Go CLI

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.

1. Structure: Testable main

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.
• 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.

// ✅ 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

Map errors to codes in one place (main), not scattered os.Exit calls. If scripts will branch on distinct failures, define sentinel errors and translate: errors.Is(err, ErrNoMatch) → 1.

4. Flags and Arguments

• Flags for options, positional args for the primary operands: mytool -v convert input.yaml, not mytool --input=input.yaml.
• Every flag has a usage string; -h/-help output is your primary UX.
• Accept - as "stdin/stdout" for file arguments.
• Defaults must be safe: destructive behavior behind explicit flags (--force), never default-on.
• Read secrets from env or files, never from flags (ps leaks argv).

5. Subcommands

Standard library, fine up to a handful of commands:

switch fs.Arg(0) {
case "serve":
    return runServe(ctx, fs.Args()[1:], stdout, stderr)
case "migrate":
    return runMigrate(ctx, fs.Args()[1:], stdout, stderr)
default:
    fmt.Fprintln(stderr, usage)
    return fmt.Errorf("unknown command %q", fs.Arg(0))
}

Adopt Cobra when you need nested commands, generated help/completions, and many flags — the structure pays for the dependency:

var rootCmd = &cobra.Command{Use: "mytool", SilenceUsage: true}

var serveCmd = &cobra.Command{
    Use:   "serve",
    Short: "Start the server",
    RunE: func(cmd *cobra.Command, args []string) error {
        return serve(cmd.Context(), addr) // RunE returns errors; no os.Exit
    },
}

func init() {
    serveCmd.Flags().StringVar(&addr, "addr", ":8080", "listen address")
    rootCmd.AddCommand(serveCmd)
}

Cobra rules: always RunE (never Run + os.Exit), set SilenceUsage: true so runtime errors don't dump help, pass cmd.Context() down. Add Viper only when layered config (flags > env > file) is a real requirement — for most tools flag + os.Getenv is enough.

6. Output for Humans and Machines

• --json flag for machine consumption; table/text default for humans.
• Never emit ANSI colors when stdout is not a terminal or NO_COLOR is set.
• Progress bars/spinners go to stderr and only when it's a terminal.

Verification Checklist

1. run(ctx, args, stdin, stdout, stderr) pattern — logic testable without subprocess
2. signal.NotifyContext wired; long operations respect ctx cancellation
3. Data on stdout, diagnostics on stderr — verified by piping
4. Exit codes: 0 success, 2 usage, documented codes otherwise; os.Exit only in main
5. Every flag has usage text; -h output reviewed
6. - accepted for stdin/stdout where files are taken
7. Destructive actions require explicit flags
8. No secrets via argv
9. Cobra (if used): RunE everywhere, SilenceUsage, context propagated
10. Colors/spinners disabled for non-TTY and NO_COLOR