golang-spf13-cobra

**Persona:** You are a Go CLI engineer building command trees that feel native to the Unix shell. You design the user-facing surface first, then wire behavior into the right hook.

**Modes:**

- **Build** — creating a new CLI from scratch: follow command tree setup, hook wiring, and flag 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: check the Common Mistakes table, verify `RunE` usage, `OutOrStdout()`, hook chain ordering, and args validation.

# Using spf13/cobra for CLI command trees in Go

Cobra is the de facto standard for Go CLI applications. It provides the command/subcommand tree, flag parsing (via `pflag`), args validation, shell completion generation, and documentation generation. It does **not** handle configuration layering — that's viper's job.

**Official Resources:**

- [pkg.go.dev/github.com/spf13/cobra](https://pkg.go.dev/github.com/spf13/cobra)
- [github.com/spf13/cobra](https://github.com/spf13/cobra)
- [cobra.dev](https://cobra.dev)

This skill is not exhaustive. Please refer to library documentation and code examples for more information. Context7 can help as a discoverability platform.

```bash
go get github.com/spf13/cobra@latest
```

## Cobra vs. viper

These libraries do fundamentally different things and can be used independently.

| Concern | cobra | viper |
| --- | --- | --- |
| Owns | Command tree, flags, arg validation, completions | Configuration value resolution |
| User-facing? | Yes — subcommands, flags, help text | No — purely a key-value resolver |
| Without the other? | Yes — a CLI with flags only needs cobra | Yes — a daemon reading YAML + env needs only viper |
| Integration seam | Hands `pflag.Flag` to viper via `BindPFlag` | Treats the cobra flag as the highest-precedence layer |

**Use cobra alone** when your binary takes flags and args but needs no config file or env resolution. **Use viper alone** when you have a long-running service reading config from YAML + env with no CLI subcommands. Use both when you need both — bind at `PersistentPreRunE` on the root command.

→ See `samber/cc-skills-golang@golang-spf13-viper` for the viper side of this integration.

## Command tree

Every cobra CLI has a root command plus zero or more subcommands registered with `AddCommand`. The root command name is the binary name.

```go
var rootCmd = &cobra.Command{
    Use:          "myapp",
    Short:        "One-line summary",
    SilenceUsage: true,  // ✓ prevents usage wall on every error
    SilenceErrors: true, // ✓ lets you control error output format
}
```

Use `AddGroup` to label subcommands in help output — register groups **before** the `AddCommand` calls that reference them; cobra does not retroactively assign groups.

## The Run\* family

Cobra commands have five run hooks executed in order:

```
PersistentPreRunE → PreRunE → RunE → PostRunE → PersistentPostRunE
```

Always use `*E` variants — the non-`E` forms cannot return errors. Key rules:

- `PersistentPreRunE` on the root runs before **every** subcommand — use it for config init and auth checks.
- A child `PersistentPreRunE` **replaces** the parent's entirely — call the parent explicitly if you need both.
- `PostRunE` runs only if `RunE` succeeded.

For the full lifecycle and inheritance rules, see [commands-and-args.md](references/commands-and-args.md).

## Args validators

Cobra validates positional arguments before `RunE` runs. Never write `len(args)` checks inside `RunE` — that bypasses cobra's standard error messages and arg count tracking.

Built-ins: `NoArgs`, `ExactArgs(n)`, `MinimumNArgs(n)`, `MaximumNArgs(n)`, `RangeArgs(min,max)`, `OnlyValidArgs`, `ExactValidArgs(n)`. Compose with `MatchAll(v1, v2)`. Custom validator: `func(cmd *cobra.Command, args []string) error`.

For the full validator set with examples and `MatchAll` patterns, see [commands-and-args.md](references/commands-and-args.md).

## Flags primer

Cobra delegates flag parsing to `pflag`. **Persistent flags** (`PersistentFlags()`) are inherited by all subcommands; **local flags** (`Flags()`) apply only to the declaring command.

```go
rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file path") // inherited by all subcommands
serveCmd.Flags().IntVar(&port, "port", 8080, "listen port")                     // local to serveCmd only
serveCmd.MarkFlagRequired("port")
serveCmd.MarkFlagsMutuallyExclusive("json", "yaml")
```

For pflag types, custom flag values, flag groups, and viper binding, see [flags.md](references/flags.md).

## Completions primer

Cobra generates shell completions automatically. Extend them with:

- **`ValidArgs []string`** — static positional arg completion.
- **`ValidArgsFunction`** — dynamic: `func(cmd, args, toComplete string) ([]string, ShellCompDirective)`. Return `ShellCompDirectiveNoFileComp` to suppress file fallback.
- **`RegisterFlagCompletionFunc(name, fn)`** — flag value completion.

For `ShellCompDirective` values, annotations, and testing, see [completions.md](references/completions.md).

## Testing commands

Test commands by executing them programmatically. **Never use `os.Stdout` / `os.Stderr` directly** in command handlers — use `cmd.OutOrStdout()` / `cmd.ErrOrStderr()` so tests can redirect output.

```go
func TestServeCmd(t *testing.T) {
    buf := new(bytes.Buffer)
    rootCmd.SetOut(buf)
    rootCmd.SetArgs([]string{"serve", "--port", "9090"})
    require.NoError(t, rootCmd.Execute())
    assert.Contains(t, buf.String(), "listening on :9090")
}
```

Cobra accumulates flag state across `Execute()` calls — build a fresh command tree per test. For isolation patterns, golden files, and testing completions, see [testing.md](references/testing.md).

## Best Practices

1. **Always use `RunE`, never `Run`** — `Run` cannot return an error; the only escape is `os.Exit` or panic, bypassing de