<p align="center"><img width="90%" src="kong.png" /></p>

Kong is a command-line parser for Go

Version 1.0.0 Release
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()
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
- Name(help) and Description(help) - set the application name description
- Configuration(loader, 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
- ConfigureHelp(HelpOptions) and Help(HelpFunc) - customising help
- Injecting values into Run() methods
- Other options

Version 1.0.0 Release

Kong has been stable for a long time, so it seemed appropriate to cut a 1.0 release.

There is one breaking change, #436, which should effect relatively few users.

Introduction

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())
  }
}

Help

Help as a user of a Kong application

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.

Defining help in Kong

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

Showing the command's detailed 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

Showing an argument's detailed 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

Command handling

There are two ways to handle commands in Kong.

Switch on the command string

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.

Attach a `Run(...) error` method to each command

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

Break leaf commands out into separate structs.
Attach a Run(...) error method to all leaf commands.
Call kong.Kong.Parse() to obtain a kong.Context.
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)
}

Hooks: BeforeReset(), BeforeResolve(), BeforeApply(), AfterApply()

If a node in the CLI, or any of its embedded fields, implements a BeforeReset(...) error, BeforeResolve (...) error, BeforeApply(...) error and/or AfterApply(...) error method, those will be called as Kong resets, resolves, validates, and assigns values to the node.

| Hook | Description

Kong

Install / Use

README