package main

import(
	. "fmt"
	"os"
	"io"
	"crypto/sha256"
)

var usage = "usage: sha256sum [file]"

func Error(s interface{}){
	Fprintln(os.Stderr,s)
	os.Exit(1)
}

func main(){
	var err error
	var bs []byte
	args := os.Args[1:]
	rd := os.Stdin
	
	switch(len(args)){
	case 0:
	case 1:
		f := args[0]
		rd,err = os.Open(f)	
		if err != nil {
			Error(err)
		}
	default:
		Error(usage)
	}
	bs,err = io.ReadAll(rd)
	if err != nil {
		Error(err)
	}
    rd.Close()
	sum := sha256.Sum256(bs)
	Printf("%x\n",sum)
}