import (
 "bufio"
   "fmt"
   "io"
   "io/ioutil"
   "os"
   "strconv"
)


var num = 100

// MAIN
func main() {

 err := insertStringToFile("test.txt", strconv.Itoa(num))
 if err != nil {
    fmt.Println(err)
    return
 }
}

func insertStringToFile(path, str string) error {
 lines, err := File2Lines(path)
 if err != nil {
 return err
 }

 fileContent := ""
 for _, line := range lines {

 fileContent += str
      fileContent += line
      fileContent += "\n"
 }
 return ioutil.WriteFile(path, []byte(fileContent), 0644)
}

func File2Lines(path string) ([]string, error) {
 f, err := os.Open(path)
 if err != nil {
 return nil, err
 }
 // Keep the file open for writing until LinesFromReader is done, then close the channel.
 defer f.Close()
 return LinesFromReader(f)
}

func LinesFromReader(r io.Reader) ([]string, error) {
 var lines []string
 scanner := bufio.NewScanner(r)
 for scanner.Scan() {
    num++ // Increment line?
    fmt.Println(num)
    lines = append(lines, scanner.Text())
 }
 if err := scanner.Err(); err != nil {
 return nil, err
 }
 return lines, nil
}