Golang: Command Line Arguments and Flags

 Command line arguments based on index.

//repeatgreetings 108
//repeatgreetings 108 | wc -l
package main

import (
	"fmt"
	"log"
	"os"
	"strconv"
)

func main() {
	var repeatCount int
	var err error
	if len(os.Args) >= 2 {
		repeatCount, err = strconv.Atoi(os.Args[1])
		if err != nil {
			log.Fatal(err)
		}
		for i := 0; i < repeatCount; i++ {
			fmt.Println("Hello Gopher!")
		}
	}
}

Command line flags which uses keynames instead of index

//personalgreetings -help
//personalgreetings
//personalgreetings -gophername Uday
package main

import (
	"flag"
	"fmt"
)

func main() {
	var gopherName string
	//Arguments: variable,flag,default value,description
	flag.StringVar(&gopherName, "gophername", "Gopher", "The name of the Gopher")
	flag.Parse()
	fmt.Println("Hello " + gopherName + "!")
}

Case study: username checker

//usernamechecker -username ^Java_Duke^
//usernamechecker -username @Uday
package main

import (
	"flag"
	"fmt"
    "log"
    "regexp"
)

const UsernameRegex string = `^@?(\w){1,15}$` //backticks instead of double/single quotes.

func main(){
    var usernameInput string
    flag.StringVar(&usernameInput,"username","Gopher","The GopherFace Username to Check")
    flag.Parse()

    fmt.Println("GopherFace Username Validation Checker")
    fmt.Println("Checking Syntax for username,\"",usernameInput,"\",resulted in: ",CheckUsernameSyntax(usernameInput),"\n")
}

func CheckUsernameSyntax(username string)bool{
    validtionResult := false
    r,err:=regexp.Compile(UsernameRegex)
    if err!=nil{
        log.Fatal(err)
    }
    validtionResult=r.MatchString(username)
    return validtionResult
}