# go-cli

Guides coding agents in building well-behaved Go CLI tools with clean signal handling, flag parsing, and Unix I/O discipline.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · ★ 83
- Author: eduardo-sl
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/eduardo-sl/go-agent-skills/tree/HEAD/skills/(architecture)/go-cli/SKILL.md
- Updated: 2026-10-15
- HTML: https://affpapa.org/en/nevibecode/skills/go-cli

## What it does

The skill structures Go command-line tools around a testable `run(ctx, args, stdin, stdout, stderr)` function. This pattern allows unit tests to execute via in-memory buffers instead of spinning up subprocesses. Graceful shutdown relies on `signal.NotifyContext` tied directly to Ctrl+C, keeping `os.Exit` restricted strictly to `main` so `defer` statements still execute.

Output rules enforce strict Unix conventions. Data streams go to `stdout`, while diagnostic logs and progress indicators route to `stderr`. The skill outlines when the standard `flag` library suffices and when adding `Cobra` makes sense, requiring `RunE` and `SilenceUsage: true` to prevent messy help dumps on runtime failures. Integration tests use `testscript` (`.txtar`) and golden files.

Running this skill requires the local Go toolchain. It focuses exclusively on command-line ergonomics, leaving API design and project scaffolding to other domain skills.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add eduardo-sl/go-agent-skills --skill go-cli -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/go-cli/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/go-cli/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add eduardo-sl/go-agent-skills --skill go-cli -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/go-cli/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add eduardo-sl/go-agent-skills --skill go-cli -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/go-cli/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install eduardo-sl/go-agent-skills go-cli
```

2. **Manually: put the files in place**
- File: `.github/skills/go-cli/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add eduardo-sl/go-agent-skills --skill go-cli -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/eduardo-sl/go-agent-skills --path 'skills/(architecture)/go-cli'
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/go-cli/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add eduardo-sl/go-agent-skills --skill go-cli -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/eduardo-sl/go-agent-skills/tree/HEAD/skills/(architecture)/go-cli/SKILL.md

## When to use it
- isolate data streams on stdout from diagnostics on stderr
- handle Ctrl+C cancellations via context without leaking defers
- test CLI arguments and streams without running subprocesses
- choose appropriately between standard flags and Cobra subcommands

## Example prompts
- Build a Go CLI tool that reads from stdin and outputs JSON using only the standard library
- Add a Cobra subcommand structure with proper signal handling and RunE
- Set up golden file tests for this command-line tool

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 9bfbd68e968c3d1e.

BEGIN THIRD-PARTY FILE 9bfbd68e968c3d1e
````text
---
name: go-cli
description: >
  Build command-line tools in Go: flag handling, subcommands, stdin/stdout
  discipline, exit codes, signal handling, and when Cobra/Viper earn their
  weight over the standard library. Use when: "build a CLI", "add a
  subcommand", "parse flags", "exit codes", "handle Ctrl+C", "cobra
  command", "read from stdin", "CLI UX".
  Not for: HTTP APIs (go-api-design), scaffolding (go-project-layout),
  service configuration (go-architecture-review).
user-invocable: true
license: MIT
compatibility: Designed for Claude Code or similar AI coding agents working on Go projects. Requires the Go toolchain.
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(gofmt:*)
metadata:
  author: eduardo-sl
  version: "1.2.0"
---

# Go CLI Design

A good CLI is a well-behaved Unix citizen: flags before magic, stdout
for data, stderr for diagnostics, exit codes that scripts can trust,
and Ctrl+C that actually stops it.

Detailed reference material, loaded on demand:

- `references/testing-and-versioning.md` — exit-code mapping in `main`,
  golden-file and testscript tests, version from build info, completions.

Read a reference file only when the summary below is not enough.

## 1. Structure: Testable main

```go
func main() {
    ctx, stop := signal.NotifyContext(context.Background(),
        os.Interrupt, syscall.SIGTERM)
    defer stop()

    if err := run(ctx, os.Args[1:], os.Stdin, os.Stdout, os.Stderr); err != nil {
        fmt.Fprintln(os.Stderr, "error:", err)
        os.Exit(1)
    }
}

func run(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
    fs := flag.NewFlagSet("mytool", flag.ContinueOnError)
    fs.SetOutput(stderr)
    verbose := fs.Bool("v", false, "verbose output")
    out := fs.String("o", "-", "output file (- for stdout)")
    if err := fs.Parse(args); err != nil {
        return err
    }
    // ...
    _ = verbose
    _ = out
    return nil
}
```

- `signal.NotifyContext` makes Ctrl+C cancel the context — every
  long operation takes `ctx` and stops cleanly. Call `stop()` once the
  context is done so a second Ctrl+C kills a stuck program immediately.
- `run` receives args and streams — tests call it directly with
  `strings.Reader`/`bytes.Buffer`, no subprocess needed.
- `os.Exit` only in `main` (it skips defers).

## 2. stdout vs stderr

- **stdout**: the program's output — data, results, the thing you pipe.
- **stderr**: logs, progress, warnings, usage errors.
- `--json` or detecting a pipe (`!term.IsTerminal(int(os.Stdout.Fd()))`)
  should silence decorations, never change the data.

```go
// ✅ Good — result to stdout, progress to stderr
fmt.Fprintf(stderr, "processed %d files\n", n)
fmt.Fprintln(stdout, result)

// ❌ Bad — mixing both into stdout breaks every pipe
fmt.Printf("processing...\ndone: %s\n", result)
```

## 3. Exit Codes

| Code | Meaning |
````
END THIRD-PARTY FILE 9bfbd68e968c3d1e

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