Purpose

Build robust Go CLI tools using Cobra for command structure, Viper for config, and idiomatic Go patterns for I/O and error handling.

When to use this skill

• creating a new Go CLI binary with subcommands
• wiring Cobra commands with Viper config file and env var binding
• adding --output json|table|text formatting
• structuring a multi-command Go CLI project

Do not use this skill when

• building an interactive terminal UI — prefer bubbletea-go
• building a Python CLI — prefer cli-development-python
• only writing shell scripts — prefer bash

Procedure

1. Scaffold with Cobra-CLI — run cobra-cli init and cobra-cli add <cmd> to generate command files.
2. Define root command — set PersistentPreRunE on root for global setup (config loading, logger init).
3. Bind flags to Viper — call viper.BindPFlag("key", cmd.Flags().Lookup("flag")) in init().
4. Load config — use viper.SetConfigName(".myapp"), viper.AddConfigPath("$HOME"), viper.AutomaticEnv().
5. Structure output — accept --output flag; use encoding/json for JSON, text/tabwriter for tables.
6. Handle errors idiomatically — return error from RunE, let Cobra print usage on bad args. Use fmt.Errorf("verb: %w", err).
7. Add completions — Cobra auto-generates completion subcommand; add custom ValidArgsFunction for dynamic completions.
8. Build and test — go build -ldflags "-X main.version=$(git describe)" for version embedding; test with cobra.Command.ExecuteC().

Project layout

myapp/
  cmd/
    root.go          # root command + global flags
    serve.go         # subcommand
    migrate.go       # subcommand
  internal/
    config/config.go # Viper wrapper
    output/output.go # JSON/table formatter
  main.go            # cmd.Execute()
  .myapp.yaml        # default config

Key patterns

var rootCmd = &cobra.Command{
    Use:   "myapp",
    Short: "Does the thing",
    PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
        viper.SetConfigName(".myapp")
        viper.AddConfigPath(".")
        viper.AutomaticEnv()
        viper.SetEnvPrefix("MYAPP")
        _ = viper.ReadInConfig()
        return nil
    },
}

func init() {
    rootCmd.PersistentFlags().StringP("output", "o", "text", "Output format: text|json|table")
    viper.BindPFlag("output", rootCmd.PersistentFlags().Lookup("output"))
}

Decision rules

• Use RunE (not Run) so errors propagate to os.Exit(1) via Cobra.
• Bind every flag to Viper so env vars and config files work as overrides.
• Write to cmd.OutOrStdout() instead of os.Stdout for testability.
• Validate args in Args: field (e.g., cobra.ExactArgs(1)) not inside RunE.
• Prefer --output flag over multiple commands (list-json, list-table).

References

• https://cobra.dev/
• https://github.com/spf13/viper

Related skills

• cobra-go — focused Cobra command patterns
• bubbletea-go — interactive TUI on top of CLI
• release-binaries — cross-compiling and distributing the binary