package main

import (
	"io/ioutil"
	"fmt"
)

func main(){
	addHeader("a,b,c", "test.csv")
}

func addHeader(header string, filePath string) error {
	data, err := ioutil.ReadFile(filePath)
	ioutil.WriteFile(filePath, append([]byte(fmt.Sprintln(header)), data...), 0777)
	return err
}