md5sum in Golang

main.go

package main

import (
	"crypto/md5"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
	"regexp"
	"strings"
)

func main() {
	if len(os.Args) < 2 {
		help()
	} else if os.Args[1] == "-h" || os.Args[1] == "--help" {
		help()
	} else if os.Args[1] == "-c" || os.Args[1] == "--check" {
		if len(os.Args) == 2 {
			help()
		}
		checkHashesInFile(os.Args[2])
	} else {
		determineHashForFile(os.Args[1:])
	}
}

func checkHashesInFile(argFile string) {
	md5bytes, err := ioutil.ReadFile(argFile)
	if err != nil {
		error(fmt.Sprintf("failed to read file %s: %v", argFile, err))
	}

	splitBySpace := regexp.MustCompile(`\s+`)
	failedFiles := 0
	totalFiles := 0

	for _, line := range strings.Split(strings.TrimSuffix(string(md5bytes), "\n"), "\n") {
		columns := splitBySpace.Split(string(line), -1)
		if len(columns) != 2 {
			error(fmt.Sprintf("could not parse check file: each line should have two columns 'md5_hash file_name' but it is: '%s'", line))
		}

		argPath := filepath.Dir(argFile)
		fullPath := ""
		if len(argPath) > 0 {
			fullPath = argPath + string(filepath.Separator)
		}
		fullPath += columns[1]
		md5sum := md5hash(fullPath)

		if md5sum == columns[0] {
			fmt.Printf("%s: OK\n", columns[1])
		} else {
			fmt.Printf("%s: FAILED\n", columns[1])
			failedFiles += 1
		}
		totalFiles += 1
	}

	if failedFiles > 0 {
		fmt.Printf("md5sum: WARNING: %d of %d computed checksums did NOT match\n", failedFiles, totalFiles)
		os.Exit(1)
	}
}

func determineHashForFile(files []string) {
	for _, arg := range files {
		md5sum := md5hash(arg)
		fmt.Printf("%s %s\n", md5sum, filepath.Base(arg))
	}
}

func md5hash(file string) string {
	f, err := os.Open(file)
	if err != nil {
		error(fmt.Sprintf("failed to open file for read: %v", err))
	}
	defer f.Close()

	h := md5.New()
	if _, err := io.Copy(h, f); err != nil {
		error(fmt.Sprintf("failed to process file content with MD5 hashing function: %v", err))
	}

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

func help() {
	fmt.Println("Usage: md5sum [<option>] <file> [<file> [...] ]")
	fmt.Println("       md5sum [<option>] --check <file>")
	fmt.Println()
	fmt.Println(" -c, --check <file>   Check MD5 sums from <file>")
	fmt.Println(" -h, --help           Display this help message and exit")
	fmt.Println()
}

func error(errMsg string) {
	fmt.Println("Error:", errMsg)
	os.Exit(1)
}