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Flæg is a Go library for building dynamically a powerful modern Command Line Interface and loading a program configuration structure from arguments. Go developers don't need to worry about keeping flags and commands updated anymore: it works by itself!


You know how boring it is to keep your CLI up-to-date. You will be glad to use Flaeg ;-) This package uses your own configuration structure to build your CLI.

You only need to describe every StructField with a StructTag, flaeg will automatically build the CLI, parse data from args, and load Go values into Configuration structure via reflection!

We developed flaeg and staert in order to simplify configuration maintenance on traefik.


  • Load your Configuration structure with program args
  • Keep your Configuration structure values unchanged if no flags called (support defaults values)
  • Many Type of StructField can be flagged :
    • type bool
    • type int (int32, int64, uint, uint64)
    • type string
    • type float (float64)
    • type time.Time
  • Many Kind of StructField in the Configuration structure are supported :
    • Sub-Structure
    • Anonymous field (on Sub-Structure)
    • Pointers on anything
  • You can add your "Parsers" on your own type like :
    • Arrays, Slices or Maps
    • Your structures
  • Pointers flags are Boolean :
    • You can give a structure of default values for those pointers
    • Pointer fields will get default values if their flag is called
  • Flags names are fields names by default, but you can overwrite it in StructTag
  • "Shorthand" flags (1 character) can be added in StructTag as well
  • Flaeg is POSIX compliant using pflag package
  • You only need to provide the root-Command which contains the function to run
  • You can add Sub-Commands to the root-Command

Getting Started


To install Flaeg, simply run:

$ go get

Configuration Structures

Flaeg works on any kind of structure, you only need to add a StructTag "description" on the fields to flag. Like this:

// Configuration is a struct which contains all differents type to field
// using parsers on string, pointer, bool, int, int64, time.Time, float64
type Configuration struct {
	Name     string        // no description struct tag, it will not be flaged
	LogLevel string        `short:"l" description:"Log level"`      // string type field, short flag "-l"
	Timeout  parse.Duration `description:"Timeout duration"`         // parse.Duration type field
	Db       *DatabaseInfo `description:"Enable database"`          // pointer type field (on DatabaseInfo)
	Owner    *OwnerInfo    `description:"Enable Owner description"` // another pointer type field (on OwnerInfo)

You can add sub-structures even if they are anonymous:

type ServerInfo struct {
	Watch  bool   `description:"Watch device"`      // bool type
	IP     string `description:"Server ip address"` // string type field
	Load   int    `description:"Server load"`       // int type field
	Load64 int64  `description:"Server load"`       // int64 type field, same description just to be sure it works

type DatabaseInfo struct {
	ServerInfo             // anonymous sub-structures
	ConnectionMax   uint   `long:"comax" description:"Number max of connections on database"` // uint type field, long flag "--comax"
	ConnectionMax64 uint64 `description:"Number max of connections on database"`              // uint64 type field, same description just to be sure it works

type OwnerInfo struct {
	Name        *string      `description:"Owner name"`                     // pointer type field on string
	DateOfBirth time.Time    `long:"dob" description:"Owner date of birth"` // time.Time type field, long flag "--dob"
	Rate        float64      `description:"Owner rate"`                     // float64 type field
	Servers     []ServerInfo `description:"Owner Server"`                   // slice of ServerInfo type field, need a custom parser


Flaeg is POSIX compliant using pflag package. Flaeg concats the names of fields to generate the flags. They are not case sensitive.

For example, the field ConnectionMax64 in OwnerInfo sub-Structure which is in Configuration Structure will be --db.connectionmax64. But you can overwrite it with the StructTag long as like as the field ConnectionMax which is flagged --db.comax.

Finally, you can add a short flag (1 character) using the StructTag short, like in the field LogLevel with the short flags -l in addition to the flag--loglevel.

Default values

Default values on fields come from the configuration structure. If it was not initialized, Golang default values are used.

For pointers, the DefaultPointers structure provides default values.


The Command structure contains program/command information (command name and description). Config must be a pointer on the configuration struct to parse (it contains default values of field). DefaultPointersConfig contains default pointers values: those values are set on pointers fields if their flags are called.

It must be the same type (struct) as Config. Run is the func which launch the program using initialized configuration structure.

type Command struct {
	Name                  string
	Description           string
	Config                interface{}
	DefaultPointersConfig interface{}
	Run                   func() error		
	Metadata              map[string]string

So, you can create Commands like this:

rootCmd := &Command{
	Name: "flaegtest",
	Description: `flaegtest is a test program made to to test flaeg library.
	Complete documentation is available at`,
	Config:                config,
	DefaultPointersConfig: defaultPointers,
	Run: func() error {
			fmt.Printf("Run flaegtest command with config : %+v\n", config)
			return nil

You have to create at least the root-Command, and you can add some sub-Command.

Metadata allows you to store some labels(Key-value) in the command and to use it elsewhere. We needed that in Stært.


The responsive help is auto-generated using the description StructTag, default value from configuration structure and/or Command structure. Flag --help and short flag -h are bound to call the helper. If the args parser fails, it will print the error and the helper will be call as well.

Here an example:

$./flaegtest --help
flaegtest is a test program made to test flaeg library.
Complete documentation is available at

Usage: flaegtest [--flag=flag_argument] [-f[flag_argument]] ...     set flag_argument to flag(s)
   or: flaegtest [--flag[=true|false| ]] [-f[true|false| ]] ...     set true/false to boolean flag(s)

Available Commands:
	version                                            Print version
Use "flaegtest [command] --help" for more information about a command.

    --db                 Enable database                       (default "false")
    --db.comax           Number max of connections on database (default "3200000000")
    --db.connectionmax64 Number max of connections on database (default "6400000000000000000")
    --db.ip              Server ip address                     (default "")
    --db.load            Server load                           (default "32")
    --db.load64          Server load                           (default "64")           Watch device                          (default "true")
-l, --loglevel           Log level                             (default "DEBUG")
    --owner              Enable Owner description              (default "true")
    --owner.dob          Owner date of birth                   (default "1993-09-12 07:32:00 +0000 UTC")         Owner name                            (default "true")
    --owner.rate         Owner rate                            (default "0.999")
    --owner.servers      Owner Server                          (default "[]")
    --timeout            Timeout duration                      (default "1s")
-h, --help               Print Help (this message) and exit

Run Flaeg

Let's run fleag now:

  • rootCmd is the root-Command
  • versionCmd is a sub-Command
	// init flaeg
	flaeg := flaeg.New(rootCmd, os.Args[1:])
	// add sub-command Version

	// run test
	if err := flaeg.Run(); err != nil {
		t.Errorf("Error %s", err.Error())

Duration Parser

There is a built in duration parser to assist with the parsing of durations. Values such as "1s", "3m", "3h2m1s" are converted into a string indicating the number of seconds, you can then convert this string to a time.Duration if needed, as shown in the example below.

A parse.Duration can be defined like this:

import (

type Configuration struct {
	Timeout  parse.Duration `description:"Timeout duration"`  // parse.Duration type field

And then converted to a time.Duration:

duration := time.Duration(configuration.Timeout)

Custom Parsers

The function flaeg.AddParser adds a custom parser for a specified type.

func (f *Flaeg) AddParser(typ reflect.Type, parser Parser)

It can be used like this:

// add custom parser to fleag
flaeg.AddParser(reflect.TypeOf([]ServerInfo{}), &sliceServerValue{})

sliceServerValue{} need to implement flaeg.Parser:

type Parser interface {

like this:

type sliceServerValue []ServerInfo

func (c *sliceServerValue) Set(s string) error {
	// could use RegExp
	srv := ServerInfo{IP: s}
	*c = append(*c, srv)
	return nil

func (c *sliceServerValue) Get() interface{} { return []ServerInfo(*c) }

func (c *sliceServerValue) String() string { return fmt.Sprintf("%v", *c) }

func (c *sliceServerValue) SetValue(val interface{}) {
	*c = sliceServerValue(val.([]ServerInfo))


  1. Fork it!
  2. Create your feature branch: git checkout -b my-new-feature
  3. Commit your changes: git commit -am 'Add some feature'
  4. Push to the branch: git push origin my-new-feature
  5. Submit a pull request :D

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