custom-flags.go

package main

import (
	"flag"
	"fmt"
)

// MyList is string slice
type MyList []string

func (ml *MyList) String() string {
	return fmt.Sprintln(*ml)
}

// Set string value in MyList
func (ml *MyList) Set(s string) error {
	*ml = append(*ml, s)
	return nil
}

func main() {
	var list MyList
	flag.Var(&list, "files", "list of files")

	flag.Parse()

	fmt.Println(list)
}