Command-line tools with flag and os

// go run . hello 42
fmt.Println(os.Args)      // [/tmp/go-build.../exe/app hello 42]
fmt.Println(os.Args[1:])  // [hello 42]  (os.Args[0] is the program itself)

package main

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

func main() {
	n := flag.Int("n", 10, "number of lines to show")
	verbose := flag.Bool("v", false, "verbose output")
	var city string
	flag.StringVar(&city, "city", "Pune", "city name")
	flag.Parse()

	files := flag.Args() // what is left after the flags
	if len(files) == 0 {
		fmt.Fprintln(os.Stderr, "usage: report [-n N] [-v] [-city NAME] FILE...")
		os.Exit(2)
	}
	fmt.Println(*n, *verbose, city, files)
}

report -n 5 -city Mumbai sales.csv
report --n=5 -v sales.csv      # -flag and --flag both work
report -h                      # prints the generated help text
report sales.csv -n 5          # careful: parsing stops at sales.csv, so -n 5 is treated as a file name

func main() {
	if err := run(os.Args[1:], os.Stdout); err != nil {
		fmt.Fprintln(os.Stderr, "error:", err)
		os.Exit(1)
	}
}

func run(args []string, out io.Writer) error {
	fs := flag.NewFlagSet("gst", flag.ContinueOnError)
	rate := fs.Int("rate", 18, "GST rate in percent")
	if err := fs.Parse(args); err != nil {
		return err
	}
	for _, a := range fs.Args() {
		paise, err := strconv.ParseInt(a, 10, 64)
		if err != nil {
			return fmt.Errorf("bad amount %q: %w", a, err)
		}
		total := paise + paise*int64(*rate)/100
		fmt.Fprintf(out, "%d -> %d\n", paise, total)
	}
	return nil
}

switch os.Args[1] {
case "add":
	fs := flag.NewFlagSet("add", flag.ExitOnError)
	amount := fs.Int64("paise", 0, "amount in paise")
	fs.Parse(os.Args[2:])
	fmt.Println("adding", *amount)
case "report":
	// ...
default:
	fmt.Fprintf(os.Stderr, "unknown command %q\n", os.Args[1])
	os.Exit(2)
}