Code:

topFlags := flag.NewFlagSet("", flag.ExitOnError)
cmdListFlags := flag.NewFlagSet("list", flag.ExitOnError)
cmdSendFlags := flag.NewFlagSet("send", flag.ExitOnError)

var cmdDefault struct {
    count int
    data  string
}

var cmdList struct {
    age  int
    name string
}

var cmdSend struct {
    dst     string
    payload string
}

topFlags.IntVar(&cmdDefault.count, "cnt", 0, "-cnt <count>")
topFlags.StringVar(&cmdDefault.data, "data", "", "-data <data string>")

cmdListFlags.IntVar(&cmdList.age, "age", -1, "-age <age>")
cmdListFlags.StringVar(&cmdList.name, "name", "", "-name <name>")

cmdSendFlags.StringVar(&cmdSend.dst, "dst", "", "-dst <destination>")
cmdSendFlags.StringVar(&cmdSend.payload, "p", "", "-p <payload>")

if cmd, err := Parse(topFlags,
    cmdListFlags,
    cmdSendFlags); err != nil {
    // show/handle error
} else {
    _ = cmd // the matched *flag.FlagSet
}

// use values of back fields for flags

Code:

topFlags := flag.NewFlagSet("", flag.ExitOnError)

var cmdDefault struct {
    count int
    data  string
}

topFlags.IntVar(&cmdDefault.count, "cnt", 0, "-cnt <count>")
topFlags.StringVar(&cmdDefault.data, "data", "", "-data <data string>")

if cmd, err := Parse(topFlags); err != nil {
    // show/handle error
} else {
    fromenv := func(set *flag.FlagSet, flagName, envName string) error {
        f := set.Lookup(flagName)
        if f == nil {
            return nil
        }
        if f.Value.String() != f.DefValue {
            return nil
        }
        e := os.Getenv(envName)
        if e == "" {
            return nil
        }
        if e == f.DefValue {
            return nil
        }
        return f.Value.Set(e)
    }
    topFlags.VisitAll(func(f *flag.Flag) {
        // sample env var name convention: APPNAME_FLAGNAME
        if err := fromenv(topFlags,
            f.Name,
            strings.ToUpper(filepath.Base(os.Args[0])+"_"+f.Name)); err != nil {
            log.Fatal(err)
        }
    })
    _ = cmd // the matched *flag.FlagSet
}

// use values of back fields for flags

Code:

topFlags := flag.NewFlagSet("", flag.ExitOnError)

var cmdDefault struct {
    count int
    data  string
}

topFlags.IntVar(&cmdDefault.count, "cnt", 0, "-cnt <count>")
topFlags.StringVar(&cmdDefault.data, "data", "", "-data <data string>")

if cmd, err := Parse(topFlags); err != nil {
    // show/handle error
} else {
    nonFlags := topFlags.Args()
    _ = cmd // the matched *flag.FlagSet
    _ = nonFlags
}

// use values of back fields for flags