flag — 命令行参数解析

var (
    name    = flag.String("name", "default", "your name")
    age     = flag.Int("age", 0, "your age")
    verbose = flag.Bool("verbose", false, "verbose output")
)

var (
    name    string
    age     int
    verbose bool
)

func init() {
    flag.StringVar(&name, "name", "default", "your name")
    flag.IntVar(&age, "age", 0, "your age")
    flag.BoolVar(&verbose, "verbose", false, "verbose output")
}

func main() {
    flag.Parse()  // ← 必须调用！

    fmt.Println("name:", *name)   // 返回指针的方式需要解引用
    fmt.Println("age:", age)      // 绑定到变量的方式直接使用
}

flag.Parse()

// 返回非标志参数
args := flag.Args()             // []string
fmt.Println("args:", args)

n := flag.NArg()                // 非标志参数个数
fmt.Println("count:", n)

m := flag.NFlag()               // 已设置的标志个数
fmt.Println("flags set:", m)

$ ./app -name alice file1.txt file2.txt
# flag.Args() = ["file1.txt", "file2.txt"]

func main() {
    if len(os.Args) < 2 {
        fmt.Println("expected 'serve' or 'migrate' subcommand")
        os.Exit(1)
    }

    switch os.Args[1] {
    case "serve":
        serveCmd(os.Args[2:])
    case "migrate":
        migrateCmd(os.Args[2:])
    default:
        fmt.Printf("unknown subcommand: %s\n", os.Args[1])
        os.Exit(1)
    }
}

func serveCmd(args []string) {
    fs := flag.NewFlagSet("serve", flag.ExitOnError)
    port := fs.Int("port", 8080, "server port")
    fs.Parse(args)
    fmt.Println("serving on port", *port)
}

flag.Usage = func() {
    fmt.Fprintf(os.Stderr, "Usage: %s [options] <args>\n", os.Args[0])
    fmt.Fprintf(os.Stderr, "Options:\n")
    flag.PrintDefaults()
}