Combine config with flags

config.go

/*
This small POC shows a possible way to combine a config file with command line flags such that the command line
flags override the settings from the config file.
*/
package main

import (
	"flag"
	"log"
	"os"

	yaml "gopkg.in/yaml.v1"
)

type config struct {
	Foo        string `yaml: "foo"`
	Bar        string `yaml: "bar"`
	Baz        string `yaml: "baz"`
	configFile string
}

var defaults = config{
	Foo: "default foo",
	Bar: "default bar",
	Baz: "default baz",
}

const fakeConfig = `
foo: configured foo
bar: configured bar
`

func main() {
	// fake args:
	os.Args = []string{"cmd", "-config-file", "config.yaml", "-foo", "foo from flags"}

	// copy defaults:
	c := defaults

	// init and parse flags:
	flag.StringVar(&c.Foo, "foo", defaults.Foo, "just foo")
	flag.StringVar(&c.Bar, "bar", defaults.Bar, "just bar")
	flag.StringVar(&c.Baz, "baz", defaults.Baz, "just baz")
	flag.StringVar(&c.configFile, "config-file", "", "optional base configuration from file")
	flag.Parse()

	// record only those flags that were provided:
	// see: https://godoc.org/flag#Visit
	// assumption: Value.String() should return a string that can be used to set the same value
	providedFlags := make(map[string]string)
	flag.Visit(func(f *flag.Flag) {
		providedFlags[f.Name] = f.Value.String()
	})

	// assume yaml file read:
	y := []byte(fakeConfig)
	if err := yaml.Unmarshal(y, &c); err != nil {
		log.Fatalf("failed to parse config file: %v", err)
	}

	// apply the flags that were provided again:
	for name, value := range providedFlags {
		f := flag.Lookup(name)
		if f == nil {
			log.Fatalf("flag lookup failed: %s", name)
		}

		if err := f.Value.Set(value); err != nil {
			log.Fatalln(err)
		}
	}

	// expecting: config{
	// 	Foo: "foo from flags",
	// 	Bar: "configured bar",
	// 	Baz: "default baz",
	// 	configFile: "config.yaml",
	// }
	log.Println(c)
}