Read command line flags

A command line tool needs named options with defaults, and options placed after a file name are not seen.

package main

import (
	"flag"
	"fmt"
)

func main() {
	name := flag.String("name", "world", "who to greet")
	times := flag.Int("n", 1, "how many times")
	loud := flag.Bool("loud", false, "shout")
	flag.Parse()
	for i := 0; i < *times; i++ {
		msg := "hello " + *name
		if *loud {
			msg += "!"
		}
		fmt.Println(msg)
	}
	fmt.Println("args:", flag.Args())
}

go build -o greet main.go
./greet -name Ada -n 2 -loud extra
echo ---
./greet extra -n 2
echo ---
./greet -loud=false -name=Bob
echo ---
./greet -n abc 2>&1 | head -2

hello Ada!
hello Ada!
args: [extra]
---
hello world
args: [extra -n 2]
---
hello Bob
args: []
---
invalid value "abc" for flag -n: parse error
Usage of ./greet:

Each constructor registers an option and returns a pointer, which is filled by flag.Parse. The first run uses the usual forms. The second shows the rule people trip on: parsing ends at the first non-flag argument, so -n 2 after extra is left in flag.Args() and the count stays at 1. A boolean flag takes a value only in the -loud=false form, never as a separate word. Both rules come from the flag package. A bad value prints the error and the usage and exits with status 2.

The package accepts one or two dashes for any option and has no short and long aliases, no grouping like -xvf and no subcommands; for those use FlagSet per subcommand, or a library such as cobra. Put the flags before the positional arguments in documentation and in scripts that call the tool.