command-line utility for SHA1 hashing - modified from Guillermo Estrada

mdsha.go

/*
  Hash - Guillermo Estrada
         (modified to demonstrate Go idioms by Kyle Lemons)

  Simple utility to obtain the MD5 and/or SHA-1
  of a file from the command line.

  2011
*/

package main

// Typically, imports are sorted in alphabetical order
import (
	"crypto/md5"
	"crypto/sha1"
	"flag"
	"fmt"
	"hash"
	"io"
	"os"
)

// Flags can be defined in multiple packages and don't need to be defined in your main()
var (
	useMD5  = flag.Bool("md5", false, "-md5 calculate md5 hash of file")
	useSHA1 = flag.Bool("sha1", false, "-sha1 calculate sha1 hash of file")
)

func main() {
	flag.Parse()

	// Blocks of var/const work just as well inside a function call
	var (
		writers []io.Writer
		hashes  []hash.Hash
		names   []string
	)

	// Closures can be used to collect common operations into a nice, clean function call
	push := func(name string, h hash.Hash) {
		writers = append(writers, h) // a Hash is a writer, so this is easy
		hashes = append(hashes, h)
		names = append(names, name)
	}

	// Now any number of hashes can be added here with very little code
	if *useMD5 {
		push("MD5", md5.New())
	}
	if *useSHA1 {
		push("SHA1", sha1.New())
	}

	if len(names) == 0 {
		flag.Usage()
		os.Exit(1)
	}

	in, err := os.Open(flag.Arg(0))
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	// The variadic expansion of a slice is really convenient.
	io.Copy(io.MultiWriter(writers...), in)

	for i, name := range names {
		fmt.Printf("%4s: %x\n", name, hashes[i].Sum())
	}
}