1// Copyright 2023 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package flag_test
6
7import (
8	"flag"
9	"fmt"
10	"time"
11)
12
13func ExampleFlagSet() {
14
15	start := func(args []string) {
16		// A real program (not an example) would use flag.ExitOnError.
17		fs := flag.NewFlagSet("start", flag.ContinueOnError)
18		addr := fs.String("addr", ":8080", "`address` to listen on")
19		if err := fs.Parse(args); err != nil {
20			fmt.Printf("error: %s", err)
21			return
22		}
23		fmt.Printf("starting server on %s\n", *addr)
24	}
25
26	stop := func(args []string) {
27		fs := flag.NewFlagSet("stop", flag.ContinueOnError)
28		timeout := fs.Duration("timeout", time.Second, "stop timeout duration")
29		if err := fs.Parse(args); err != nil {
30			fmt.Printf("error: %s", err)
31			return
32		}
33		fmt.Printf("stopping server (timeout=%v)\n", *timeout)
34	}
35
36	main := func(args []string) {
37		subArgs := args[2:] // Drop program name and command.
38		switch args[1] {
39		case "start":
40			start(subArgs)
41		case "stop":
42			stop(subArgs)
43		default:
44			fmt.Printf("error: unknown command - %q\n", args[1])
45			// In a real program (not an example) print to os.Stderr and exit the program with non-zero value.
46		}
47	}
48
49	main([]string{"httpd", "start", "-addr", ":9999"})
50	main([]string{"httpd", "stop"})
51	main([]string{"http", "start", "-log-level", "verbose"})
52
53	// Output:
54	// starting server on :9999
55	// stopping server (timeout=1s)
56	// error: flag provided but not defined: -log-level
57}
58