Can I add commands with Go flags?

I know I can define flags like this:

initPtr := flag.String("init", "config.json", "Initialize with config filename")

Which I will call like this:

myapp --init=myconfig.json

I wonder though if I can define something like commands so that to call the app like this?

myapp init --filename=myconfig.json
myapp check --filename=myconfig.json
myapp run

Yes, you can.

Take a look at https://golang.org/pkg/flag/ for FlagSet documentation.

And for a working example: https://github.com/constabulary/gb/blob/master/cmd/gb/main.go

And an example as requested by miltonb:

% go run flagset.go init --filename=foo.json foo bar
init foo.json [foo bar]
% go run flagset.go check --filename=bar.json 1 2 3
check bar.json [1 2 3]
% go run flagset.go run
run
% cat flagset.go
package main

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

func main() {
	init := flag.NewFlagSet("init", flag.ExitOnError)
	initFile := init.String("filename", "myconfig.json", "configuration file")

	check := flag.NewFlagSet("check", flag.ExitOnError)
	checkFile := check.String("filename", "myconfig.json", "configuration file")

	if len(os.Args) <= 1 {
		flag.Usage()
		os.Exit(1)
	}

	switch os.Args[1] {
	case "init":
		if err := init.Parse(os.Args[2:]); err == nil {
			fmt.Println("init", *initFile, init.Args())
		}
	case "check":
		if err := check.Parse(os.Args[2:]); err == nil {
			fmt.Println("check", *checkFile, check.Args())
		}
	case "run":
		fmt.Println("run")
	}
}

You can do that using a combination of command line arguments and flags.

Eg.

configFilePtr := flag.String("filename", "", "Config Filename")
flag.Parse()

n := len(os.Args)
if n > 1 {
    switch os.Args[1] {
        case "init":
            initApp(*configFilePtr)
        case "check":
            checkApp(*configFilePtr)
        case "run":
            runApp()
    }
}

Another option is using something like spf13's cobra.

Update :

If you require the use of different flags as per command you can use a FlagSet as mentioned in Kare Nuorteva's answer.

Eg.

f1 := flag.NewFlagSet("f1", flag.ContinueOnError)
silent := f1.Bool("silent", false, "")
f2 := flag.NewFlagSet("f2", flag.ContinueOnError)
loud := f2.Bool("loud", false, "")

switch os.Args[1] {
  case "apply":
    if err := f1.Parse(os.Args[2:]); err == nil {
      fmt.Println("apply", *silent)
    }
  case "reset":
    if err := f2.Parse(os.Args[2:]); err == nil {
      fmt.Println("reset", *loud)
    }
}

Reference