CLI: echo "checksum *filename" | sha256sum -c -, but in Golang

main.go

package main

import (
	"io"
	"os"
	"log"
	"fmt"
	"flag"
	"crypto/sha256"
	"encoding/hex"
)

var hash, file string

func init () {
	flag.StringVar(&hash, "hash", "", "sha256 sum")
	flag.StringVar(&file, "file", "", "targer file")
}

func main() {
	flag.Parse()
	if len(hash) == 0 || len(file) == 0 {
		flag.PrintDefaults()
		os.Exit(2)
	}

	h, err := checksum(file)
	if err != nil {
		log.Fatal(err)
		os.Exit(1)
	}

	if hash != h {
		fmt.Println("error: checksum verification failed")
		os.Exit(1)
	}

	fmt.Println("OK!")
}

func checksum(filename string) (string, error) {
	f, err := os.Open(filename)
	if err != nil {
		return "", err
	}
	defer f.Close()

	h := sha256.New()
	if _, err := io.Copy(h, f); err != nil {
		return "", err
	}

	hash := h.Sum(nil)
	return hex.EncodeToString(hash), nil
}