subcommand.go

package main

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

var usage = `Usage argtest [Globals] <Command> [Options]
Commands:
  put Put something
  get Get something

Globals Options:
`

var subcommandOptionsUsageFormat = "\nOptions for subcommand \"%s\":\n"

func subcommandUsageFunc(subcommand string, fs *flag.FlagSet) func() {
	return func() {
		flag.Usage()
		fmt.Printf(subcommandOptionsUsageFormat, subcommand)
		fs.PrintDefaults()
	}
}

func main() {
	var (
		opt1 = flag.String("opt1", "default-value", "First string option")
		opt2 = flag.String("opt2", "default-value", "Second string option")
	)

	flag.Usage = func() {
		fmt.Print(usage)
		flag.PrintDefaults()
	}
	flag.Parse()
	args := flag.Args()
	if len(args) == 0 {
		flag.Usage()
		os.Exit(1)
	}
	fmt.Println("opt1:", *opt1)
	fmt.Println("opt2:", *opt2)
	switch args[0] {
	case "put":
		PutCommand(args[1:])
	case "get":
		GetCommand(args[1:])
	default:
		flag.Usage()
		os.Exit(1)
	}
}

func PutCommand(args []string) {
	fs := flag.NewFlagSet("put", flag.ExitOnError)
	putOpt := fs.String("put-opt", "default-value", "Put option")
	fs.Usage = subcommandUsageFunc("put", fs)
	fs.Parse(args)
	if len(args) == 0 {
		fs.Usage()
		os.Exit(1)
	}

	fmt.Println("args:", fs.Args())
	fmt.Println("putopt:", *putOpt)
}
func GetCommand(args []string) {
	fs := flag.NewFlagSet("get", flag.ExitOnError)
	getOpt := fs.String("get-opt", "default-value", "Get option")
	fs.Usage = subcommandUsageFunc("get", fs)
	fs.Parse(args)
	if len(args) == 0 {
		fs.Usage()
		os.Exit(1)
	}

	fmt.Println("args:", fs.Args())
	fmt.Println("getopt:", *getOpt)
}

Here are example usage outputs:

$ go build -o flag-example
$ ./flag-example -h
Usage argtest [Globals] <Command> [Options]
Commands:
  put Put something
  get Get something

Globals Options:
  -opt1 string
        First string option (default "default-value")
  -opt2 string
        Second string option (default "default-value")

$ ./flag-example put -h
opt1: default-value
opt2: default-value
Usage argtest [Globals] <Command> [Options]
Commands:
  put Put something
  get Get something

Globals Options:
  -opt1 string
        First string option (default "default-value")
  -opt2 string
        Second string option (default "default-value")

Options for subcommand "put":
  -put-opt string
        Put option (default "default-value")

$ ./flag-example get -h
opt1: default-value
opt2: default-value
Usage argtest [Globals] <Command> [Options]
Commands:
  put Put something
  get Get something

Globals Options:
  -opt1 string
        First string option (default "default-value")
  -opt2 string
        Second string option (default "default-value")

Options for subcommand "get":
  -get-opt string
        Get option (default "default-value")