Cobra CLI Framework

Cobra is a library for creating powerful modern CLI applications in Go, used by Kubernetes, Hugo, GitHub CLI, and many more.

Core Concepts

Commands represent actions, Args are things, Flags are modifiers. Follow the pattern: APPNAME COMMAND ARG --FLAG.

Project Structure

code

appName/
  cmd/
    root.go
    subcommand.go
  main.go

Creating Commands

Root Command

package cmd

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

var rootCmd = &cobra.Command{
    Use:   "myapp",
    Short: "Brief description",
    Long:  `Detailed description...`,
    Run: func(cmd *cobra.Command, args []string) {
        // Action here
    },
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

Subcommands

var versionCmd = &cobra.Command{
    Use:   "version",
    Short: "Print version",
    Run: func(cmd *cobra.Command, args []string) {
        fmt.Println("v1.0.0")
    },
}

func init() {
    rootCmd.AddCommand(versionCmd)
}

Error Handling with RunE

var cmd = &cobra.Command{
    Use: "delete [name]",
    RunE: func(cmd *cobra.Command, args []string) error {
        if len(args) == 0 {
            return fmt.Errorf("name required")
        }
        return deleteResource(args[0])
    },
}

Flags

Persistent Flags (cascade to children)

var verbose bool

func init() {
    rootCmd.PersistentFlags().BoolVarP(&verbose, "verbose", "v", false, "verbose output")
}

Local Flags (command-specific)

var source string

func init() {
    cmd.Flags().StringVarP(&source, "source", "s", "", "source directory")
}

Required Flags

func init() {
    rootCmd.Flags().StringVarP(&region, "region", "r", "", "AWS region")
    rootCmd.MarkFlagRequired("region")
}

Flag Groups

func init() {
    rootCmd.Flags().StringVarP(&user, "username", "u", "", "Username")
    rootCmd.Flags().StringVarP(&pass, "password", "p", "", "Password")
    rootCmd.MarkFlagsRequiredTogether("username", "password")
    rootCmd.MarkFlagsMutuallyExclusive("json", "yaml")
}

Count/Array Flags

var verbose int
var files []string

func init() {
    rootCmd.PersistentFlags().CountVarP(&verbose, "verbose", "v", "verbosity (-v, -vv, -vvv)")
    rootCmd.Flags().StringArrayVarP(&files, "file", "f", []string{}, "input files")
}

Argument Validation

var cmd = &cobra.Command{
    Use:  "create [name]",
    Args: cobra.ExactArgs(1),
    Run:  func(cmd *cobra.Command, args []string) { ... },
}

Built-in validators:

•NoArgs - no positional args allowed
•ArbitraryArgs - any number (default)
•MinimumNArgs(int) - at least N args
•MaximumNArgs(int) - at most N args
•ExactArgs(int) - exactly N args
•RangeArgs(min, max) - between min and max
•OnlyValidArgs - only values in ValidArgs

Combined validation:

Args: cobra.MatchAll(cobra.ExactArgs(2), cobra.OnlyValidArgs),

PreRun/PostRun Hooks

Execution order: PersistentPreRun -> PreRun -> Run -> PostRun -> PersistentPostRun

var rootCmd = &cobra.Command{
    Use: "myapp",
    PersistentPreRun: func(cmd *cobra.Command, args []string) {
        // Runs before any child command
    },
    PreRun: func(cmd *cobra.Command, args []string) {
        // Runs before this command's Run
    },
    Run: func(cmd *cobra.Command, args []string) {
        // Main logic
    },
    PostRun: func(cmd *cobra.Command, args []string) {
        // Runs after Run
    },
    PersistentPostRun: func(cmd *cobra.Command, args []string) {
        // Runs after any child command
    },
}

Shell Completions

Static Completions

var cmd = &cobra.Command{
    Use:       "get [resource]",
    ValidArgs: []string{"pod", "service", "deployment"},
    Run:       func(cmd *cobra.Command, args []string) { ... },
}

Dynamic Completions

var cmd = &cobra.Command{
    Use: "status [release]",
    ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]cobra.Completion, cobra.ShellCompDirective) {
        return getReleases(toComplete), cobra.ShellCompDirectiveNoFileComp
    },
    Run: func(cmd *cobra.Command, args []string) { ... },
}

Flag Completions

cmd.RegisterFlagCompletionFunc("output", func(cmd *cobra.Command, args []string, toComplete string) ([]cobra.Completion, cobra.ShellCompDirective) {
    return []cobra.Completion{"json", "yaml", "table"}, cobra.ShellCompDirectiveNoFileComp
})

ShellCompDirective Values

•ShellCompDirectiveDefault - default behavior
•ShellCompDirectiveNoSpace - no space after completion
•ShellCompDirectiveNoFileComp - no file completion
•ShellCompDirectiveFilterFileExt - filter by file extension
•ShellCompDirectiveFilterDirs - directory names only

Help Customization

cmd.SetHelpCommand(customHelpCmd)
cmd.SetHelpFunc(func(cmd *cobra.Command, args []string) { ... })
cmd.SetHelpTemplate(customTemplate)
cmd.SetUsageFunc(func(cmd *cobra.Command) error { ... })
cmd.SetUsageTemplate(customTemplate)

Version Flag

var rootCmd = &cobra.Command{
    Use:     "myapp",
    Version: "1.0.0",
}

Command Groups

func init() {
    rootCmd.AddGroup(
        &cobra.Group{ID: "management", Title: "Management Commands:"},
        &cobra.Group{ID: "query", Title: "Query Commands:"},
    )
    
    listCmd.GroupID = "query"
    createCmd.GroupID = "management"
    rootCmd.AddCommand(listCmd, createCmd)
}

Plugin Support (kubectl-style)

rootCmd := &cobra.Command{
    Use: "kubectl-myplugin",
    Annotations: map[string]string{
        cobra.CommandDisplayNameAnnotation: "kubectl myplugin",
    },
}

Common Patterns

Traverse Children Flags

rootCmd := &cobra.Command{
    Use:              "myapp",
    TraverseChildren: true, // Parse parent flags before child command
}

Silence Errors/Usage

cmd.SilenceErrors = true // Don't print error prefix
cmd.SilenceUsage = true  // Don't print usage on error

Hidden Commands

cmd.Hidden = true

Deprecated Commands

cmd.Deprecated = "use 'newcmd' instead"

Examples Index

File	Description
references/_examples/basic_app.go	Minimal CLI application
references/_examples/subcommands.go	Nested subcommands
references/_examples/flags.go	Flag patterns
references/_examples/hooks.go	PreRun/PostRun hooks
references/_examples/completions.go	Shell completions

Integration with Viper

import "github.com/spf13/viper"

func init() {
    rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file")
    rootCmd.PersistentFlags().StringP("author", "a", "", "author name")
    viper.BindPFlag("author", rootCmd.PersistentFlags().Lookup("author"))
}