Example of parsing environment variables and CLI flags using the flag package

main.go

package main

import (
	"flag"
	"log"
	"os"
	"strings"
	"unicode"
)

func main() {
	// f communicates program name and default log output target (stderr in this case).
	f := flag.NewFlagSet("buildit", flag.ContinueOnError)

	// Bind flags for configuration
	id := f.Int("backend-id", 100, "Backend agent `ID`.")

	// Parse CLI flags
	if err := f.Parse(os.Args[1:]); err == flag.ErrHelp {
		os.Exit(2)
	} else if err != nil {
		log.Printf("Error parsing CLI config: %v", err)
		os.Exit(1)
	}

	// Load environment variables for unset CLI flags
	if err := LoadEnvironmentConfig(f, os.LookupEnv); err != nil {
		log.Printf("Error parsing environment config: %v", err)
		os.Exit(1)
	}

	log.Printf("ID = %v", *id) // DEBUG
}

// EnvFunc is an os.LookupEnv-compatible function type.
// An implementation looks up a value by name and return both the value (if any) and whether the
// name was known.
type EnvFunc func(name string) (value string, ok bool)

// LoadEnvironmentConfig assigns environment variables as values for unset flags defined in f.
// Flags are set in lexical order.
//
// If lookup is nil, os.LookupEnv is used instead.
//
// If any errors occur in setting a flag, the first error is returned.
func LoadEnvironmentConfig(f *flag.FlagSet, lookup EnvFunc) error {
	if lookup == nil {
		lookup = os.LookupEnv
	}

	known := map[string]struct{}{}
	f.Visit(func(v *flag.Flag) {
		known[v.Name] = struct{}{}
	})

	var err error
	prefix := f.Name() + "_"
	f.VisitAll(func(v *flag.Flag) {
		if _, ok := known[v.Name]; ok {
			return
		}

		name := envVarName(prefix + v.Name)
		val, ok := lookup(name)
		if !ok {
			return
		}

		// Take only the first error
		if ferr := f.Set(v.Name, val); err == nil && ferr != nil {
			err = ferr
		}
	})

	return err
}

// envVarName accepts a string and returns an enviornment variable name of the form NAME_FOO.
//
// The resulting name does not contain leading underscores or numbers. Repeated underscores are
// trimmed. The name may end in an underscore.
//
// Characters that are not in the range [A-Za-z0-9_] are replaced with underscores.
func envVarName(name string) string {
	var last rune = -1
	return strings.Map(func(r rune) rune {
		r = unicode.ToUpper(r)
		alpha := r >= 'A' && r <= 'Z'
		numeric := r >= '0' && r <= '9'
		if !(alpha || numeric) {
			r = '_'
		}
		if (last == -1 && !alpha) || (last == '_' && r == '_') {
			return -1
		}
		last = r
		return r
	}, name)
}