Cobra Modularity

Cobra Modular CLI Architecture

Build scalable, maintainable CLI applications using Cobra with modular command registration patterns.

Your Role: CLI Architect

You help structure CLI applications with clean, modular architecture. You:

✅ Design modular command structure - Self-contained command modules ✅ Implement Register pattern - Commands register themselves ✅ Handle flags properly - Persistent vs local, parsing, validation ✅ Structure subcommands - Nested command hierarchies ✅ Apply Cobra idioms - RunE for errors, PreRun hooks, etc. ✅ Follow project patterns - Use existing module conventions

❌ Do NOT centralize commands - Keep modules self-contained ❌ Do NOT modify root unnecessarily - Add via registration only ❌ Do NOT ignore errors - Use RunE, not Run

Core Principles

1. Modular Command Registration

Commands live in their own packages and register with parent commands.

✅ GOOD - Self-registering module:

// modules/demo/cmd.go
package demo

import "github.com/spf13/cobra"

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "demo",
        Short: "Demo commands",
    }
    parent.AddCommand(cmd)

    // Register subcommands
    spinner.Register(cmd)
    list.Register(cmd)
}

❌ BAD - Centralized registration:

// cmd/root.go - DON'T do this
func init() {
    RootCmd.AddCommand(demoCmd)
    RootCmd.AddCommand(listCmd)
    RootCmd.AddCommand(spinnerCmd)
    // Root knows too much
}

2. Command Structure Pattern

Standard module layout:

modules/demo/
├── cmd.go              # Register function
├── spinner/
│   ├── cmd.go         # spinner.Register()
│   └── spinner.go     # Implementation
└── list/
    ├── cmd.go         # list.Register()
    └── list.go        # Implementation

Benefits:

• Module is self-contained
• Easy to add/remove commands
• No changes to root when adding modules
• Testable in isolation

3. Error Handling

Use RunE (not Run):

✅ GOOD:

cmd := &cobra.Command{
    Use:   "fetch",
    Short: "Fetch data",
    RunE:  run,  // Returns error
}

func run(cmd *cobra.Command, args []string) error {
    if err := doWork(); err != nil {
        return fmt.Errorf("fetch failed: %w", err)
    }
    return nil
}

❌ BAD:

cmd := &cobra.Command{
    Run: func(cmd *cobra.Command, args []string) {
        doWork()  // Error ignored
    },
}

Cobra Command Structure

Basic Command

package mycommand

import (
    "github.com/spf13/cobra"
)

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "mycommand",
        Short: "Short description",
        Long:  `Long description with examples`,
        RunE:  run,
    }
    parent.AddCommand(cmd)
}

func run(cmd *cobra.Command, args []string) error {
    // Implementation
    return nil
}

With Flags

Local flags (command-specific):

var (
    flagName   string
    flagCount  int
    flagVerbose bool
)

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "process",
        Short: "Process data",
        RunE:  run,
    }

    // String flag with shorthand
    cmd.Flags().StringVarP(&flagName, "name", "n", "", "Name (required)")
    cmd.MarkFlagRequired("name")

    // Int flag
    cmd.Flags().IntVarP(&flagCount, "count", "c", 10, "Count")

    // Bool flag
    cmd.Flags().BoolVarP(&flagVerbose, "verbose", "v", false, "Verbose output")

    parent.AddCommand(cmd)
}

func run(cmd *cobra.Command, args []string) error {
    // Use flagName, flagCount, flagVerbose
    return nil
}

Persistent flags (inherited by subcommands):

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "server",
        Short: "Server commands",
    }

    // Available to all subcommands
    cmd.PersistentFlags().StringP("config", "c", "", "Config file")

    parent.AddCommand(cmd)

    // Register subcommands
    start.Register(cmd)  // Can access --config
    stop.Register(cmd)   // Can access --config
}

With Arguments

Exact args:

cmd := &cobra.Command{
    Use:   "delete <id>",
    Short: "Delete item by ID",
    Args:  cobra.ExactArgs(1),  // Requires exactly 1 arg
    RunE:  run,
}

func run(cmd *cobra.Command, args []string) error {
    id := args[0]
    return deleteItem(id)
}

Range of args:

Args: cobra.RangeArgs(1, 3),  // 1 to 3 args
Args: cobra.MinimumNArgs(1),  // At least 1 arg
Args: cobra.MaximumNArgs(2),  // At most 2 args

Custom validation:

Args: func(cmd *cobra.Command, args []string) error {
    if len(args) != 1 {
        return fmt.Errorf("requires exactly one arg")
    }
    if !isValidID(args[0]) {
        return fmt.Errorf("invalid ID: %s", args[0])
    }
    return nil
},

Module Patterns

Demo Module Pattern (CLY)

Parent command - modules/demo/cmd.go:

package demo

import (
    "github.com/spf13/cobra"
    spinner "github.com/yurifrl/cly/modules/demo/spinner"
)

var DemoCmd = &cobra.Command{
    Use:   "demo",
    Short: "Demo TUI components",
}

func Register(parent *cobra.Command) {
    parent.AddCommand(DemoCmd)
}

func init() {
    // Register all demo subcommands
    spinner.Register(DemoCmd)
    // ... more demos
}

Subcommand - modules/demo/spinner/cmd.go:

package spinner

import (
    tea "github.com/charmbracelet/bubbletea"
    "github.com/spf13/cobra"
)

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "spinner",
        Short: "Spinner demo",
        RunE:  run,
    }
    parent.AddCommand(cmd)
}

func run(cmd *cobra.Command, args []string) error {
    p := tea.NewProgram(initialModel())
    if _, err := p.Run(); err != nil {
        return err
    }
    return nil
}

Utility Module Pattern (CLY)

Standalone command - modules/uuid/cmd.go:

package uuid

import (
    tea "github.com/charmbracelet/bubbletea"
    "github.com/spf13/cobra"
)

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "uuid",
        Short: "Generate UUIDs",
        Long:  "Interactive UUID generator with history",
        RunE:  run,
    }
    parent.AddCommand(cmd)
}

func run(cmd *cobra.Command, args []string) error {
    p := tea.NewProgram(initialModel())
    _, err := p.Run()
    return err
}

Registered in root - cmd/root.go:

import (
    "github.com/yurifrl/cly/modules/uuid"
    "github.com/yurifrl/cly/modules/demo"
)

func init() {
    uuid.Register(RootCmd)
    demo.Register(RootCmd)
}

Advanced Patterns

PreRun Hooks

Validate before run:

cmd := &cobra.Command{
    Use:  "deploy",
    PreRunE: func(cmd *cobra.Command, args []string) error {
        // Validation
        if !fileExists(configFile) {
            return fmt.Errorf("config not found: %s", configFile)
        }
        return nil
    },
    RunE: run,
}

Setup before run:

PreRunE: func(cmd *cobra.Command, args []string) error {
    // Setup database connection
    db, err = connectDB()
    return err
},

PostRun Hooks

Cleanup after run:

PostRun: func(cmd *cobra.Command, args []string) {
    // Cleanup
    db.Close()
    tempFile.Remove()
},

Persistent PreRun (Inherited)

cmd := &cobra.Command{
    Use: "api",
    PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
        // Runs before ALL subcommands
        return loadConfig()
    },
}

Flag Dependencies

Require flag if another present:

cmd.Flags().String("format", "", "Output format")
cmd.Flags().String("output", "", "Output file")

cmd.MarkFlagsRequiredTogether("format", "output")

Mutually exclusive flags:

cmd.Flags().Bool("json", false, "JSON output")
cmd.Flags().Bool("yaml", false, "YAML output")

cmd.MarkFlagsMutuallyExclusive("json", "yaml")

Subcommand Groups

parent := &cobra.Command{
    Use:   "api",
    Short: "API commands",
}

// Group 1
parent.AddGroup(&cobra.Group{
    ID:    "server",
    Title: "Server Commands:",
})

cmd1 := &cobra.Command{
    Use:     "start",
    GroupID: "server",
}

cmd2 := &cobra.Command{
    Use:     "stop",
    GroupID: "server",
}

parent.AddCommand(cmd1, cmd2)

Configuration Integration

With Viper

import (
    "github.com/spf13/cobra"
    "github.com/spf13/viper"
)

var cfgFile string

func Register(parent *cobra.Command) {
    cmd := &cobra.Command{
        Use:   "server",
        Short: "Run server",
        PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
            return initConfig()
        },
        RunE: run,
    }

    cmd.PersistentFlags().StringVar(&cfgFile, "config", "", "Config file")

    // Bind flag to viper
    viper.BindPFlag("config", cmd.PersistentFlags().Lookup("config"))

    parent.AddCommand(cmd)
}

func initConfig() error {
    if cfgFile != "" {
        viper.SetConfigFile(cfgFile)
    } else {
        viper.SetConfigName("config")
        viper.SetConfigType("yaml")
        viper.AddConfigPath(".")
        viper.AddConfigPath("$HOME/.myapp")
    }

    viper.AutomaticEnv()
    return viper.ReadInConfig()
}

func run(cmd *cobra.Command, args []string) error {
    port := viper.GetInt("server.port")
    // Use config
    return nil
}

Best Practices

1. Keep Commands Focused

One responsibility per command:

// ✅ GOOD
cly uuid              # Generate UUIDs
cly demo spinner      # Show spinner demo

// ❌ BAD
cly utils  # Does everything

2. Use Meaningful Names

// ✅ GOOD
Use: "generate",
Use: "list-users",
Use: "deploy-app",

// ❌ BAD
Use: "do",
Use: "run",
Use: "execute",

3. Provide Good Help

cmd := &cobra.Command{
    Use:   "deploy <environment>",
    Short: "Deploy application",
    Long: `Deploy the application to specified environment.

Environments: dev, staging, prod

Examples:
  cly deploy dev
  cly deploy prod --version v1.2.3`,
    Example: `  cly deploy dev
  cly deploy prod --version v1.2.3`,
}

4. Validate Early

PreRunE: func(cmd *cobra.Command, args []string) error {
    // Validate flags
    if port < 1 || port > 65535 {
        return fmt.Errorf("invalid port: %d", port)
    }

    // Check prerequisites
    if !commandExists("docker") {
        return fmt.Errorf("docker not found")
    }

    return nil
},

5. Handle Interrupts

import (
    "context"
    "os/signal"
    "syscall"
)

func run(cmd *cobra.Command, args []string) error {
    ctx, stop := signal.NotifyContext(
        context.Background(),
        os.Interrupt,
        syscall.SIGTERM,
    )
    defer stop()

    return runWithContext(ctx)
}

Testing Commands

Test Registration

func TestRegister(t *testing.T) {
    parent := &cobra.Command{Use: "root"}
    Register(parent)

    require.Len(t, parent.Commands(), 1)
    require.Equal(t, "mycommand", parent.Commands()[0].Use)
}

Test Command Execution

func TestRun(t *testing.T) {
    cmd := &cobra.Command{
        Use:  "test",
        RunE: run,
    }

    cmd.SetArgs([]string{"arg1", "arg2"})

    err := cmd.Execute()
    require.NoError(t, err)
}

Test with Flags

func TestRunWithFlags(t *testing.T) {
    cmd := &cobra.Command{
        Use:  "test",
        RunE: run,
    }

    var flagValue string
    cmd.Flags().StringVar(&flagValue, "flag", "", "test flag")

    cmd.SetArgs([]string{"--flag", "value"})

    err := cmd.Execute()
    require.NoError(t, err)
    require.Equal(t, "value", flagValue)
}

Common Pitfalls

❌ Using Run instead of RunE:

Run: func(cmd *cobra.Command, args []string) {
    // Can't return errors!
    doWork()
}

✅ Use RunE:

RunE: func(cmd *cobra.Command, args []string) error {
    return doWork()
}

❌ Not validating args:

RunE: func(cmd *cobra.Command, args []string) error {
    id := args[0]  // Panic if no args!
    return nil
}

✅ Validate with Args:

cmd := &cobra.Command{
    Args: cobra.ExactArgs(1),
    RunE: run,
}

❌ Centralizing all commands:

// root.go
RootCmd.AddCommand(cmd1)
RootCmd.AddCommand(cmd2)
RootCmd.AddCommand(cmd3)
// Tight coupling

✅ Module registration:

// Each module registers itself
module1.Register(RootCmd)
module2.Register(RootCmd)

Checklist

• Command uses RunE (not Run)
• Register() function for modularity
• Args validated with Args field
• Flags bound to variables
• Required flags marked
• Good Short description
• Detailed Long description
• Examples provided
• Errors wrapped with context
• Tests for command execution

Reference

CLY Project Structure:

cmd/
└── root.go              # Root command, imports modules

modules/
├── demo/
│   ├── cmd.go          # demo.Register()
│   └── spinner/
│       └── cmd.go      # spinner.Register()
└── uuid/
    └── cmd.go          # uuid.Register()

Pattern: Each module registers itself, no central registration.

Resources

• Cobra Docs
• Cobra User Guide
• Viper Config
• CLY examples: modules/demo/, modules/uuid/