Alec Thomas
442 lines
14 KiB
Markdown
442 lines
14 KiB
Markdown
<!-- markdownlint-disable MD013 MD033 -->
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<p align="center"><img width="90%" src="kong.png" /></p>
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# Kong is a command-line parser for Go [](https://circleci.com/gh/alecthomas/kong)
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<!-- MarkdownTOC -->
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1. [Introduction](#introduction)
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1. [Help](#help)
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1. [Command handling](#command-handling)
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1. [Switch on the command string](#switch-on-the-command-string)
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1. [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
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1. [Flags](#flags)
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1. [Commands and sub-commands](#commands-and-sub-commands)
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1. [Branching positional arguments](#branching-positional-arguments)
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1. [Terminating positional arguments](#terminating-positional-arguments)
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1. [Slices](#slices)
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1. [Maps](#maps)
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1. [Custom named types](#custom-named-types)
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1. [Supported tags](#supported-tags)
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1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
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1. [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
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1. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
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1. [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
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1. [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
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1. [`HelpOptions(HelpPrinterOptions)` and `Help(HelpFunc)` - customising help](#helpoptionshelpprinteroptions-and-helphelpfunc---customising-help)
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1. [`Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed](#hookfield-hookfunc---callback-hooks-to-execute-when-the-command-line-is-parsed)
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1. [Other options](#other-options)
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<!-- /MarkdownTOC -->
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## Introduction
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Kong aims to support arbitrarily complex command-line structures with as little developer effort as possible.
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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.
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For example, the following command-line:
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shell rm [-f] [-r] <paths> ...
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shell ls [<paths> ...]
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Can be represented by the following command-line structure:
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```go
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package main
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import "github.com/alecthomas/kong"
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var CLI struct {
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Rm struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg name:"path" help:"Paths to remove." type:"path"`
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} `cmd help:"Remove files."`
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Ls struct {
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Paths []string `arg optional name:"path" help:"Paths to list." type:"path"`
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} `cmd help:"List paths."`
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}
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func main() {
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cmd := kong.Parse(&CLI)
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switch cmd {
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case "rm <path>":
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case "ls":
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default:
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panic(cmd)
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}
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}
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```
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## Help
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Help is automatically generated. With no other arguments provided, help will display a full summary of all available commands.
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eg.
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$ shell --help
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usage: shell <command>
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A shell-like example app.
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Flags:
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--help Show context-sensitive help.
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--debug Debug mode.
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Commands:
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rm <path> ...
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Remove files.
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ls [<path> ...]
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List paths.
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If a command is provided, the help will show full detail on the command including all available flags.
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eg.
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$ shell --help rm
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usage: shell rm <paths> ...
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Remove files.
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Arguments:
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<paths> ... Paths to remove.
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Flags:
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--debug Debug mode.
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-f, --force Force removal.
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-r, --recursive Recursively remove files.
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## Command handling
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There are two ways to handle commands in Kong.
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### Switch on the command string
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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:
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There's an example of this pattern [here](https://github.com/alecthomas/kong/blob/master/_examples/shell/main.go).
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eg.
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```go
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package main
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import "github.com/alecthomas/kong"
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var CLI struct {
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Rm struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg name:"path" help:"Paths to remove." type:"path"`
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} `cmd help:"Remove files."`
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Ls struct {
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Paths []string `arg optional name:"path" help:"Paths to list." type:"path"`
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} `cmd help:"List paths."`
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}
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func main() {
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cmd := kong.Parse(&CLI)
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switch cmd {
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case "rm <path>":
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case "ls":
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default:
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panic(cmd)
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}
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}
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```
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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.
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### Attach a `Run(...) error` method to each command
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A more robust approach is to break each command out into their own structs:
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1. Break leaf commands out into separate structs.
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2. Attach a `Run(...) error` method to all leaf commands (`Run()` signatures must match).
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3. Call `kong.Kong.Parse()` to obtain a `kong.Context`.
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4. Call `kong.Context.Run(params...)` to call the selected parsed command.
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There's a full example emulating part of the Docker CLI [here](https://github.com/alecthomas/kong/tree/master/_examples/docker).
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eg.
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```go
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type RmCmd struct {
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Force bool `help:"Force removal."`
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Recursive bool `help:"Recursively remove files."`
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Paths []string `arg name:"path" help:"Paths to remove." type:"path"`
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}
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func (r *RmCmd) Run(debug bool) error {
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fmt.Println("rm", r.Paths)
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return nil
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}
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type LsCmd struct {
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Paths []string `arg optional name:"path" help:"Paths to list." type:"path"`
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}
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func (l *LsCmd) Run(debug bool) error {
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fmt.Println("ls", l.Paths)
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return nil
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}
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var cli struct {
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Debug bool `help:"Enable debug mode."`
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Rm RmCmd `cmd help:"Remove files."`
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Ls LsCmd `cmd help:"List paths."`
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}
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func main() {
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parser := kong.Must(&cli)
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// Parse and apply the command-line.
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ctx, err := parser.Parse(os.Args[1:])
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parser.FatalIfErrorf(err)
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// Call the Run() method of the selected parsed command.
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err = ctx.Run(cli.Debug)
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parser.FatalIfErrorf(err)
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}
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```
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## Flags
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Any [mapped](#mapper---customising-how-the-command-line-is-mapped-to-go-values) field in the command structure *not* tagged with `cmd` or `arg` will be a flag. Flags are optional by default.
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eg. The command-line `app [--flag="foo"]` can be represented by the following.
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```go
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type CLI struct {
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Flag string
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}
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```
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## Commands and sub-commands
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Sub-commands are specified by tagging a struct field with `cmd`. Kong supports arbitrarily nested commands.
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eg. The following struct represents the CLI structure `command [--flag="str"] sub-command`.
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```go
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type CLI struct {
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Command struct {
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Flag string
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SubCommand struct {
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} `cmd`
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} `cmd`
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}
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```
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## Branching positional arguments
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In addition to sub-commands, structs can also be configured as branching positional arguments.
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This is achieved by tagging an [unmapped](#mapper---customising-how-the-command-line-is-mapped-to-go-values) nested struct field with `arg`, then including a positional argument field inside that struct _with the same name_. For example, the following command structure:
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app rename <name> to <name>
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Can be represented with the following:
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```go
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var CLI struct {
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Rename struct {
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Name struct {
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Name string `arg` // <-- NOTE: identical name to enclosing struct field.
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To struct {
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Name struct {
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Name string `arg`
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} `arg`
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} `cmd`
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} `arg`
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} `cmd`
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}
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```
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This looks a little verbose in this contrived example, but typically this will not be the case.
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## Terminating positional arguments
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If a [mapped type](#mapper---customising-how-the-command-line-is-mapped-to-go-values) is tagged with `arg` it will be treated as the final positional values to be parsed on the command line.
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If a positional argument is a slice, all remaining arguments will be appended to that slice.
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## Slices
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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.
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To represent the following command-line:
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cmd ls <file> <file> ...
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You would use the following:
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```go
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var CLI struct {
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Ls struct {
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Files []string `arg type:"existingfile"`
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} `cmd`
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}
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```
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## Maps
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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 `=`.
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To represent the following command-line:
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cmd config set <key>=<value> <key>=<value> ...
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You would use the following:
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```go
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var CLI struct {
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Config struct {
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Set struct {
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Config map[string]float64 `arg type:"file:"`
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} `cmd`
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} `cmd`
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}
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```
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## Custom named types
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Kong includes a number of builtin custom type mappers. These can be used by
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specifying the tag `type:"<type>"`. They are registered with the option
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function `NamedMapper(name, mapper)`.
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| Name | Description |
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|-------------------|---------------------------------------------------|
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| `file` | A path. ~ expansion is applied. |
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| `existingfile` | An existing path. ~ expansion is applied. |
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| `existingdir` | An existing directory. ~ expansion is applied. |
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Slices and maps treat type tags specially. For slices, the `type:""` tag
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specifies the element type. For maps, the tag has the format
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`tag:"[<key>]:[<value>]"` where either may be omitted.
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## Supported tags
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Tags can be in two forms:
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1. Standard Go syntax, eg. `kong:"required,name='foo'"`.
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2. Bare tags, eg. `required name:"foo"`
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Both can coexist with standard Tag parsing.
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| Tag | Description |
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| -----------------------| ------------------------------------------- |
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| `cmd` | If present, struct is a command. |
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| `arg` | If present, field is an argument. |
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| `env:"X"` | Specify envar to use for default value.
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| `name:"X"` | Long name, for overriding field name. |
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| `help:"X"` | Help text. |
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| `type:"X"` | Specify [named types](#custom-named-types) to use. |
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| `placeholder:"X"` | Placeholder text. |
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| `default:"X"` | Default value. |
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| `short:"X"` | Short name, if flag. |
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| `required` | If present, flag/arg is required. |
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| `optional` | If present, flag/arg is optional. |
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| `hidden` | If present, flag is hidden. |
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| `format:"X"` | Format for parsing input, if supported. |
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| `sep:"X"` | Separator for sequences (defaults to ","). May be `none` to disable splitting. |
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## Modifying Kong's behaviour
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Each Kong parser can be configured via functional options passed to `New(cli interface{}, options...Option)`.
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The full set of options can be found [here](https://godoc.org/github.com/alecthomas/kong#Option).
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### `Name(help)` and `Description(help)` - set the application name description
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Set the application name and/or description.
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The name of the application will default to the binary name, but can be overridden with `Name(name)`.
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As with all help in Kong, text will be wrapped to the terminal.
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### `Configuration(loader, paths...)` - load defaults from configuration files
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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.
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eg.
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```go
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kong.Parse(&cli, kong.Configuration(kong.JSON, "/etc/myapp.json", "~/.myapp.json"))
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```
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### `Resolver(...)` - support for default values from external sources
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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.
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Example resolvers can be found in [resolver.go](https://github.com/alecthomas/kong/blob/master/resolver.go).
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### `*Mapper(...)` - customising how the command-line is mapped to Go values
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Command-line arguments are mapped to Go values via the Mapper interface:
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```go
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// A Mapper knows how to map command-line input to Go.
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type Mapper interface {
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// Decode scan into target.
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//
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// "ctx" contains context about the value being decoded that may be useful
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// to some mapperss.
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Decode(ctx *MapperContext, scan *Scanner, target reflect.Value) error
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}
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```
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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:
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1. `NamedMapper(string, Mapper)` and using the tag key `type:"<name>"`.
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2. `KindMapper(reflect.Kind, Mapper)`.
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3. `TypeMapper(reflect.Type, Mapper)`.
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4. `ValueMapper(interface{}, Mapper)`, passing in a pointer to a field of the grammar.
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### `HelpOptions(HelpPrinterOptions)` and `Help(HelpFunc)` - customising help
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The default help output is usually sufficient, but if not there are two solutions.
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1. Use `HelpOptions(HelpPrinterOptions)` to configure how help is formatted (see [HelpPrinterOptions](https://godoc.org/github.com/alecthomas/kong#HelpPrinterOptions) for details).
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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.
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### `Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed
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Hooks are callback functions that are bound to a node in the command-line and executed at parse time, before structural validation and assignment.
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eg.
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```go
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app := kong.Must(&CLI, kong.Hook(&CLI.Debug, func(ctx *Context, path *Path) error {
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log.SetLevel(DEBUG)
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return nil
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}))
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```
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Note: it is generally more advisable to use an imperative approach to building command-lines, eg.
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```go
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if CLI.Debug {
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log.SetLevel(DEBUG)
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}
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```
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But under some circumstances, hooks are the right choice.
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### Other options
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The full set of options can be found [here](https://godoc.org/github.com/alecthomas/kong#Option).
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