Configuration with Viper

Layer configuration sources for flexible CLI behavior.

Configuration Precedence

Flags override environment variables override config files override defaults. This layering gives users flexibility while maintaining sane defaults.

---

Configuration Hierarchy

Viper integrates seamlessly with Cobra for layered configuration:

graph LR
    Flags[Command-line Flags] --> Env[Environment Variables]
    Env --> Config[Config File]
    Config --> Defaults[Defaults]

    %% Ghostty Hardcore Theme
    style Flags fill:#f92572,color:#1b1d1e
    style Env fill:#fd971e,color:#1b1d1e
    style Config fill:#65d9ef,color:#1b1d1e
    style Defaults fill:#5e7175,color:#f8f8f3

---

Implementation

package cmd

import (
    "fmt"
    "os"
    "strings"

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

var cfgFile string

var rootCmd = &cobra.Command{
    Use:   "myctl",
    Short: "Kubernetes orchestration CLI",
    PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
        return initConfig()
    },
}

func init() {
    rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default: $HOME/.myctl.yaml)")
    rootCmd.PersistentFlags().StringP("namespace", "n", "default", "Kubernetes namespace")
    rootCmd.PersistentFlags().String("kubeconfig", "", "Path to kubeconfig file")
    rootCmd.PersistentFlags().Bool("verbose", false, "Enable verbose output")

    // Bind flags to viper
    viper.BindPFlag("namespace", rootCmd.PersistentFlags().Lookup("namespace"))
    viper.BindPFlag("kubeconfig", rootCmd.PersistentFlags().Lookup("kubeconfig"))
    viper.BindPFlag("verbose", rootCmd.PersistentFlags().Lookup("verbose"))
}

func initConfig() error {
    if cfgFile != "" {
        viper.SetConfigFile(cfgFile)
    } else {
        home, err := os.UserHomeDir()
        if err != nil {
            return err
        }
        viper.AddConfigPath(home)
        viper.AddConfigPath(".")
        viper.SetConfigName(".myctl")
        viper.SetConfigType("yaml")
    }

    // Environment variable binding
    viper.SetEnvPrefix("MYCTL")
    viper.SetEnvKeyReplacer(strings.NewReplacer("-", "_", ".", "_"))
    viper.AutomaticEnv()

    // Read config file (ignore if not found)
    if err := viper.ReadInConfig(); err != nil {
        if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
            return fmt.Errorf("error reading config: %w", err)
        }
    }

    return nil
}

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

---

Example Config File

# ~/.myctl.yaml
namespace: production
kubeconfig: ~/.kube/config
verbose: false

cache:
  ttl: 300
  backend: redis
  redis:
    host: localhost
    port: 6379

---

Accessing Configuration

func runCheck(cmd *cobra.Command, args []string) error {
    namespace := viper.GetString("namespace")
    verbose := viper.GetBool("verbose")
    cacheTTL := viper.GetInt("cache.ttl")

    if verbose {
        fmt.Printf("Using namespace: %s\n", namespace)
        fmt.Printf("Cache TTL: %d seconds\n", cacheTTL)
    }

    // ... check logic
    return nil
}

---

Environment Variable Patterns

Standard Kubernetes Variables

Support the variables users expect:

func init() {
    // Standard kubeconfig handling
    if kubeconfig := os.Getenv("KUBECONFIG"); kubeconfig != "" {
        viper.SetDefault("kubeconfig", kubeconfig)
    }

    // Namespace from environment (Kubernetes convention)
    if ns := os.Getenv("NAMESPACE"); ns != "" {
        viper.SetDefault("namespace", ns)
    }
}

In-Cluster Detection

func isInCluster() bool {
    _, err := os.Stat("/var/run/secrets/kubernetes.io/serviceaccount/token")
    return err == nil
}

func getDefaultNamespace() string {
    // In-cluster: read from mounted secret
    if isInCluster() {
        if ns, err := os.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/namespace"); err == nil {
            return string(ns)
        }
    }
    // Out-of-cluster: use default
    return "default"
}

---

Configuration Best Practices

Practice	Description
Environment prefix	Use MYCTL_ prefix for all env vars
Nested config	Support cache.ttl style nested values
Config file optional	Never require a config file to exist
Respect KUBECONFIG	Always honor the standard env var
Document all options	List all config options in --help

---

Layer configuration for flexibility: flags override env vars override config files.

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