Dart CLI Application ArchitectureSkill

---
name: dart-build-cli-app
description: >-
  Architectural patterns, entrypoint structure, exit codes, stream routing, and subprocess spawning for Dart command-line interface (CLI) applications. Use when building CLI tools, console utilities, scripts, argument parsing with `package:args` (ArgParser or CommandRunner), handling exit codes, configuring executables in pubspec.yaml, spawning Dart subprocesses, or compiling native CLI binaries. Don't use for Flutter UI widgets, web applications, or standalone HTTP backend servers.
---

# Building Dart CLI Applications

## Contents
* [1. Core Architecture & Process Lifecycle](#1-core-architecture--process-lifecycle)
* [2. Streams, Diagnostics & Formatting](#2-streams-diagnostics--formatting)
* [3. Project Configuration & Packaging](#3-project-configuration--packaging)
* [4. Argument Parsing & Command Routing](#4-argument-parsing--command-routing)
* [5. Native Async & Modern Stack Traces](#5-native-async--modern-stack-traces)
* [6. Subprocess Spawning & AOT Resilience](#6-subprocess-spawning--aot-resilience)
* [7. Signal Handling & Terminal Teardown](#7-signal-handling--terminal-teardown)
* [8. Testing CLI Applications](#8-testing-cli-applications)
* [9. Modern Compilation & Distribution](#9-modern-compilation--distribution)
* [10. Workflows & Audit Checklist](#10-workflows--audit-checklist)
* [References & Examples](#references--examples)

---

## 1. Core Architecture & Process Lifecycle

### Avoid Destructive Exits (`exit(N)`)
Calling `dart:io`'s `exit(int code)` invokes `Platform::Exit(code)` in the C++ runtime. It immediately terminates the OS process without unwinding the Dart stack:
* **Debugger Disconnect**: When launched with `--pause-isolates-on-exit`, the VM Service pauses isolates before shutdown to allow IDE inspection. `exit()` terminates the OS process before the VM Service can pause or inspect state.
* **Coverage Loss**: `package:coverage` queries execution lines over VM Service RPCs during the paused-on-exit state. `exit()` destroys the process before RPC extraction, yielding 0% coverage.
* **Buffer Truncation**: `stdout` and `stderr` are buffered asynchronous `IOSink` streams. `exit()` drops unflushed bytes.
* **Resource Leaks**: `finally` blocks (closing locks, deleting temp directories) are bypassed.

**Rule**: Avoid calling `exit(code)` directly during normal execution; set `exitCode = code` or return an integer exit code from `CommandRunner<int>` (from `package:args`) and allow the asynchronous `main()` function to return naturally. Do not call `exit()` on unhandled errors; throw an unhandled `Error` or exception so the runtime unwinds cleanly and exits with a non-zero status.

Standard POSIX exit codes (`/usr/include/sysexits.h`):
* `0`: Success (`EX_OK` / `ExitCode.success.code`)
* `64`: Command-line usage error (`EX_USAGE` / `ExitCode.usage.code`)
* `65`: Data format error (`EX_DATAERR` / `ExitCode.data.code`)
* `70`: Internal software crash (`EX_SOFTWARE` / `ExitCode.software.code`)
* `78`: Configuration error (`EX_CONFIG` / `ExitCode.config.code`)

*Note*: Prefer importing `package:io/io.dart` and using `ExitCode` constants
(e.g., `ExitCode.usage.code`, `ExitCode.software.code`) rather than magic
integer literals. For minimal standalone scripts without package dependencies,
standard POSIX integer literals (`0`, `64`, `70`) may be used.

```dart
import 'dart:io';
import 'package:args/command_runner.dart';
import 'package:io/io.dart' show ExitCode; // Provides standard POSIX ExitCode constants

Future<void> main(List<String> args) async {
  final runner = CommandRunner<int>('tool', 'CLI tool description.');
  try {
    final status = await runner.run(args);
    exitCode = status ?? ExitCode.success.code;
  } on UsageException catch (e) {
    stderr
      ..writeln(e.message)
      ..writeln(e.usage);
    exitCode = ExitCode.usage.code;
  }
}
```

### The Thin Entrypoint Pattern (`bin/` vs. `lib/src/`)
Keep `bin/*.dart` files strictly as minimal entrypoint trampolines (instantiate runner, pass `args`, await exit code). Place all command definitions, argument parsers, formatters, and business logic inside `lib/src/`.

* **Rationale**: Code in `bin/` cannot be cleanly imported via `package:` URIs. Moving logic into `lib/src/` allows the entire command runner, subcommand hierarchy, and business logic to be unit-tested in-memory in milliseconds (`< 2ms`) without spawning OS subprocesses.

```dart
// bin/my_cli.dart — Thin entrypoint trampoline
import 'dart:io';
import 'package:my_cli/src/cli.dart';

Future<void> main(List<String> args) async {
  exitCode = await runCli(args);
}
```

---

## 2. Output, Diagnostics & Formatting

* **Data vs. Diagnostics**: Write intended program results and machine-readable data exclusively to `stdout`. Write warnings, error messages, and debug logs exclusively to `stderr`.
* **The Error Usage Rule**: When an argument parsing or mandatory option error
  occurs (`FormatException`, `UsageException`, or `ArgumentError` thrown when
  accessing a missing `mandatory: true` option via `results.option(...)`), **both
  the error message and the usage text must write to `stderr`**, and exit code
  `64` (`EX_USAGE` / `ExitCode.usage.code`) must be returned. `stdout` should
  ONLY receive usage help when the user explicitly requests it via `--help` or
  `-h`.
* **No `print()` in Error Handlers**: `print()` routes to `stdout`. Use `stderr.writeln()` for all failure notifications. For standard output, prefer `stdout.writeln()` over `print()` to comply with the [`avoid_print`](https://dart.dev/tools/linter-rules/avoid_print) lint rule (unless `analysis_options.yaml` explicitly configures `avoid_print: false`).
* **Terminal Capability Detection & `NO_COLOR`**: Verify `stdout.hasTerminal`, `stdout.supportsAnsiEscapes`, and `!Platform.environment.containsKey('NO_COLOR')` before emitting ANSI color or cursor escape codes:
  ```dart
  bool get useAnsi =>
      stdout.hasTerminal &&
      stdout.supportsAnsiEscapes &&
      !Platform.environment.containsKey('NO_COLOR');
  ```
* **Machine-Readable Modes**: When `--json` or `--machine` flags are passed, format data as JSON to `stdout` and route logs to `stderr`.

---

## 3. Project Configuration & Packaging

### Scaffolding & Pubspec Executable Mapping (`executables:`)
Scaffold new command-line projects using `dart create -t console <package_name>`, which initializes the standard `bin/` and `lib/` layout. Always declare executables in `pubspec.yaml` under `executables:` to map command names to scripts in `bin/`, enabling clean invocation via `dart run <command>` (without specifying `bin/...dart`) and configuring global binary symlinks for `dart install`:

```yaml
name: my_cli
description: High-performance CLI tool.
version: 1.0.0

executables:
  my_cli: # Maps to bin/my_cli.dart
  secondary_cmd: helper # Maps to bin/helper.dart
```

### Single-Source Versioning (`package:build_version`)
Avoid hardcoding `--version` strings in `bin/*.dart` or manually synchronizing constant files. Use `package:build_version` to generate `lib/src/version.dart` containing `const packageVersion = 'x.y.z';` directly from `pubspec.yaml` during builds.

### Caching Conventions
Store transient cache files in `.dart_tool/<package_name>/`. Never write persistent cache files directly to the project root.

---

## 4. Argument Parsing & Command Routing

Import `package:args` to manage command-line arguments:

* **Simple Scripts**: Use `ArgParser` directly with `addFlag()` and `addOption()`.
* **Multi-Command Tools**: Implement `CommandRunner<int>` and extend `Command<int>` for each subcommand, returning POSIX exit codes directly.
* **Type-Safe Accessors**: Use `results.flag('name')`, `results.option('name')`, and `results.multiOption('name')` (available in `package:args` 2.5+) instead of map indexing `operator []` to eliminate manual type casts (`as bool`, `as String?`).
* **Complex Options Models**: For applications with extensive flags, use `package:build_cli` to generate strongly-typed options classes. Leverage named default overrides (e.g. `{String? hostDefaultOverride}`) to cleanly merge configuration files with CLI flags.

---

## 5. Native Async & Modern Stack Traces

* **Avoid `Chain.capture()`**: The Dart VM natively preserves asynchronous stack frames across `await` suspension points. `Chain.capture` wraps the event loop in custom Zones, incurring substantial allocation overhead and trapping errors across Zone boundaries.
* **Sanitize with `Trace.from(st).terse`**: Use static utilities from `package:stack_trace` on uncaught errors without capturing zones:

```dart
import 'dart:io';
import 'package:io/io.dart' show ExitCode;
import 'package:stack_trace/stack_trace.dart';

Future<void> runMain(List<String> args) async {
  try {
    await executeLogic(args);
    exitCode = ExitCode.success.code;
  } catch (e, st) {
    stderr.writeln('Fatal error: $e');
    if (args.contains('-v') || args.contains('--verbose')) {
      stderr.writeln(Trace.from(st).terse);
    }
    exitCode = ExitCode.software.code;
  }
}
```

---

## 6. Subprocess Spawning & AOT Resilience

When spawning Dart SDK subprocesses or executing other Dart tools (e.g., `dart format`, `dart test`, `build_runner`):

* **Do not assume `Platform.resolvedExecutable` or `Platform.executable` points to the `dart` command-line executable**: In standalone AOT-compiled binaries (`dart install` / `dart compile exe`), `resolvedExecutable` points to the compiled application binary itself, causing recursive self-invocation loops or flag rejection crashes.
* **Use `package:cli_util`**: Resolve the Dart SDK executable using `cli_util.dartExecutable` or `cli_util.sdkPath` instead of writing custom PATH or directory scrapers.
* See version requirements and detailed technical guide in [references/aot_sdk_discovery.md](references/aot_sdk_discovery.md).

---

## 7. Signal Handling & Terminal Teardown

If your CLI alters terminal modes, displays spinners, or opens listening sockets:

* **Windows Signal Guard**: On Windows, `ProcessSignal.sigterm.watch()` throws `UnsupportedError`. Guard `sigterm` with `if (!Platform.isWindows)`.
* **Echo & Line Mode Teardown**: If setting `stdin.echoMode = false` or `stdin.lineMode = false`, check `if (!stdin.hasTerminal) return;` first, and install a `SIGINT` listener and `finally` block to restore them so user keystrokes remain visible after exit.
* **Cursor Visibility**: If emitting ANSI hide-cursor (`\x1B[?25l`), always restore cursor visibility (`\x1B[?25h`) on exit or cancellation.
* **Socket Cleanup**: Explicitly close listening `HttpServer` or `ServerSocket` instances (`server.close(force: true)`) on termination signals to immediately release OS ports.
* See detailed patterns in [references/signals_and_terminal.md](references/signals_and_terminal.md).

---

## 8. Testing CLI Applications

Structure testing across two distinct layers:

1. **Unit Tests (In-Memory, `< 5ms`)**: Test command classes, option parsing, and business logic directly by importing `package:<pkg>/src/...` in `test/`.
2. **Integration Tests (Subprocesses)**: Use `package:test_process` and `package:test_descriptor` to verify end-to-end binary execution, process I/O streaming, and OS exit codes:

```dart
import 'package:test/test.dart';
import 'package:test_descriptor/test_descriptor.dart' as d;
import 'package:test_process/test_process.dart';

void main() {
  test('CLI processes input and exits cleanly', () async {
    await d.file('input.txt', 'hello').create();

    final process = await TestProcess.start('dart', [
      'run',
      'bin/my_cli.dart',
      '--input',
      d.path('input.txt'),
    ]);

    await expectLater(process.stdout, emitsThrough('Processing complete.'));
    await process.shouldExit(0);
  });
}
```

---

## 9. Modern Compilation & Distribution

Dart 3.12+ standardizes CLI distribution around `dart run` and `dart install` (moving away from `dart pub global activate`):

* **Ephemeral Execution (JIT)**: `dart run <package>@<version> [args]` downloads and runs the CLI on demand.
* **Global Installation (Native AOT)**: `dart install <package>` compiles the package entrypoint to a fast native standalone binary in `~/.dart/install/bin/`.
* **Local Development**: Use `dart run <command>` (resolves via `executables:` in `pubspec.yaml`) or `dart run bin/cli.dart`.
* **Bundling Dynamic Libraries & Code Assets**: Use `dart build cli`. Outputs bundle to `build/cli/_/bundle/`.
* **Standalone Executable Compilation**: Use `dart compile exe bin/cli.dart -o <output_path>`.

---

## 10. Workflows & Audit Checklist

### Implementation Workflow
- [ ] Declare entry points in `pubspec.yaml` under `executables:`.
- [ ] Keep `bin/*.dart` as a thin entrypoint; place command logic in `lib/src/`.
- [ ] Return integer exit codes or set `exitCode = N`; avoid raw `exit(N)`.
- [ ] Direct errors, warnings, and usage on parse failure to `stderr`.
- [ ] Use `results.flag()`, `results.option()`, and `results.multiOption()` for type safety.
- [ ] Validate `useAnsi` (checking `stdout.hasTerminal`, `supportsAnsiEscapes`, and `NO_COLOR`) before emitting ANSI codes.
- [ ] Spawn child tools using `cli_util.dartExecutable`, never `Platform.resolvedExecutable`.
- [ ] Unit-test command runners in-memory; test end-to-end binary execution with `test_process`.

---

## References & Examples

* **Single-Command Tool Template**: [examples/single_command_tool.dart](examples/single_command_tool.dart)
* **Multi-Command Runner Template**: [examples/multi_command_runner.dart](examples/multi_command_runner.dart)
* **AOT SDK Discovery & Subprocess Spawning**: [references/aot_sdk_discovery.md](references/aot_sdk_discovery.md)
* **Signal Handling & Terminal Teardown**: [references/signals_and_terminal.md](references/signals_and_terminal.md)