How to calculate sha256 file checksum in Go

The SHA256 hasher implements the io.Writer interface, so one option would be to use the io.Copy() function to copy the data from an appropriate io.Reader in blocks. Something like this should do:

f, err := os.Open(os.Args[1])
if err != nil {
    log.Fatal(err)
}
defer f.Close()
if _, err := io.Copy(hasher, f); err != nil {
    log.Fatal(err)
}

Full example of md5sum:

func md5sum(filePath string) (string, error) {
    file, err := os.Open(filePath)
    if err != nil {
        return "", err
    }
    defer file.Close()

    hash := md5.New()
    if _, err := io.Copy(hash, file); err != nil {
        return "", err
    }
    return hex.EncodeToString(hash.Sum(nil)), nil
}

EncodeToString does not omits leading 0 bytes, so fmt.Println(hex.EncodeToString([]byte{0x00, 0x00, 0xA, 0xB, 0xC})) gives 00000a0b0c

The crypto/sha256 godoc actually has a snippet that shows how to do that (it's basically the same code as James):

package main

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

func main() {
  f, err := os.Open("file.txt")
  if err != nil {
    log.Fatal(err)
  }
  defer f.Close()

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

  fmt.Printf("%x", h.Sum(nil))
}