Add BOM

main.go

package main

import (
	"bufio"
	"bytes"
	"errors"
	"io"
	"log"
	"os"
)

var bom = []byte(string([]rune{'\uFEFF'}))

func parseArgs(args []string) (string, error) {
	if len(args) != 2 {
		return "", errors.New("Specify a file path")
	}
	return args[1], nil
}

func write(in io.Reader, out io.Writer) error {
	bufIn := bufio.NewReader(in)
	bufOut := bufio.NewWriter(out)

	prefix, err := bufIn.Peek(3)
	if err != nil {
		return err
	}

	if !bytes.Equal(prefix, bom) {
		if _, err := bufOut.Write(bom); err != nil {
			return err
		}
	}

	if _, err := io.Copy(bufOut, bufIn); err != nil {
		return err
	}

	return bufOut.Flush()
}

func main() {
	path, err := parseArgs(os.Args)
	if err != nil {
		log.Fatal(err)
	}

	file, err := os.Open(path)
	if err != nil {
		log.Fatal(err)
	}
	defer file.Close()

	if err := write(file, os.Stdout); err != nil {
		log.Fatal(err)
	}
}