go的flag解析 生成密码

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package main

import (
	"flag"
	"fmt"
	"math/rand"
	"time"
)

var (
	length  int
	charset string
)

const (
	NumStr  = "0123456789"
	CharStr = "ABCDEFGHIKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
	SpecStr = "!?$%^&*()_-+={[}]:;@~#|<,>./"
)

func parseArgs() {
	flag.IntVar(&length, "l", 16, "-l 用户生成密码的长度")
	flag.StringVar(&charset, "t", "num",
		`-t 制定生成密码的字符集,
		num: 只使用数字[0-9],
		char:只使用英文字母[a-zA-A], 
		mix 使用数字和字母, 
		advance:使用数字,字母以及特殊字符`)
	flag.Parse()
}

func generatePasswd() string {
	var passwd []byte = make([]byte, length, length)
	var sourceStr string
	if charset == "num" {
		sourceStr = NumStr
	} else if charset == "char" {
		sourceStr = CharStr
	} else if charset == "mix" {
		sourceStr = fmt.Sprintf("%s%s", NumStr, CharStr)
	} else if charset == "advance" {
		sourceStr = fmt.Sprintf("%s%s%s", NumStr, CharStr, SpecStr)
	} else {
		sourceStr = NumStr
	}
	fmt.Println("source:", sourceStr)
	for i := 0; i < length; i++ {
		index := rand.Intn(len(sourceStr))
		passwd[i] = sourceStr[index]
	}
	return string(passwd)
}

func main() {
	rand.Seed(time.Now().UnixNano())
	parseArgs()
	fmt.Printf("length:%d charset:%s\n", length, charset)
	passwd := generatePasswd()
	fmt.Println("passwd>>",passwd)
}

输出:

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API server listening at: 127.0.0.1:8345
length:16 charset:num
source: 0123456789
passwd>> 3441011573221112