External Configuration

Use ParseWithDefaults to load configuration from external sources.

Variables are evaluated in the following order (from highest precedence to lowest):

1. command line flags
2. flag defaults from external sources (such as JSON, YAML, etc.) set via the map supplied to the ParseWithDefaults function
3. environment variables
4. defaults set via the WithDefaultValue function or via struct tag annotations

Example

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
    "github.com/napalu/goopt"
)

func getDefaults() (map[string]string, error) {
	extension := filepath.Ext(os.Args[0])
	configFile := strings.TrimRight(os.Args[0], extension) + ".config.json"

	if _, err := os.Stat(configFile); err == nil {
		jsonFile, err := os.Open(configFile)
		if err != nil {
			return nil, err
		}
		defer jsonFile.Close()

		jsonBytes, _ := io.ReadAll(jsonFile)
		var result map[string]interface{}
		err = json.Unmarshal(jsonBytes, &result)
		if err != nil {
			return nil, err
		}

		defaults := make(map[string]string, len(result))
		for k, v := range result {
			defaults[k] = fmt.Sprint(v)
		}

		return defaults, nil
	}

	return nil, fmt.Errorf("no config file found")
}

func main() {
    parser := goopt.NewParser()

    var success bool
    if defaults, err := getDefaults(); err == nil {
        // whatever command line flags are not set will be set to the defaults if present
        success = parser.ParseWithDefaults(defaults, os.Args)
    } else {
        success = parser.Parse(os.Args)
    }

    if !success {
        fmt.Println("Failed to parse flags")
    }
}