Golang : Print how to use flag for your application example

Just a short tutorial on how to usage flag package to check for the number of flag arguments require by your application. If the flag is misspelled or has missing parameters, print out the application usage instruction.

Here you go!

 package main

 import (
 "flag"
 "fmt"
 "os"
 )

 func usage() {
 fmt.Printf("usage : %s -inputValue=123\n", os.Args[0])
 os.Exit(0)
 }

 var input = flag.String("inputValue", "", "String value to display")

 func main() {
 flag.Usage = usage
 flag.Parse()

 //fmt.Println("Args : ", flag.NFlag())

 if flag.NFlag() == 0 {
 usage()
 os.Exit(-1)
 }

 fmt.Println("String value to display is : ", *input)
 }

Sample output:

$ ./usageflags -inputValue=xzyy 123

String value to display is : xzyy

$ ./usageflags -inputValue=" asjjdd 123 adsqd"

String value to display is : asjjdd 123 adsqd

$ ./usageflags -inputValue=2121121 -fff

flag provided but not defined: -fff

$ ./usageflags

usage : ./usageflags -inputValue=123

If you prefer not to use the flag package, os package can do the same as well, but os package can only handle direct parameter/arguments. Here is a code fragment on how to use os package to display your application usage instruction.

 if len(os.Args) <= 2 {
 fmt.Printf("USAGE : %s <target_directory> <target_filename or part of filename> \n", os.Args[0])
 os.Exit(0)
  }

  targetDirectory := os.Args[1] // get the target directory
  fileName := os.Args[2:] // to handle wildcard such as filename*.go

Happy coding!

Reference:

https://www.socketloop.com/references/golang-flag-nflag-function-example