Generate random passwords

secret.go

package main

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

func main() {
	c := flag.String("custom", "", "User-provided string")
	d := flag.Bool("decimal", false, "Digits 0 - 9")
	lc := flag.Bool("lowercase", false, "ASCII lowercase characters")
	uc := flag.Bool("uppercase", false, "ASCII uppercase characters")
	p := flag.Bool("punctuation", false, "All ASCII punctuation characters")
	s := flag.Bool("special", false, "ASCII punctuation without quotes or backslashes")
	l := flag.Int("length", 10, "Length of string")
	flag.Parse()

	if *p && *s {
		fmt.Println("Error: Special and Punctuation cannot be combined.")
		flag.PrintDefaults()
		os.Exit(1)
	}
	special := "!#$%&()*+,-./:;<=>?@[]^_`{|}~"

	// default pool to use if no args
	pool := "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" + special

	// if an arg is provided build custom pool
	if !(*c == "" && !*d && !*lc && !*uc && !*p && !*s) {
		pool = ""
		pool += *c
		if *d {
			pool += "0123456789"
		}
		if *lc {
			pool += "abcdefghijklmnopqrstuvwxyz"
		}
		if *uc {
			pool += "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
		}
		if *p {
			pool += "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
		}
		if *s {
			pool += special
		}
	}

	// https://yourbasic.org/golang/generate-random-string/
	rand.Seed(time.Now().UnixNano())
	chars := []rune(pool)
	var b strings.Builder
	for i := 0; i < *l; i++ {
		b.WriteRune(chars[rand.Intn(len(chars))])
	}
	str := b.String()
	fmt.Print(str)
}

Read: https://blog.golang.org/strings

Docs: https://golang.org/pkg/strings/#Builder.String