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Command-line options parsing library, POSIX/GNU compliant, supports struct tags as well as the Go's flag approach.
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flaq is a command-line options parsing library for Go.

The package follows POSIX / GNU conventions, it notably supports:

  • short and long options
  • options with required, optional, or no argument
  • parsing options intermixed with args
  • defining options in a struct
  • customizable help usage
  • option abbreviations


As an example, we will specify a --name option with a -n shorthand, accepting a string argument. We will also define a --yell bool option.

There are mainly 2 approaches when using flaq.

Using a struct

The first approach is to define options through a struct:

package main

import (


type Options struct {
	Name string `flaq:"-n, --name string  name of the person to greet"`
	Yell bool   `flaq:"    --yell         greet the person loudly"`

var opts = Options{Name: "world"}

func main() {

	if opts.Yell {
		opts.Name = strings.ToUpper(opts.Name)
	fmt.Printf("Hello %s!\n", opts.Name)

Using a struct creates a self-documented piece of code. It also plays well with other projects such as caarlos0/env, if you want for example to read values from environment variables as well.

See the struct fields section for more information about this.

Using a variable per option

The second approach relies on a similar API than the flag package:

package main

import (


var name = "world"
var yell = false

func main() {
	flaq.String(&name, "name", "n", "name of the person to greet")
	flaq.Bool(&yell, "yell", "", "whether to yell or not", false)

	if yell {
		name = strings.ToUpper(name)
	fmt.Printf("Hello %s!\n", name)

Help usage

By default, help usage is printed whenever a --help or -h option is seen:

$ greet --help
Usage: greet [options]

  -h, --help           show usage help
  -n, --name <string>  name of the person to greet
      --yell           greet loudly

This behavior can be removed or customized.

Struct fields

Struct fields tags are expected to follow the -s, --long type description pattern, where:

  • -s is the option shortand.
  • --long is the option long form.
  • type is the type of argument the option accepts, if any.
  • description is the option description.

In order to ease alignment for readability, there can be an arbitrary number of whitespaces between the different sections.

Here is a reference of the supported field types:

type Options struct {
	String   string        `flaq:"-n, --name string         a string eg. --string=world"`
	Switch   bool          `flaq:"    --switch              --switch will set the value to true"`
	Bool     bool          `flaq:"    --bool bool           --bool, --bool=true or --bool=false"`
	Int      int           `flaq:"    --int int             an int value eg. --int=100"`
	Float64  float64       `flaq:"    --float64 float64     a float value eg. --float64=3.14159"`
	Count    int           `flaq:"-c, --count count         -ccc will set this count value to 3"`
	Duration time.Duration `flaq:"    --duration duration   a duration eg. --duration=5min"`
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