Get input from command-line arguments, or fallback to STDIN

flags.go

package main

import (
	"bufio"
	"flag"
	"os"
	"strings"
)

func main() {
	flag.Parse()
	input := GetInput(flag.Args(), os.Stdin)
	
	// Iterate over each input value.
	for input.Scan() {
		arg := input.Text()
		
		// ...do something with arg
	}
	if err := input.Err(); err != nil {
		log.Fatalln(err)
	}
}

func GetInput(args []string, fallback io.Reader) *bufio.Scanner {
	if len(args) > 0 {
		return bufio.NewScanner(strings.NewReader(strings.Join(args, "\n")))
	}
	
	return bufio.NewScanner(fallback)
}