is 8MB in size and does NOT require Dart to be installed.

## Dependencies

So far we haven't actually used the DCli API in our hello.dart program. Let's now set up dependency management so we can use the DCli API and any other Dart package.

{% hint style="info" %}
Search [pub.dev](https://pub.dev/) for third party package. You can only use packages labeled 'DART | NATIVE'
{% endhint %}

Dart uses a special file called `pubspec.yaml` to control the set of packages accessible to your application.

Dart's pubspec.yaml is equivalent to a makefile, pom.xml, gradle.build or package.json, in that it defines the set of dependencies for your application.

To use the DCli API or any other Dart package you need to add the dependency to your pubspec.yaml.

Create and edit your first 'pubspec.yaml' file using your preferred editor:

{% hint style="info" %}
Check [pub.dev](https://pub.dev/packages/dcli/install) for the latest version no. of DCli.
{% endhint %}

```bash
name: hello_world
description: My first app that does bugger all.

dependencies:
  dcli: ^7.0.0  # update the version no. to the latest; https://pub.dev/packages/dcli
```

{% hint style="info" %}
The pubspec.yaml lives at the top of your projects directory tree. We refer to this directory as your package root.
{% endhint %}

Whenever you change your 'pubspec.yaml' you must run 'dart pub get' to download the required dependencies:

```bash
dart pub get
Resolving dependencies... 
Got dependencies!
```

## Writing a DCli script

Now that we have added DCli to our pubspec.yaml we can modify hello.dart to make calls to the DCli API.

Edit your hello.dart script as follows:

```bash
import 'package:dcli/dcli.dart';

void main() {
    print("Now let's do someting useful.");

    var username =  ask( 'username:');
    print('username: $username');

    var password = ask( 'password:', hidden: true);
    print('password: $password');

    // create a directory
    if (!exists('tmp')) {
        createDir('tmp');
    }

    // Truncate any existing content
    // of the file 'tmp/text.txt' and write
    // 'Hello world' to the file.
    'tmp/text.txt'.write('Hello world');

    // append 'My second line' to the file 'tmp/text.txt'.
    'tmp/text.txt'.append('My second line');

    // and another append to the same file.
    'tmp/text.txt'.append('My third line');

    // now copy the file tmp/text.txt to second.txt
    copy('tmp/text.txt', 'tmp/second.txt', overwrite: true);

    // lets dump the file we just created to the console
    read('tmp/second.txt').forEach((line) => print(line));

    //let's prove that both files exist by running
    // a recursive find.
    find('*.txt').forEach((file) => print('Found $file'));

    // Now let's tail the file using the OS tail command.
    // DCli uses Dart extensions on the String class allowing us to
    // treat a string
    // as an OS command and `.run` that command as 
    // a child process.
    // Stdout and stderr output are written
    // directly to the console.
    'tail tmp/text.txt'.run;

    //Let's do a word count capturing stdout,
    // stderr will will be swallowed.
    'wc tmp/second.txt'.forEach((line) => print('Captured $line'));

    if (confirm( "Should I delete 'tmp'? (y/n):")) {
        // Now let's clean up
        delete('tmp/text.txt');
        delete('tmp/second.txt');
        deleteDir('tmp');
    }

}
```

Now run our script.

```bash
dart hello.dart
Now let's do someting useful.
username: auser
username: auser
password: *********
password: apassword
Hello world
My second line
My third line
Found /tmp/fred/tmp/second.txt
Found /tmp/fred/tmp/text.txt
Hello world
My second line
My third line
Captured  3  8 41 tmp/second.txt
Should I delete 'tmp'? (y/n): (y/n): y
```

You are now officially a DCli guru.

Go forth young man (or gal) and create.