[root@byol-host ~]# cat main.go
package main

import (
 "fmt"
 "os"
 "flag"
 "strings"
 "strconv"
 "encoding/csv"
 "encoding/json"
 "io"
)

type User struct {
 Id     int     `json:"id"`
 Name   string  `json:"name"`
 Home   string  `json:"home"`
 Shell  string  `json:"shell"`
}

func main() {
 path, format := parseFlags()
 users := collectUsers()

 var output io.Writer

 if path != "" {
  f, err := os.Create(path)
  handleError(err)
  defer f.Close()

  output = f
 } else {
  output = os.Stdout
 }

 if format == "json" {
  data, err := json.MarshalIndent(users, "", " ")
  handleError(err)
  output.Write(data)
 } else if format == "csv" {
   output.Write([]byte("name,id,home,shell\n"))
   writer := csv.NewWriter(output)
   for _, user := range users {
    err := writer.Write([]string{user.Name, strconv.Itoa(user.Id), user.Home, user.Shell})
    handleError(err)
   }
   writer.Flush()
 }
}

func handleError(err error) {
 if err != nil {
  fmt.Println("Error: ", err)
  os.Exit(1)
 }
}

func collectUsers() (users []User) {
 f, err := os.Open("/etc/passwd")

 handleError(err)

 defer f.Close()

 reader := csv.NewReader(f)
 reader.Comma = ':'

 lines, err := reader.ReadAll()
 handleError(err)

 for _, line := range lines {
  id, err := strconv.ParseInt(line[2], 10, 64)
  handleError(err)
  if id < 1000 {
   continue
  }

  user := User{
   Name:  line[0],
   Id:    int(id),
   Home:  line[5],
   Shell: line[6],
  }

  users = append(users, user)
 }
 return
}

func parseFlags() (path, format string) {
 flag.StringVar(&path, "path", "", "the path to the export file.")
 flag.StringVar(&format, "format", "json", "output format file")

 flag.Parse()

 format = strings.ToLower(format)

 if format != "csv" && format != "json" {
  fmt.Println("Error. Invalid format")
  flag.Usage()
  os.Exit(1)

 }
 return
}