This library provides similar functionality to the builtin flag library of go, but provides much more functionality and nicer formatting. From the documentation:

Package flags provides an extensive command line option parser. The flags package is similar in functionality to the go builtin flag package but provides more options and uses reflection to provide a convenient and succinct way of specifying command line options.

Supported features:

• Options with short names (-v)
• Options with long names (--verbose)
• Options with and without arguments (bool v.s. other type)
• Options with optional arguments and default values
• Multiple option groups each containing a set of options
• Generate and print well-formatted help message
• Passing remaining command line arguments after -- (optional)
• Ignoring unknown command line options (optional)
• Supports -I/usr/include -I=/usr/include -I /usr/include option argument specification
• Supports multiple short options -aux
• Supports all primitive go types (string, int{8..64}, uint{8..64}, float)
• Supports same option multiple times (can store in slice or last option counts)
• Supports maps
• Supports function callbacks

The flags package uses structs, reflection and struct field tags to allow users to specify command line options. This results in very simple and consise specification of your application options. For example:

type Options struct {
    Verbose []bool `short:"v" long:"verbose" description:"Show verbose debug information"`
}

This specifies one option with a short name -v and a long name --verbose. When either -v or --verbose is found on the command line, a 'true' value will be appended to the Verbose field. e.g. when specifying -vvv, the resulting value of Verbose will be {[true, true, true]}.

Slice options work exactly the same as primitive type options, except that whenever the option is encountered, a value is appended to the slice.

Map options from string to primitive type are also supported. On the command line, you specify the value for such an option as key:value. For example

type Options struct {
    AuthorInfo string[string] `short:"a"`
}

Then, the AuthorInfo map can be filled with something like -a name:Jesse -a "surname:van den Kieboom".

Available field tags:

• short: the short name of the option (single character)
• long: the long name of the option
• description: the description of the option (optional)
• optional: whether an argument of the option is optional (optional)
• default: the default argument value if the option occurs without an argument (optional)
• base: a base used to convert strings to integer values (optional)
• value-name: the name of the argument value (to be shown in the help, optional)

Either short: or long: must be specified to make the field eligible as an option.

var opts struct {
	// Slice of bool will append 'true' each time the option
	// is encountered (can be set multiple times, like -vvv)
	Verbose []bool `short:"v" long:"verbose" description:"Show verbose debug information"`

	// Example of automatic marshalling to desired type (uint)
	Offset uint `long:"offset" description:"Offset"`

	// Example of a callback, called each time the option is found.
	Call func(string) `short:"c" description:"Call phone number"`
}

// Callback which will invoke callto:<argument> to call a number.
// Note that this works just on OS X (and probably only with
// Skype) but it shows the idea.
opts.Call = func(num string) {
	cmd := exec.Command("open", "callto:" + num)
	cmd.Start()
	cmd.Process.Release()
}

// Parse flags
args, err := flags.Parse(&opts)

if err != nil {
	os.Exit(1)
}

fmt.Printf("Verbosity: %d\n", len(opts.Verbose))
fmt.Printf("Offset: %d\n", opts.Offset)
fmt.Printf("Remaining args: %s\n", strings.Join(args, " "))

// Output: Verbosity: 0
// Offset: 0
// Remaining args:

More information can be found in the godocs: http://go.pkgdoc.org/github.com/jessevdk/go-flags