golang-cli-cobra-viper

// cmd/deploy.go
package cmd
 
import (
    "fmt"
 
    "github.com/spf13/cobra"
    "github.com/spf13/viper"
)
 
var deployCmd = &cobra.Command{
    Use:   "deploy [environment]",
    Short: "Deploy application to specified environment",
    Long: `Deploy the application to the specified environment.
Supports: dev, staging, production`,
    Args: cobra.ExactArgs(1),
    ValidArgs: []string{"dev", "staging", "production"},
    PreRunE: func(cmd *cobra.Command, args []string) error {
        // Validation logic runs before RunE
        env := args[0]
        if env == "production" && !viper.GetBool("force") {
            return fmt.Errorf("production deploys require --force flag")
        }
        return nil
    },
    RunE: func(cmd *cobra.Command, args []string) error {
        env := args[0]
        region := viper.GetString("region")
        force := viper.GetBool("force")
 
        fmt.Printf("Deploying to %s in region %s (force=%v)\n", env, region, force)
 
        // Actual deployment logic
        return deploy(env, region, force)
    },
    PostRunE: func(cmd *cobra.Command, args []string) error {
        // Cleanup or notifications
        fmt.Println("Deployment complete")
        return nil
    },
}
 
func init() {
    rootCmd.AddCommand(deployCmd)
 
    // Local flags (only for this command)
    deployCmd.Flags().StringP("region", "r", "us-east-1", "AWS region")
    deployCmd.Flags().BoolP("force", "f", false, "Force deployment without confirmation")
 
    // Bind flags to viper
    viper.BindPFlag("region", deployCmd.Flags().Lookup("region"))
    viper.BindPFlag("force", deployCmd.Flags().Lookup("force"))
}
 
func deploy(env, region string, force bool) error {
    // Implementation
    return nil
}

// Persistent flags: Available to command and all subcommands
rootCmd.PersistentFlags().String("config", "", "config file path")
rootCmd.PersistentFlags().Bool("verbose", false, "verbose output")
 
// Local flags: Only available to this specific command
deployCmd.Flags().String("region", "us-east-1", "deployment region")
deployCmd.Flags().Bool("force", false, "force deployment")
 
// Required flags
deployCmd.MarkFlagRequired("region")
 
// Flag dependencies
deployCmd.MarkFlagsRequiredTogether("username", "password")
deployCmd.MarkFlagsMutuallyExclusive("json", "yaml")

var serverCmd = &cobra.Command{
    Use:   "server",
    Short: "Start API server",
 
    // Execution order (all optional):
    PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
        // Runs before PreRunE, inherited by subcommands
        return setupLogging()
    },
    PreRunE: func(cmd *cobra.Command, args []string) error {
        // Validation and setup before RunE
        return validateConfig()
    },
    RunE: func(cmd *cobra.Command, args []string) error {
        // Main command logic
        return startServer()
    },
    PostRunE: func(cmd *cobra.Command, args []string) error {
        // Cleanup after RunE
        return cleanup()
    },
    PersistentPostRunE: func(cmd *cobra.Command, args []string) error {
        // Runs after PostRunE, inherited by subcommands
        return flushLogs()
    },
}

func initConfig() {
    // 1. Set defaults (lowest priority)
    viper.SetDefault("port", 8080)
    viper.SetDefault("database.host", "localhost")
    viper.SetDefault("database.port", 5432)
 
    // 2. Config file locations (checked in order)
    viper.SetConfigName("config")
    viper.SetConfigType("yaml")
    viper.AddConfigPath("/etc/myapp/")
    viper.AddConfigPath("$HOME/.myapp")
    viper.AddConfigPath(".")
 
    // 3. Environment variables (prefix + automatic mapping)
    viper.SetEnvPrefix("MYAPP")
    viper.AutomaticEnv() // MYAPP_PORT, MYAPP_DATABASE_HOST, etc.
 
    // 4. Read config file (optional)
    if err := viper.ReadInConfig(); err != nil {
        if _, ok := err.(viper.ConfigFileNotFoundError); ok {
            // Config file not found - use defaults and env vars
        } else {
            // Config file found but error reading it
            return err
        }
    }
 
    // 5. Flags will be bound in init() functions (highest priority)
}

viper.SetEnvPrefix("MYAPP") // Prefix for env vars
viper.AutomaticEnv()        // Enable automatic mapping
 
// Config key → Environment variable
// "port"              → MYAPP_PORT
// "database.host"     → MYAPP_DATABASE_HOST
// "database.port"     → MYAPP_DATABASE_PORT
// "aws.s3.region"     → MYAPP_AWS_S3_REGION

viper.BindEnv("database.host", "DB_HOST")          // Custom env var name
viper.BindEnv("database.password", "DB_PASSWORD")  // Different naming convention

port: 8080
log_level: info
 
database:
  host: localhost
  port: 5432
  user: postgres
  ssl_mode: require
 
aws:
  region: us-east-1
  s3:
    bucket: my-app-bucket

port := viper.GetInt("port")                    // 8080
dbHost := viper.GetString("database.host")      // "localhost"
s3Bucket := viper.GetString("aws.s3.bucket")    // "my-app-bucket"
 
// Type-safe access
if viper.IsSet("database.ssl_mode") {
    sslMode := viper.GetString("database.ssl_mode")
}
 
// Unmarshal into struct
type Config struct {
    Port     int    `mapstructure:"port"`
    LogLevel string `mapstructure:"log_level"`
    Database struct {
        Host    string `mapstructure:"host"`
        Port    int    `mapstructure:"port"`
        User    string `mapstructure:"user"`
        SSLMode string `mapstructure:"ssl_mode"`
    } `mapstructure:"database"`
}
 
var config Config
if err := viper.Unmarshal(&config); err != nil {
    return err
}

// cmd/root.go
func init() {
    cobra.OnInitialize(initConfig) // Load config before command execution
 
    // Define flags
    rootCmd.PersistentFlags().String("config", "", "config file")
    rootCmd.PersistentFlags().String("log-level", "info", "log level")
    rootCmd.PersistentFlags().Int("port", 8080, "server port")
 
    // Bind flags to Viper (critical step!)
    viper.BindPFlag("config", rootCmd.PersistentFlags().Lookup("config"))
    viper.BindPFlag("log_level", rootCmd.PersistentFlags().Lookup("log-level"))
    viper.BindPFlag("port", rootCmd.PersistentFlags().Lookup("port"))
}
 
func initConfig() {
    // This runs BEFORE command execution via cobra.OnInitialize
    if cfgFile := viper.GetString("config"); cfgFile != "" {
        viper.SetConfigFile(cfgFile)
    } else {
        viper.AddConfigPath("$HOME/.myapp")
        viper.AddConfigPath(".")
        viper.SetConfigName("config")
    }
 
    viper.SetEnvPrefix("MYAPP")
    viper.AutomaticEnv()
 
    viper.ReadInConfig() // Ignore errors - config file is optional
}

// Strategy 1: Bind each flag individually (explicit)
viper.BindPFlag("log_level", rootCmd.Flags().Lookup("log-level"))
 
// Strategy 2: Bind all flags automatically (convenient)
viper.BindPFlags(rootCmd.Flags())
 
// Strategy 3: Hybrid approach (recommended)
// - Bind persistent flags globally
// - Bind local flags in each command's init()
rootCmd.PersistentFlags().String("config", "", "config file")
viper.BindPFlags(rootCmd.PersistentFlags())
 
deployCmd.Flags().String("region", "us-east-1", "AWS region")
viper.BindPFlag("deploy.region", deployCmd.Flags().Lookup("region"))

var rootCmd = &cobra.Command{
    Use:   "myapp",
    Short: "My application",
    PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
        // Runs before ALL commands (inherited by subcommands)
 
        // 1. Validate required config
        if !viper.IsSet("api_key") {
            return fmt.Errorf("API key not configured (set MYAPP_API_KEY or add to config file)")
        }
 
        // 2. Setup logging based on config
        logLevel := viper.GetString("log_level")
        if err := setupLogging(logLevel); err != nil {
            return fmt.Errorf("invalid log level: %w", err)
        }
 
        // 3. Initialize clients/services
        apiKey := viper.GetString("api_key")
        if err := initAPIClient(apiKey); err != nil {
            return fmt.Errorf("failed to initialize API client: %w", err)
        }
 
        return nil
    },
}

// cmd/completion.go
package cmd
 
import (
    "os"
 
    "github.com/spf13/cobra"
)
 
var completionCmd = &cobra.Command{
    Use:   "completion [bash|zsh|fish|powershell]",
    Short: "Generate shell completion script",
    Long: `Generate shell completion script for myapp.
 
To load completions:
 
Bash:
  $ source <(myapp completion bash)
  # To load automatically, add to ~/.bashrc:
  $ echo 'source <(myapp completion bash)' >> ~/.bashrc
 
Zsh:
  $ source <(myapp completion zsh)
  # To load automatically, add to ~/.zshrc:
  $ echo 'source <(myapp completion zsh)' >> ~/.zshrc
 
Fish:
  $ myapp completion fish | source
  # To load automatically:
  $ myapp completion fish > ~/.config/fish/completions/myapp.fish
 
PowerShell:
  PS> myapp completion powershell | Out-String | Invoke-Expression
  # To load automatically, add to PowerShell profile.
`,
    DisableFlagsInUseLine: true,
    ValidArgs:             []string{"bash", "zsh", "fish", "powershell"},
    Args:                  cobra.ExactValidArgs(1),
    RunE: func(cmd *cobra.Command, args []string) error {
        switch args[0] {
        case "bash":
            return cmd.Root().GenBashCompletion(os.Stdout)
        case "zsh":
            return cmd.Root().GenZshCompletion(os.Stdout)
        case "fish":
            return cmd.Root().GenFishCompletion(os.Stdout, true)
        case "powershell":
            return cmd.Root().GenPowerShellCompletionWithDesc(os.Stdout)
        }
        return nil
    },
}
 
func init() {
    rootCmd.AddCommand(completionCmd)
}

var deployCmd = &cobra.Command{
    Use:   "deploy [environment]",
    Short: "Deploy to environment",
    Args:  cobra.ExactArgs(1),
    ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
        // Return available environments
        envs := []string{"dev", "staging", "production"}
        return envs, cobra.ShellCompDirectiveNoFileComp
    },
    RunE: func(cmd *cobra.Command, args []string) error {
        return deploy(args[0])
    },
}
 
// Custom flag completion
deployCmd.RegisterFlagCompletionFunc("region", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
    regions := []string{"us-east-1", "us-west-2", "eu-west-1"}
    return regions, cobra.ShellCompDirectiveNoFileComp
})

// ❌ BAD: Technical jargon
return fmt.Errorf("db connection failed: EOF")
 
// ✅ GOOD: Actionable error message
return fmt.Errorf("cannot connect to database at %s:%d\nPlease check:\n  - Database is running\n  - Credentials are correct (MYAPP_DB_PASSWORD)\n  - Network connectivity", host, port)
 
// ✅ GOOD: Suggest remediation
if !viper.IsSet("api_key") {
    return fmt.Errorf("API key not configured\nSet environment variable: export MYAPP_API_KEY=your-key\nOr add to config file: ~/.myapp.yaml")
}

import "github.com/briandowns/spinner"
 
func deploy(env string) error {
    s := spinner.New(spinner.CharSets[11], 100*time.Millisecond)
    s.Suffix = " Deploying to " + env + "..."
    s.Start()
    defer s.Stop()
 
    // Deployment logic
    if err := performDeployment(env); err != nil {
        s.Stop()
        return err
    }
 
    s.Stop()
    fmt.Println("✓ Deployment successful")
    return nil
}

import (
    "encoding/json"
    "github.com/olekukonko/tablewriter"
)
 
func displayResults(items []Item, format string) error {
    switch format {
    case "json":
        enc := json.NewEncoder(os.Stdout)
        enc.SetIndent("", "  ")
        return enc.Encode(items)
 
    case "table":
        table := tablewriter.NewWriter(os.Stdout)
        table.SetHeader([]string{"ID", "Name", "Status"})
        for _, item := range items {
            table.Append([]string{item.ID, item.Name, item.Status})
        }
        table.Render()
        return nil
 
    default:
        return fmt.Errorf("unknown format: %s (use json or table)", format)
    }
}

import (
    "log"
    "os"
)
 
var (
    // User-facing output (stdout)
    out = os.Stdout
 
    // Logging and errors (stderr)
    logger = log.New(os.Stderr, "[myapp] ", log.LstdFlags)
)
 
func RunCommand() error {
    // User output: stdout
    fmt.Fprintln(out, "Processing files...")
 
    // Debug/verbose logging: stderr
    if viper.GetBool("verbose") {
        logger.Println("Reading config from", viper.ConfigFileUsed())
    }
 
    // Errors: stderr
    if err := process(); err != nil {
        fmt.Fprintf(os.Stderr, "Error: %v\n", err)
        return err
    }
 
    // Success message: stdout
    fmt.Fprintln(out, "✓ Complete")
    return nil
}

// cmd/deploy_test.go
package cmd
 
import (
    "bytes"
    "testing"
 
    "github.com/spf13/cobra"
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/require"
)
 
func executeCommand(root *cobra.Command, args ...string) (output string, err error) {
    buf := new(bytes.Buffer)
    root.SetOut(buf)
    root.SetErr(buf)
    root.SetArgs(args)
 
    err = root.Execute()
    return buf.String(), err
}
 
func TestDeployCommand(t *testing.T) {
    tests := []struct {
        name    string
        args    []string
        wantErr bool
        wantOut string
    }{
        {
            name:    "deploy to dev",
            args:    []string{"deploy", "dev"},
            wantErr: false,
            wantOut: "Deploying to dev",
        },
        {
            name:    "deploy to production without force",
            args:    []string{"deploy", "production"},
            wantErr: true,
            wantOut: "production deploys require --force flag",
        },
        {
            name:    "deploy to production with force",
            args:    []string{"deploy", "production", "--force"},
            wantErr: false,
            wantOut: "Deploying to production",
        },
    }
 
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            output, err := executeCommand(rootCmd, tt.args...)
 
            if tt.wantErr {
                require.Error(t, err)
            } else {
                require.NoError(t, err)
            }
 
            assert.Contains(t, output, tt.wantOut)
        })
    }
}

func TestCommandWithConfig(t *testing.T) {
    // Reset viper state before each test
    viper.Reset()
 
    // Set test configuration
    viper.Set("region", "eu-west-1")
    viper.Set("api_key", "test-key-123")
 
    output, err := executeCommand(rootCmd, "deploy", "staging")
 
    require.NoError(t, err)
    assert.Contains(t, output, "eu-west-1")
}

func TestOutputFormat(t *testing.T) {
    // Capture stdout
    oldStdout := os.Stdout
    r, w, _ := os.Pipe()
    os.Stdout = w
    defer func() { os.Stdout = oldStdout }()
 
    // Execute command
    err := listCmd.RunE(listCmd, []string{})
    require.NoError(t, err)
 
    // Read output
    w.Close()
    var buf bytes.Buffer
    io.Copy(&buf, r)
    output := buf.String()
 
    assert.Contains(t, output, "ID")
    assert.Contains(t, output, "Name")
}

var deployCmd = &cobra.Command{
    Use: "deploy",
    Run: func(cmd *cobra.Command, args []string) {
        deploy(args[0]) // Ignores error!
    },
}

var deployCmd = &cobra.Command{
    Use: "deploy",
    RunE: func(cmd *cobra.Command, args []string) error {
        return deploy(args[0]) // Proper error handling
    },
}

// Confusing: Which takes precedence?
config.Port = viper.GetInt("port")
if os.Getenv("PORT") != "" {
    config.Port = atoi(os.Getenv("PORT"))
}
if *flagPort != 0 {
    config.Port = *flagPort
}

// Clear: Viper handles precedence automatically
viper.BindPFlag("port", rootCmd.Flags().Lookup("port"))
viper.SetEnvPrefix("MYAPP")
viper.AutomaticEnv()
viper.SetDefault("port", 8080)
 
config.Port = viper.GetInt("port") // Respects: flag > env > config > default

rootCmd.Flags().String("region", "us-east-1", "AWS region")
// Flag not bound to Viper - won't respect precedence!
 
func deploy() {
    region := viper.GetString("region") // Always returns config file value
}

rootCmd.Flags().String("region", "us-east-1", "AWS region")
viper.BindPFlag("region", rootCmd.Flags().Lookup("region")) // Bind it!
 
func deploy() {
    region := viper.GetString("region") // Respects flag > env > config
}

// No tests for CLI commands - bugs slip through

func TestDeployCommand(t *testing.T) {
    output, err := executeCommand(rootCmd, "deploy", "staging", "--region", "eu-west-1")
    require.NoError(t, err)
    assert.Contains(t, output, "Deploying to staging")
    assert.Contains(t, output, "eu-west-1")
}

return fmt.Errorf("connection failed") // Unhelpful

return fmt.Errorf("cannot connect to database at %s:%d\nCheck:\n  - Database is running\n  - Credentials (MYAPP_DB_PASSWORD)\n  - Firewall rules", host, port)

var rootCmd = &cobra.Command{
    Use: "myapp",
    Run: func(cmd *cobra.Command, args []string) {
        if err := execute(); err != nil {
            fmt.Println(err) // Error not propagated
        }
    },
}

var rootCmd = &cobra.Command{
    Use: "myapp",
    RunE: func(cmd *cobra.Command, args []string) error {
        return execute() // Cobra handles error display and exit code
    },
}

myapp/
├── cmd/
│   ├── root.go          # Root command + config loading
│   ├── deploy.go        # Deploy subcommand
│   ├── status.go        # Status subcommand
│   └── completion.go    # Shell completion
├── main.go              # Entry point
├── config.yaml          # Example config file
└── go.mod

package main
 
import "myapp/cmd"
 
func main() {
    cmd.Execute()
}

# Development
go run main.go deploy staging --region us-west-2
 
# Production build
go build -o myapp
 
# Install globally
go install
 
# Enable shell completion
myapp completion bash > /etc/bash_completion.d/myapp