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
• github.com/spf13/cobra
• 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.

go get github.com/spf13/cobra@latest

Cobra vs. viper

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

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.

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.

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.

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.

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.

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.

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.

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.

Best Practices

1. Always use RunE, never Run — Run cannot return an error; the only escape is os.Exit or panic, bypassing defers.
2. Put config initialization in PersistentPreRunE — it runs before every subcommand; the right place for viper binding and auth checks.
3. Validate positional args with Args, not inside RunE — Args gives cobra's standard error messages; MatchAll composes validators.
4. Use cmd.OutOrStdout() / cmd.ErrOrStderr() for all output — direct os.Stdout writes cannot be captured by tests.
5. Re-create the command tree per test — cobra accumulates flag state across Execute() calls on the same instance.

Common Mistakes

Further Reading

• commands-and-args.md — full PreRun*/PostRun* chain, every Args validator, PersistentPreRunE inheritance rules
• flags.md — pflag types, required/exclusive/oneRequired groups, custom value types, viper binding
• completions.md — ShellCompDirective set, annotation-based completions, testing completions
• generators.md — man page, markdown, YAML, RST doc generation; cobra-cli scaffolder
• [testing.md](ref