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# Kong is a command-line parser for Go [![](https://godoc.org/github.com/alecthomas/kong?status.svg)](http://godoc.org/github.com/alecthomas/kong) [![CircleCI](https://img.shields.io/circleci/project/github/alecthomas/kong.svg)](https://circleci.com/gh/alecthomas/kong)
<!-- MarkdownTOC -->
<!-- TOC -->
1. [Introduction](#introduction)
1. [Help](#help)
1. [Command handling](#command-handling)
2. [Help](#help)
3. [Command handling](#command-handling)
1. [Switch on the command string](#switch-on-the-command-string)
1. [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
1. [BeforeApply\(\), AfterApply\(\) and the Bind\(\) option](#BeforeApply-AfterApply-and-the-bind-option)
1. [Flags](#flags)
1. [Commands and sub-commands](#commands-and-sub-commands)
1. [Branching positional arguments](#branching-positional-arguments)
1. [Terminating positional arguments](#terminating-positional-arguments)
1. [Slices](#slices)
1. [Maps](#maps)
1. [Custom named decoders](#custom-named-decoders)
1. [Custom decoders \(mappers\)](#custom-decoders-mappers)
1. [Supported tags](#supported-tags)
1. [Variable interpolation](#variable-interpolation)
1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
2. [Attach a `Run(...) error` method to each command](#attach-a-run-error-method-to-each-command)
4. [Hooks: BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option](#hooks-beforeresolve-beforeapply-afterapply-and-the-bind-option)
5. [Flags](#flags)
6. [Commands and sub-commands](#commands-and-sub-commands)
7. [Branching positional arguments](#branching-positional-arguments)
8. [Terminating positional arguments](#terminating-positional-arguments)
9. [Slices](#slices)
10. [Maps](#maps)
11. [Custom named decoders](#custom-named-decoders)
12. [Custom decoders (mappers)](#custom-decoders-mappers)
13. [Supported tags](#supported-tags)
14. [Variable interpolation](#variable-interpolation)
15. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
1. [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
1. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
1. [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
1. [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
1. [`ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help](#configurehelphelpoptions-and-helphelpfunc---customising-help)
1. [`Bind(...)` - bind values for callback hooks and Run\(\) methods](#bind---bind-values-for-callback-hooks-and-run-methods)
1. [Other options](#other-options)
2. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
3. [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
4. [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
5. [`ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help](#configurehelphelpoptions-and-helphelpfunc---customising-help)
6. [`Bind(...)` - bind values for callback hooks and Run() methods](#bind---bind-values-for-callback-hooks-and-run-methods)
7. [Other options](#other-options)
<!-- /MarkdownTOC -->
<!-- /TOC -->
<a id="markdown-introduction" name="introduction"></a>
## Introduction
Kong aims to support arbitrarily complex command-line structures with as little developer effort as possible.
@@ -74,6 +75,7 @@ func main() {
}
```
<a id="markdown-help" name="help"></a>
## Help
Help is automatically generated. With no other arguments provided, help will display a full summary of all available commands.
@@ -114,10 +116,12 @@ eg.
-f, --force Force removal.
-r, --recursive Recursively remove files.
<a id="markdown-command-handling" name="command-handling"></a>
## Command handling
There are two ways to handle commands in Kong.
<a id="markdown-switch-on-the-command-string" name="switch-on-the-command-string"></a>
### 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:
@@ -157,6 +161,7 @@ func main() {
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 id="markdown-attach-a-run-error-method-to-each-command" name="attach-a-run-error-method-to-each-command"></a>
### Attach a `Run(...) error` method to each command
A more robust approach is to break each command out into their own structs:
@@ -212,7 +217,8 @@ func main() {
```
## Hooks: BeforeResolve(), BeforeSet(), AfterSet() and the Bind() option
<a id="markdown-hooks-beforeresolve-beforeapply-afterapply-and-the-bind-option" name="hooks-beforeresolve-beforeapply-afterapply-and-the-bind-option"></a>
## Hooks: BeforeResolve(), BeforeApply(), AfterApply() and the Bind() option
If a node in the grammar has a `BeforeResolve(...)`, `BeforeApply(...) error` and/or `AfterApply(...) error` method, those methods will be called before validation/assignment and after validation/assignment, respectively.
@@ -245,6 +251,7 @@ func main() {
}
```
<a id="markdown-flags" name="flags"></a>
## Flags
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.
@@ -257,6 +264,7 @@ type CLI struct {
}
```
<a id="markdown-commands-and-sub-commands" name="commands-and-sub-commands"></a>
## Commands and sub-commands
Sub-commands are specified by tagging a struct field with `cmd`. Kong supports arbitrarily nested commands.
@@ -274,6 +282,7 @@ type CLI struct {
}
```
<a id="markdown-branching-positional-arguments" name="branching-positional-arguments"></a>
## Branching positional arguments
In addition to sub-commands, structs can also be configured as branching positional arguments.
@@ -301,12 +310,14 @@ var CLI struct {
This looks a little verbose in this contrived example, but typically this will not be the case.
<a id="markdown-terminating-positional-arguments" name="terminating-positional-arguments"></a>
## Terminating positional arguments
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.
If a positional argument is a slice, all remaining arguments will be appended to that slice.
<a id="markdown-slices" name="slices"></a>
## Slices
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.
@@ -325,6 +336,7 @@ var CLI struct {
}
```
<a id="markdown-maps" name="maps"></a>
## Maps
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 `=`.
@@ -347,6 +359,7 @@ var CLI struct {
For flags, multiple key+value pairs should be separated by `;` eg. `--set="key1=value1;key2=value2"`.
<a id="markdown-custom-named-decoders" name="custom-named-decoders"></a>
## Custom named decoders
Kong includes a number of builtin custom type mappers. These can be used by
@@ -365,11 +378,13 @@ specifies the element type. For maps, the tag has the format
`tag:"[<key>]:[<value>]"` where either may be omitted.
<a id="markdown-custom-decoders-mappers" name="custom-decoders-mappers"></a>
## Custom decoders (mappers)
If a field implements the [MapperValue](https://godoc.org/github.com/alecthomas/kong#MapperValue)
interface it will be used to decode arguments into the field.
<a id="markdown-supported-tags" name="supported-tags"></a>
## Supported tags
Tags can be in two forms:
@@ -399,6 +414,7 @@ Both can coexist with standard Tag parsing.
| `group:"X"` | Logical group for a flag or command. |
| `prefix:"X"` | Prefix for all sub-flags. |
<a id="markdown-variable-interpolation" name="variable-interpolation"></a>
## Variable interpolation
Kong supports limited variable interpolation into help strings, enum lists and
@@ -433,12 +449,14 @@ func main() {
}
```
<a id="markdown-modifying-kongs-behaviour" name="modifying-kongs-behaviour"></a>
## Modifying Kong's behaviour
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](https://godoc.org/github.com/alecthomas/kong#Option).
<a id="markdown-namehelp-and-descriptionhelp---set-the-application-name-description" name="namehelp-and-descriptionhelp---set-the-application-name-description"></a>
### `Name(help)` and `Description(help)` - set the application name description
Set the application name and/or description.
@@ -447,6 +465,7 @@ The name of the application will default to the binary name, but can be overridd
As with all help in Kong, text will be wrapped to the terminal.
<a id="markdown-configurationloader-paths---load-defaults-from-configuration-files" name="configurationloader-paths---load-defaults-from-configuration-files"></a>
### `Configuration(loader, paths...)` - load defaults from configuration files
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.
@@ -457,12 +476,14 @@ eg.
kong.Parse(&cli, kong.Configuration(kong.JSON, "/etc/myapp.json", "~/.myapp.json"))
```
<a id="markdown-resolver---support-for-default-values-from-external-sources" name="resolver---support-for-default-values-from-external-sources"></a>
### `Resolver(...)` - support for default values from external sources
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](https://github.com/alecthomas/kong/blob/master/resolver.go).
<a id="markdown-mapper---customising-how-the-command-line-is-mapped-to-go-values" name="mapper---customising-how-the-command-line-is-mapped-to-go-values"></a>
### `*Mapper(...)` - customising how the command-line is mapped to Go values
Command-line arguments are mapped to Go values via the Mapper interface:
@@ -485,6 +506,7 @@ All builtin Go types (as well as a bunch of useful stdlib types like `time.Time`
3. `TypeMapper(reflect.Type, Mapper)`.
4. `ValueMapper(interface{}, Mapper)`, passing in a pointer to a field of the grammar.
<a id="markdown-configurehelphelpoptions-and-helphelpfunc---customising-help" name="configurehelphelpoptions-and-helphelpfunc---customising-help"></a>
### `ConfigureHelp(HelpOptions)` and `Help(HelpFunc)` - customising help
The default help output is usually sufficient, but if not there are two solutions.
@@ -492,10 +514,12 @@ The default help output is usually sufficient, but if not there are two solution
1. Use `ConfigureHelp(HelpOptions)` to configure how help is formatted (see [HelpOptions](https://godoc.org/github.com/alecthomas/kong#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.
<a id="markdown-bind---bind-values-for-callback-hooks-and-run-methods" name="bind---bind-values-for-callback-hooks-and-run-methods"></a>
### `Bind(...)` - bind values for callback hooks and Run() methods
See the [section on hooks](#BeforeApply-AfterApply-and-the-bind-option) for details.
<a id="markdown-other-options" name="other-options"></a>
### Other options
The full set of options can be found [here](https://godoc.org/github.com/alecthomas/kong#Option).