How to print usage for positional argument with Go's flag package?

Question

Given this simple Go program, which requires exactly one command line argument, how can I improve it so that flag.Usage() gives useful output?

package main

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

func main() {
    flag.Parse()
    if len(flag.Args()) == 0 { 
        flag.Usage()
        os.Exit(1)
    }

    args := flag.Args()
    fmt.Println(args[0])
}

Current output with no arguments given:

$ ./args
Usage of ./args:

(i.e. usage is empty, as I can find no way to tell the usage() function which parameters are required).

I can remove flag.Usage() and replace it with something like this:

fmt.Fprintln(os.Stderr, "Usage:", os.Args[0], "<argument>")

However, I don't want to reinvent the wheel if there's already a good way with flag.Usage(). Especially as it's already handling optional arguments:

$ ./args -foo
flag provided but not defined: -foo
Usage of ./args:

Answer 1

flag.Usage() will only give you useful information about the defined flags. So you either define your arguments as flags via var foo = flag.Int(...).

Another option would be to define your own usage handler. see below for a simple example which will print a custom message and the defaults for all defined flags. This way you custom Usage will be printed in case flag.Parse() fails.

package main
import (
    "flag"
    "fmt"
    "os"
)

func myUsage() {
     fmt.Printf("Usage: %s [OPTIONS] argument ...\n", os.Args[0])
     flag.PrintDefaults()
}

func main() {
     flag.Usage = myUsage
     /* ... */
}