How to parse flags from a configuration file and override them with command-line flags in Go?

To parse flags from a configuration file and override them with command-line flags in Go, you can use the flag package along with a configuration package, such as viper. Here's a step-by-step guide to achieve this:

Step 1: Install the required packages

go get -u github.com/spf13/viper

Step 2: Create a configuration file Assume you have a configuration file named config.yaml with some initial values:

database:
  host: localhost
  port: 5432
  username: root
  password: password123

Step 3: Import the necessary packages

import (
	"flag"
	"fmt"
	"log"

	"github.com/spf13/viper"
)

Step 4: Define a struct to hold the configuration values

type Config struct {
	Database struct {
		Host     string
		Port     int
		Username string
		Password string
	}
}

Step 5: Load and parse the configuration file

func loadConfig() (*Config, error) {
	var cfg Config

	viper.SetConfigName("config")
	viper.SetConfigType("yaml")
	viper.AddConfigPath(".")
	err := viper.ReadInConfig()
	if err != nil {
		return nil, fmt.Errorf("failed to read config file: %w", err)
	}

	err = viper.Unmarshal(&cfg)
	if err != nil {
		return nil, fmt.Errorf("failed to unmarshal config file: %w", err)
	}

	return &cfg, nil
}

Step 6: Define and parse command-line flags

func parseFlags(cfg *Config) {
	flag.StringVar(&cfg.Database.Host, "db_host", cfg.Database.Host, "Database host")
	flag.IntVar(&cfg.Database.Port, "db_port", cfg.Database.Port, "Database port")
	flag.StringVar(&cfg.Database.Username, "db_user", cfg.Database.Username, "Database username")
	flag.StringVar(&cfg.Database.Password, "db_pass", cfg.Database.Password, "Database password")

	flag.Parse()
}

Step 7: Implement main function

func main() {
	cfg, err := loadConfig()
	if err != nil {
		log.Fatal(err)
	}

	parseFlags(cfg)

	// Use the configuration values

	fmt.Println("Database Host:", cfg.Database.Host)
	fmt.Println("Database Port:", cfg.Database.Port)
	fmt.Println("Database Username:", cfg.Database.Username)
	fmt.Println("Database Password:", cfg.Database.Password)
}

Now you can run your program and provide command-line flags to override the values from the configuration file:

go run main.go --db_host=localhost --db_port=5433 --db_user=admin --db_pass=admin123

The command-line flags will override the corresponding values from the configuration file when provided.