golang-cli

Files (13)

• SKILL.md
• args.go · assets/examples/
• cli_test.go · assets/examples/
• completion.go · assets/examples/
• config.go · assets/examples/
• exit_codes.go · assets/examples/
• flags.go · assets/examples/
• main.go · assets/examples/
• output.go · assets/examples/
• root.go · assets/examples/
• serve.go · assets/examples/
• signal.go · assets/examples/
• evals.json · evals/

SKILL.md (first lines)

**Persona:** You are a Go CLI engineer. You build tools that feel native to the Unix shell — composable, scriptable, and predictable under automation.

**Modes:**

- **Build** — creating a new CLI from scratch: follow the project structure, root command setup, flag binding, and version embedding sections sequentially.
- **Extend** — adding subcommands, flags, or completions to an existing CLI: read the current command tree first, then apply changes consistent with the existing structure.
- **Review** — auditing an existing CLI for correctness: check the Common Mistakes table, verify `SilenceUsage`/`SilenceErrors`, flag-to-Viper binding, exit codes, and stdout/stderr discipline.

# Go CLI Best Practices

Use Cobra + Viper as the default stack for Go CLI applications. Cobra provides the command/subcommand/flag structure and Viper handles configuration from files, environment variables, and flags with automatic layering. This combination powers kubectl, docker, gh, hugo, and most production Go CLIs.

When using Cobra or Viper, refer to the library's official documentation and code examples for current API signatures. 

For trivial single-purpose tools with no subcommands and few flags, stdlib `flag` is sufficient.

## Quick Reference

| Concern             | Package / Tool                       |
| ------------------- | ------------------------------------ |
| Commands & flags    | `github.com/spf13/cobra`             |
| Configuration       | `github.com/spf13/viper`             |
| Flag parsing        | `github.com/spf13/pflag` (via Cobra) |
| Colored output      | `github.com/fatih/color`             |
| Table output        | `github.com/olekukonez/tablewriter`  |
| Interactive prompts | `github.com/charmbracelet/bubbletea` |
| Version injection   | `go build -ldflags`                  |
| Distribution        | `goreleaser`                         |

## Project Structure

Organize CLI commands in `cmd/myapp/` with one file per command. Keep `main.go` minimal — it only calls `Execute()`.
`main.go` should be minimal — see [assets/examples/main.go](assets/examples/main.go).

```
myapp/
├── cmd/
│   └── myapp/
│       ├── main.go              # package main, only calls Execute()
│       ├── root.go              # Root command + Viper init
│       ├── serve.go             # "serve" subcommand
│       ├── migrate.go           # "migrate" subcommand
│       └── version.go           # "version" subcommand
├── src/
│   ├── server/                  # HTTP server logic, middleware, handlers
│   ├── database/                # DB connection, queries, migrations
│   └── aws/                     # AWS SDK clients and service wrappers
├── go.mod
└── go.sum
```

Business logic lives in `src/`, which sits as a sibling to `cmd/`. Each subdirectory groups functionality by concern — `server/` for HTTP handling, `database/` for persistence, `aws/` for cloud service wrappers — keeping implementation details cleanly separated from command wiring.

This layout is designed for **maximum separation of concerns**: commands only parse flags and delegate to `src/` packages; they never contain business logic themselves. It also **aids AI-assisted development** — focused, single-responsibility packages are easier for an AI agent to reason about, modify, and test in isolation, reducing the risk of unintended side-effects across the codebase.

## Root Command Setup

The root command initializes Viper configuration and sets up global behavior via `PersistentPreRunE`. See [assets/examples/root.go](assets/examples/root.go).

Key points:

- `SilenceUsage: true` MUST be set — prevents printing the full usage text on every error
- `SilenceErrors: true` MUST be set — lets you control error output format yourself
- `PersistentPreRunE` runs before every subcommand, so config is always initialized
- Logs go to stderr, output goes to stdout

## Subcommands

Add subcommands by creating separate files in `cmd/myapp/` and registering them in `init()`. See [assets/examples/serve.go](assets/examples/serve.go) for a complete subcommand example including command groups.

## Flags

See [assets/examples/flags.go](assets/examples/flags.go) for all flag patterns:

### Persistent vs Local

- **Persistent** flags are inherited by all subcommands (e.g., `--config`)
- **Local** flags only apply to the command they're defined on (e.g., `--port`)

### Required Flags

Use `MarkFlagRequired`, `MarkFlagsMutuallyExclusive`, and `MarkFlagsOneRequired` for flag constraints.

### Flag Validation with RegisterFlagCompletionFunc

Provide completion suggestions for flag values.

### Always Bind Flags to Viper

This ensures `viper.GetInt("port")` returns the flag value, env var `MYAPP_PORT`, or config file value — whichever has highest precedence.

## Argument Validation

Cobra provides built-in validators for positional arguments. See [assets/examples/args.go](assets/examples/args.go) for both built-in and custom validation examples.

| Validator                   | Description                          |
| --------------------------- | ------------------------------------ |
| `cobra.NoArgs`              | Fails if any args provided           |
| `cobra.ExactArgs(n)`        | Requires exactly n args              |
| `cobra.MinimumNArgs(n)`     | Requires at least n args             |
| `cobra.MaximumNArgs(n)`     | Allows at most n args                |
| `cobra.RangeArgs(min, max)` | Requires between min and max         |
| `cobra.ExactValidArgs(n)`   | Exactly n args, must be in ValidArgs |

## Configuration with Viper

Viper resolves configuration values in this order (highest to lowest precedence):

1. **CLI flags** (explicit user input)
2. **Environment variables** (deployment config)
3. **Config file** (persistent settings)
4. **Defaults** (set in code)

See [assets/examples/config.go](assets/examples/config.go) for complete Viper integration including struct unmarshaling and config file watching.

### Example Config File (.myapp.yaml)

```yaml
port: 8080

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