structflag

structflag is a Golang package that lets you conveniently use the standard library's flag package with structs. In other words, structflag CLI-ifies your config structs with a single line of code.

Just create a struct for your config, optionally put a `flag:"..."` and/or `usage:"..."` tag on the fields you want to populate from flags, and then call structflag.Load with a pointer to your struct.

The main benefits of this package are:

1. Putting all your config in a single struct. You can compose this struct on top of "sub-configs", and you can use these config structs in tests too.
2. A terser syntax for invoking the standard library's flag package.

Installation

To use structflag in your program, run:

go get github.com/ucarion/structflag

Basic Usage

In its simplest form, you can usually just invoke structflag.Load with an existing config to CLI-ify it.

type config struct {
	FirstName string
	LastName  string
}

func main() {
	var conf config
	structflag.Load(&conf)
	flag.Parse()

	fmt.Println(conf)
}

This is already in ./examples/minimal/main.go in this repo, so you can run it as:

$ go run ./examples/minimal/... --help
Usage of /[snip]/minimal:
  -FirstName string

  -LastName string

exit status 2

$ go run ./examples/minimal/... -FirstName john -LastName doe
{john doe}

Advanced Usage

For the common case of a relatively simple struct that you want to customize flag and usage details for, and where you already have some default values, you can do something like this:

type name struct {
	FirstName string `flag:"first" usage:"first name"`
	LastName  string `flag:"last" usage:"last name"`
}

func main() {
	config := struct {
		Name  name          `flag:"name"`
		Count int           `flag:"count" usage:"how many times to say hello"`
		Wait  time.Duration `flag:"wait" usage:"how long to wait before greeting"`
	}{
		Count: 3,
		Wait:  1 * time.Second,
	}

	structflag.Load(&config)
	flag.Parse()

	time.Sleep(config.Wait)
	for i := 0; i < config.Count; i++ {
		fmt.Println("hello", config.Name)
	}
}

This is already in ./examples/detailed/main.go in this repo, so you can run it as:

$ go run ./examples/detailed/... --help
Usage of /[snip]/detailed:
  -count int
    	how many times to say hello (default 3)
  -name-first string
    	first name
  -name-last string
    	last name
  -wait duration
    	how long to wait before greeting (default 1s)

$ go run ./examples/detailed/... --count 5 --name-first muhammad --name-last al-khwarizmi
hello {muhammad al-khwarizmi}
hello {muhammad al-khwarizmi}
hello {muhammad al-khwarizmi}
hello {muhammad al-khwarizmi}
hello {muhammad al-khwarizmi}