• Introduction
• Help
  • Help as a user of a Kong application
  • Defining help in Kong
• Command handling
  • Switch on the command string
  • Attach a Run... error method to each command
• Hooks: BeforeReset, BeforeResolve, BeforeApply, AfterApply and the Bind option
• Flags
• Commands and sub-commands
• Branching positional arguments
• Positional arguments
• Slices
• Maps
• Pointers
• Nested data structure
• Custom named decoders
• Supported field types
• Custom decoders mappers
• Supported tags
• Plugins
• Dynamic Commands
• Variable interpolation
• Validation
• Modifying Kong's behaviour
  • Namehelp and Descriptionhelp - set the application name description
  • Configurationloader, paths... - load defaults from configuration files
  • Resolver... - support for default values from external sources
  • *Mapper... - customising how the command-line is mapped to Go values
  • ConfigureHelpHelpOptions and HelpHelpFunc - customising help
  • Bind... - bind values for callback hooks and Run methods
  • Other options

Kong aims to support arbitrarily complex command-line structures with as little developer effort as possible.

To achieve that, command-lines are expressed as Go types, with the structure and tags directing how the command line is mapped onto the struct.

For example, the following command-line:

shell rm [-f] [-r] <paths> ...
shell ls [<paths> ...]

Can be represented by the following command-line structure:

package main

import "github.com/alecthomas/kong"

var CLI struct {
  Rm struct {
    Force     bool `help:"Force removal."`
    Recursive bool `help:"Recursively remove files."`

    Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
  } `cmd:"" help:"Remove files."`

  Ls struct {
    Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
  } `cmd:"" help:"List paths."`
}

func main() {
  ctx := kong.Parse(&CLI)
  switch ctx.Command() {
  case "rm <path>":
  case "ls":
  default:
    panic(ctx.Command())
  }
}

Every Kong application includes a --help flag that will display auto-generated help.

eg.

$ shell --help
usage: shell <command>

A shell-like example app.

Flags:
  --help   Show context-sensitive help.
  --debug  Debug mode.

Commands:
  rm <path> ...
    Remove files.

  ls [<path> ...]
    List paths.

If a command is provided, the help will show full detail on the command including all available flags.

eg.

$ shell --help rm
usage: shell rm <paths> ...

Remove files.

Arguments:
  <paths> ...  Paths to remove.

Flags:
      --debug        Debug mode.

  -f, --force        Force removal.
  -r, --recursive    Recursively remove files.

Help is automatically generated from the command-line structure itself, including help:"" and other tags. Variables will also be interpolated into the help string.

Finally, any command, or argument type implementing the interface Help() string will have this function called to retrieve more detail to augment the help tag. This allows for much more descriptive text than can fit in Go tags. See _examples/shell/help

A command's additional help text is not shown from top-level help, but can be displayed within contextual help:

Top level help

 $ go run ./_examples/shell/help --help
Usage: help <command>

An app demonstrating HelpProviders

Flags:
  -h, --help    Show context-sensitive help.
      --flag    Regular flag help

Commands:
  echo    Regular command help

Contextual

 $ go run ./_examples/shell/help echo --help
Usage: help echo <msg>

Regular command help

🚀 additional command help

Arguments:
  <msg>    Regular argument help

Flags:
  -h, --help    Show context-sensitive help.
      --flag    Regular flag help

Custom help will only be shown for positional arguments with named fields (see the README section on positional arguments for more details on what that means)

Contextual argument help

 $ go run ./_examples/shell/help msg --help
Usage: help echo <msg>

Regular argument help

📣 additional argument help

Flags:
  -h, --help    Show context-sensitive help.
      --flag    Regular flag help

There are two ways to handle commands in Kong.

When you call kong.Parse() it will return a unique string representation of the command. Each command branch in the hierarchy will be a bare word and each branching argument or required positional argument will be the name surrounded by angle brackets. Here's an example:

There's an example of this pattern here.

eg.

package main

import "github.com/alecthomas/kong"

var CLI struct {
  Rm struct {
    Force     bool `help:"Force removal."`
    Recursive bool `help:"Recursively remove files."`

    Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
  } `cmd:"" help:"Remove files."`

  Ls struct {
    Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
  } `cmd:"" help:"List paths."`
}

func main() {
  ctx := kong.Parse(&CLI)
  switch ctx.Command() {
  case "rm <path>":
  case "ls":
  default:
    panic(ctx.Command())
  }
}

This has the advantage that it is convenient, but the downside that if you modify your CLI structure, the strings may change. This can be fragile.

A more robust approach is to break each command out into their own structs:

1. Break leaf commands out into separate structs.
2. Attach a Run(...) error method to all leaf commands.
3. Call kong.Kong.Parse() to obtain a kong.Context.
4. Call kong.Context.Run(bindings...) to call the selected parsed command.

Once a command node is selected by Kong it will search from that node back to the root. Each encountered command node with a Run(...) error will be called in reverse order. This allows sub-trees to be re-used fairly conveniently.

In addition to values bound with the kong.Bind(...) option, any values passed through to kong.Context.Run(...) are also bindable to the target's Run() arguments.

Finally, hooks can also contribute bindings via kong.Context.Bind() and kong.Context.BindTo().

There's a full example emulating part of the Docker CLI here.

eg.

type Context struct {
  Debug bool
}

type RmCmd struct {
  Force     bool `help:"Force removal."`
  Recursive bool `help:"Recursively remove files."`

  Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
}

func (r *RmCmd) Run(ctx *Context) error {
  fmt.Println("rm", r.Paths)
  return nil
}

type LsCmd struct {
  Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
}

func (l *LsCmd) Run(ctx *Context) error {
  fmt.Println("ls", l.Paths)
  return nil
}

var cli struct {
  Debug bool `help:"Enable debug mode."`

  Rm RmCmd `cmd:"" help:"Remove files."`
  Ls LsCmd `cmd:"" help:"List paths."`
}

func main() {
  ctx := kong.Parse(&cli)
  // Call the Run() method of the selected parsed command.
  err := ctx.Run(&Context{Debug: cli.Debug})
  ctx.FatalIfErrorf(err)
}

If a node in the grammar has a BeforeReset(...), BeforeResolve (...), BeforeApply(...) error and/or AfterApply(...) error method, those methods will be called before values are reset, before validation/assignment, and after validation/assignment, respectively.

The --help flag is implemented with a BeforeReset hook.

Arguments to hooks are provided via the Run(...) method or Bind(...) option. *Kong, *Context and *Path are also bound and finally, hooks can also contribute bindings via kong.Context.Bind() and kong.Context.BindTo().

eg.

// A flag with a hook that, if triggered, will set the debug loggers output to stdout.
type debugFlag bool

func (d debugFlag) BeforeApply(logger *log.Logger) error {
  logger.SetOutput(os.Stdout)
  return nil
}

var cli struct {
  Debug debugFlag `help:"Enable debug logging."`
}

func main() {
  // Debug logger going to discard.
  logger := log.New(io.Discard, "", log.LstdFlags)

  ctx := kong.Parse(&cli, kong.Bind(logger))

  // ...
}

Another example of using hooks is load the env-file:

package main

import (
  "fmt"
  "github.com/alecthomas/kong"
  "github.com/joho/godotenv"
)

type EnvFlag string

// BeforeResolve loads env file.
func (c EnvFlag) BeforeReset(ctx *kong.Context, trace *kong.Path) error {
  path := string(ctx.FlagValue(trace.Flag).(EnvFlag)) // nolint
  path = kong.ExpandPath(path)
  if err := godotenv.Load(path); err != nil {
    return err
  }
  return nil
}

var CLI struct {
  EnvFile EnvFlag
  Flag `env:"FLAG"`
}

func main() {
  _ = kong.Parse(&CLI)
  fmt.Println(CLI.Flag)
}

Any mapped field in the command structure not tagged with cmd or arg will be a flag. Flags are optional by default.

eg. The command-line app [--flag="foo"] can be represented by the following.

type CLI struct {
  Flag string
}

Sub-commands are specified by tagging a struct field with cmd. Kong supports arbitrarily nested commands.

eg. The following struct represents the CLI structure command [--flag="str"] sub-command.

type CLI struct {
  Command struct {
    Flag string

    SubCommand struct {
    } `cmd`
  } `cmd`
}

If a sub-command is tagged with default:"1" it will be selected if there are no further arguments. If a sub-command is tagged with default:"withargs" it will be selected even if there are further arguments or flags and those arguments or flags are valid for the sub-command. This allows the user to omit the sub-command name on the CLI if its arguments/flags are not ambiguous with the sibling commands or flags.

In addition to sub-commands, structs can also be configured as branching positional arguments.

This is achieved by tagging an unmapped nested struct field with arg, then including a positional argument field inside that struct with the same name. For example, the following command structure:

app rename <name> to <name>

Can be represented with the following:

var CLI struct {
  Rename struct {
    Name struct {
      Name string `arg` // <-- NOTE: identical name to enclosing struct field.
      To struct {
        Name struct {
          Name string `arg`
        } `arg`
      } `cmd`
    } `arg`
  } `cmd`
}

This looks a little verbose in this contrived example, but typically this will not be the case.

If a field is tagged with arg:"" it will be treated as the final positional value to be parsed on the command line. By default positional arguments are required, but specifying optional:"" will alter this.

If a positional argument is a slice, all remaining arguments will be appended to that slice.

Slice values are treated specially. First the input is split on the sep:"<rune>" tag (defaults to ,), then each element is parsed by the slice element type and appended to the slice. If the same value is encountered multiple times, elements continue to be appended.

To represent the following command-line:

cmd ls <file> <file> ...

You would use the following:

var CLI struct {
  Ls struct {
    Files []string `arg:"" type:"existingfile"`
  } `cmd`
}

Maps are similar to slices except that only one key/value pair can be assigned per value, and the sep tag denotes the assignment character and defaults to =.

To represent the following command-line:

cmd config set <key>=<value> <key>=<value> ...

You would use the following:

var CLI struct {
  Config struct {
    Set struct {
      Config map[string]float64 `arg:"" type:"file:"`
    } `cmd`
  } `cmd`
}

For flags, multiple key+value pairs should be separated by mapsep:"rune" tag (defaults to ;) eg. --set="key1=value1;key2=value2".

Pointers work like the underlying type, except that you can differentiate between the presence of the zero value and no value being supplied.

For example:

var CLI struct {
	Foo *int
}

Would produce a nil value for Foo if no --foo argument is supplied, but would have a pointer to the value 0 if the argument --foo=0 was supplied.

Kong support a nested data structure as well with embed:"". You can combine embed:"" with prefix:"":

var CLI struct {
  Logging struct {
    Level string `enum:"debug,info,warn,error" default:"info"`
    Type string `enum:"json,console" default:"console"`
  } `embed:"" prefix:"logging."`
}

This configures Kong to accept flags --logging.level and --logging.type.

Kong includes a number of builtin custom type mappers. These can be used by specifying the tag type:"<type>". They are registered with the option function NamedMapper(name, mapper).

Slices and maps treat type tags specially. For slices, the type:"" tag specifies the element type. For maps, the tag has the format tag:"[<key>]:[<value>]" where either may be omitted.

Any field implementing encoding.TextUnmarshaler or json.Unmarshaler will use those interfaces for decoding values. Kong also includes builtin support for many common Go types:

For more fine-grained control, if a field implements the MapperValue interface it will be used to decode arguments into the field.

Tags can be in two forms:

1. Standard Go syntax, eg. kong:"required,name='foo'".
2. Bare tags, eg. required:"" name:"foo"

Both can coexist with standard Tag parsing.

Kong CLI's can be extended by embedding the kong.Plugin type and populating it with pointers to Kong annotated structs. For example:

var pluginOne struct {
  PluginOneFlag string
}
var pluginTwo struct {
  PluginTwoFlag string
}
var cli struct {
  BaseFlag string
  kong.Plugins
}
cli.Plugins = kong.Plugins{&pluginOne, &pluginTwo}

Additionally if an interface type is embedded, it can also be populated with a Kong annotated struct.

While plugins give complete control over extending command-line interfaces, Kong also supports dynamically adding commands via kong.DynamicCommand().

Kong supports limited variable interpolation into help strings, enum lists and default values.

Variables are in the form:

${<name>}
${<name>=<default>}

Variables are set with the Vars{"key": "value", ...} option. Undefined variable references in the grammar without a default will result in an error at construction time.

Variables can also be set via the set:"K=V" tag. In this case, those variables will be available for that node and all children. This is useful for composition by allowing the same struct to be reused.

When interpolating into flag or argument help strings, some extra variables are defined from the value itself:

${default}
${enum}

For flags with associated environment variables, the variable ${env} can be interpolated into the help string. In the absence of this variable in the help string, Kong will append ($$${env}) to the help string.

eg.

type cli struct {
  Config string `type:"path" default:"${config_file}"`
}

func main() {
  kong.Parse(&cli,
    kong.Vars{
      "config_file": "~/.app.conf",
    })
}

Kong does validation on the structure of a command-line, but also supports extensible validation. Any node in the tree may implement the following interface:

type Validatable interface {
    Validate() error
 }

If one of these nodes is in the active command-line it will be called during normal validation.

Each Kong parser can be configured via functional options passed to New(cli interface{}, options...Option).

The full set of options can be found here.

Set the application name and/or description.

The name of the application will default to the binary name, but can be overridden with Name(name).

As with all help in Kong, text will be wrapped to the terminal.

This option provides Kong with support for loading defaults from a set of configuration files. Each file is opened, if possible, and the loader called to create a resolver for that file.

eg.

kong.Parse(&cli, kong.Configuration(kong.JSON, "/etc/myapp.json", "~/.myapp.json"))

See the tests for an example of how the JSON file is structured.

• YAML
• HCL
• TOML
• JSON

Resolvers are Kong's extension point for providing default values from external sources. As an example, support for environment variables via the env tag is provided by a resolver. There's also a builtin resolver for JSON configuration files.

Example resolvers can be found in resolver.go.

Command-line arguments are mapped to Go values via the Mapper interface:

// A Mapper represents how a field is mapped from command-line values to Go.
//
// Mappers can be associated with concrete fields via pointer, reflect.Type, reflect.Kind, or via a "type" tag.
//
// Additionally, if a type implements the MapperValue interface, it will be used.
type Mapper interface {
	// Decode ctx.Value with ctx.Scanner into target.
	Decode(ctx *DecodeContext, target reflect.Value) error
}

All builtin Go types (as well as a bunch of useful stdlib types like time.Time) have mappers registered by default. Mappers for custom types can be added using kong.??Mapper(...) options. Mappers are applied to fields in four ways:

1. NamedMapper(string, Mapper) and using the tag key type:"<name>".
2. KindMapper(reflect.Kind, Mapper).
3. TypeMapper(reflect.Type, Mapper).
4. ValueMapper(interface{}, Mapper), passing in a pointer to a field of the grammar.

The default help output is usually sufficient, but if not there are two solutions.

1. Use ConfigureHelp(HelpOptions) to configure how help is formatted (see HelpOptions for details).
2. Custom help can be wired into Kong via the Help(HelpFunc) option. The HelpFunc is passed a Context, which contains the parsed context for the current command-line. See the implementation of PrintHelp for an example.
3. Use ValueFormatter(HelpValueFormatter) if you want to just customize the help text that is accompanied by flags and arguments.
4. Use Groups([]Group) if you want to customize group titles or add a header.

See the section on hooks for details.

The full set of options can be found here.