- Overview
- Features
- User-visible changes between v1 and v2
- API changes between v1 and v2
- Versions
- Change History
- Examples
- Reference Documentation
Kingpin is a fluent-style, type-safe command-line parser. It supports flags, nested commands, and positional arguments.
Install it with:
$ go get gopkg.in/alecthomas/kingpin.v2
It looks like this:
var (
verbose = kingpin.Flag("verbose", "Verbose mode.").Short('v').Bool()
name = kingpin.Arg("name", "Name of user.").Required().String()
)
func main() {
kingpin.Parse()
fmt.Printf("%v, %s\n", *verbose, *name)
}
More examples are available.
Second to parsing, providing the user with useful help is probably the most
important thing a command-line parser does. Kingpin tries to provide detailed
contextual help if --help
is encountered at any point in the command line
(excluding after --
).
- Help output that isn't as ugly as sin.
- Fully customisable help, via Go templates.
- Parsed, type-safe flags (
kingpin.Flag("f", "help").Int()
) - Parsed, type-safe positional arguments (
kingpin.Arg("a", "help").Int()
). - Parsed, type-safe, arbitrarily deep commands (
kingpin.Command("c", "help")
). - Support for required flags and required positional arguments (
kingpin.Flag("f", "").Required().Int()
). - Support for arbitrarily nested default commands (
command.Default()
). - Callbacks per command, flag and argument (
kingpin.Command("c", "").Action(myAction)
). - POSIX-style short flag combining (
-a -b
->-ab
). - Short-flag+parameter combining (
-a parm
->-aparm
). - Read command-line from files (
@<file>
). - Automatically generate man pages (
--help-man
).
Flags can be specified at any point after their definition, not just immediately after their associated command. From the chat example below, the following used to be required:
$ chat --server=chat.server.com:8080 post --image=~/Downloads/owls.jpg pics
But the following will now work:
$ chat post --server=chat.server.com:8080 --image=~/Downloads/owls.jpg pics
Previously, if a short flag was used, any argument to that flag would have to be separated by a space. That is no longer the case.
ParseWithFileExpansion()
is gone. The new parser directly supports expanding@<file>
.- Added
FatalUsage()
andFatalUsageContext()
for displaying an error + usage and terminating. Dispatch()
renamed toAction()
.- Added
ParseContext()
for parsing a command line into its intermediate context form without executing. - Added
Terminate()
function to override the termination function. - Added
UsageForContextWithTemplate()
for printing usage via a custom template. - Added
UsageTemplate()
for overriding the default template to use. Two templates are included:DefaultUsageTemplate
- default template.CompactUsageTemplate
- compact command template for larger applications.
Kingpin uses gopkg.in for versioning.
The current stable version is gopkg.in/alecthomas/kingpin.v2. The previous version, gopkg.in/alecthomas/kingpin.v1, is deprecated and in maintenance mode.
V2 is the current stable version
Installation:
$ go get gopkg.in/alecthomas/kingpin.v2
V1 is the OLD stable version
Installation:
$ go get gopkg.in/alecthomas/kingpin.v1
-
2015-09-19 -- Stable v2.1.0 release.
- Added
command.Default()
to specify a default command to use if no other command matches. This allows for convenient user shortcuts. - Exposed
HelpFlag
andVersionFlag
for further customisation. Action()
andPreAction()
added and both now support an arbitrary number of callbacks.kingpin.SeparateOptionalFlagsUsageTemplate
.--help-long
and--help-man
(hidden by default) flags.- Flags are "interspersed" by default, but can be disabled with
app.Interspersed(false)
. - Added flags for all simple builtin types (int8, uint16, etc.) and slice variants.
- Use
app.Writer(os.Writer)
to specify the default writer for all output functions. - Dropped
os.Writer
prefix from all printf-like functions.
- Added
-
2015-05-22 -- Stable v2.0.0 release.
- Initial stable release of v2.0.0.
- Fully supports interspersed flags, commands and arguments.
- Flags can be present at any point after their logical definition.
- Application.Parse() terminates if commands are present and a command is not parsed.
- Dispatch() -> Action().
- Actions are dispatched after all values are populated.
- Override termination function (defaults to os.Exit).
- Override output stream (defaults to os.Stderr).
- Templatised usage help, with default and compact templates.
- Make error/usage functions more consistent.
- Support argument expansion from files by default (with @).
- Fully public data model is available via .Model().
- Parser has been completely refactored.
- Parsing and execution has been split into distinct stages.
- Use
go generate
to generate repeated flags. - Support combined short-flag+argument: -fARG.
-
2015-01-23 -- Stable v1.3.4 release.
- Support "--" for separating flags from positional arguments.
- Support loading flags from files (ParseWithFileExpansion()). Use @FILE as an argument.
- Add post-app and post-cmd validation hooks. This allows arbitrary validation to be added.
- A bunch of improvements to help usage and formatting.
- Support arbitrarily nested sub-commands.
-
2014-07-08 -- Stable v1.2.0 release.
- Pass any value through to
Strings()
when final argument. Allows for values that look like flags to be processed. - Allow
--help
to be used with commands. - Support
Hidden()
flags. - Parser for units.Base2Bytes
type. Allows for flags like
--ram=512MB
or--ram=1GB
. - Add an
Enum()
value, allowing only one of a set of values to be selected. eg.Flag(...).Enum("debug", "info", "warning")
.
- Pass any value through to
-
2014-06-27 -- Stable v1.1.0 release.
- Bug fixes.
- Always return an error (rather than panicing) when misconfigured.
OpenFile(flag, perm)
value type added, for finer control over opening files.- Significantly improved usage formatting.
-
2014-06-19 -- Stable v1.0.0 release.
- Support cumulative positional arguments.
- Return error rather than panic when there are fatal errors not caught by the type system. eg. when a default value is invalid.
- Use gokpg.in.
-
2014-06-10 -- Place-holder streamlining.
- Renamed
MetaVar
toPlaceHolder
. - Removed
MetaVarFromDefault
. Kingpin now uses heuristics to determine what to display.
- Renamed
Kingpin can be used for simple flag+arg applications like so:
$ ping --help
usage: ping [<flags>] <ip> [<count>]
Flags:
--debug Enable debug mode.
--help Show help.
-t, --timeout=5s Timeout waiting for ping.
Args:
<ip> IP address to ping.
[<count>] Number of packets to send
$ ping 1.2.3.4 5
Would ping: 1.2.3.4 with timeout 5s and count 0
From the following source:
package main
import (
"fmt"
"gopkg.in/alecthomas/kingpin.v2"
)
var (
debug = kingpin.Flag("debug", "Enable debug mode.").Bool()
timeout = kingpin.Flag("timeout", "Timeout waiting for ping.").Default("5s").OverrideDefaultFromEnvar("PING_TIMEOUT").Short('t').Duration()
ip = kingpin.Arg("ip", "IP address to ping.").Required().IP()
count = kingpin.Arg("count", "Number of packets to send").Int()
)
func main() {
kingpin.Version("0.0.1")
kingpin.Parse()
fmt.Printf("Would ping: %s with timeout %s and count %d", *ip, *timeout, *count)
}
Kingpin can also produce complex command-line applications with global flags, subcommands, and per-subcommand flags, like this:
$ chat --help
usage: chat [<flags>] <command> [<flags>] [<args> ...]
A command-line chat application.
Flags:
--help Show help.
--debug Enable debug mode.
--server=127.0.0.1 Server address.
Commands:
help [<command>]
Show help for a command.
register <nick> <name>
Register a new user.
post [<flags>] <channel> [<text>]
Post a message to a channel.
$ chat help post
usage: chat [<flags>] post [<flags>] <channel> [<text>]
Post a message to a channel.
Flags:
--image=IMAGE Image to post.
Args:
<channel> Channel to post to.
[<text>] Text to post.
$ chat post --image=~/Downloads/owls.jpg pics
...
From this code:
package main
import (
"os"
"strings"
"gopkg.in/alecthomas/kingpin.v2"
)
var (
app = kingpin.New("chat", "A command-line chat application.")
debug = app.Flag("debug", "Enable debug mode.").Bool()
serverIP = app.Flag("server", "Server address.").Default("127.0.0.1").IP()
register = app.Command("register", "Register a new user.")
registerNick = register.Arg("nick", "Nickname for user.").Required().String()
registerName = register.Arg("name", "Name of user.").Required().String()
post = app.Command("post", "Post a message to a channel.")
postImage = post.Flag("image", "Image to post.").File()
postChannel = post.Arg("channel", "Channel to post to.").Required().String()
postText = post.Arg("text", "Text to post.").Strings()
)
func main() {
switch kingpin.MustParse(app.Parse(os.Args[1:])) {
// Register user
case register.FullCommand():
println(*registerNick)
// Post message
case post.FullCommand():
if *postImage != nil {
}
text := strings.Join(*postText, " ")
println("Post:", text)
}
}
Kingpin exports a set of functions to provide consistent errors and usage information to the user.
Error messages look something like this:
<app>: error: <message>
The functions on Application
are:
Function | Purpose |
---|---|
Errorf(format, args) |
Display a printf formatted error to the user. |
Fatalf(format, args) |
As with Errorf, but also call the termination handler. |
FatalUsage(format, args) |
As with Fatalf, but also print contextual usage information. |
FatalUsageContext(context, format, args) |
As with Fatalf, but also print contextual usage information from a ParseContext . |
FatalIfError(err, format, args) |
Conditionally print an error prefixed with format+args, then call the termination handler |
There are equivalent global functions in the kingpin namespace for the default
kingpin.CommandLine
instance.
Kingpin supports nested sub-commands, with separate flag and positional arguments per sub-command. Note that positional arguments may only occur after sub-commands.
For example:
var (
deleteCommand = kingpin.Command("delete", "Delete an object.")
deleteUserCommand = deleteCommand.Command("user", "Delete a user.")
deleteUserUIDFlag = deleteUserCommand.Flag("uid", "Delete user by UID rather than username.")
deleteUserUsername = deleteUserCommand.Arg("username", "Username to delete.")
deletePostCommand = deleteCommand.Command("post", "Delete a post.")
)
func main() {
switch kingpin.Parse() {
case "delete user":
case "delete post":
}
}
Kingpin supports both flag and positional argument parsers for converting to
Go types. For example, some included parsers are Int()
, Float()
,
Duration()
and ExistingFile()
(see parsers.go for a complete list of included parsers).
Parsers conform to Go's flag.Value
interface, so any existing implementations will work.
For example, a parser for accumulating HTTP header values might look like this:
type HTTPHeaderValue http.Header
func (h *HTTPHeaderValue) Set(value string) error {
parts := strings.SplitN(value, ":", 2)
if len(parts) != 2 {
return fmt.Errorf("expected HEADER:VALUE got '%s'", value)
}
(*http.Header)(h).Add(parts[0], parts[1])
return nil
}
func (h *HTTPHeaderValue) String() string {
return ""
}
As a convenience, I would recommend something like this:
func HTTPHeader(s Settings) (target *http.Header) {
target = &http.Header{}
s.SetValue((*HTTPHeaderValue)(target))
return
}
You would use it like so:
headers = HTTPHeader(kingpin.Flag("header", "Add a HTTP header to the request.").Short('H'))
Depending on the Value
they hold, some flags may be repeated. The
IsCumulative() bool
function on Value
tells if it's safe to call Set()
multiple times or if an error should be raised if several values are passed.
The built-in Value
s returning slices and maps, as well as Counter
are
examples of Value
s that make a flag repeatable.
Boolean values are uniquely managed by Kingpin. Each boolean flag will have a negative complement:
--<name>
and --no-<name>
.
The default value is the zero value for a type. This can be overridden with
the Default(value...)
function on flags and arguments. This function accepts
one or several strings, which are parsed by the value itself, so they must
be compliant with the format expected.
The place-holder value for a flag is the value used in the help to describe the value of a non-boolean flag.
The value provided to PlaceHolder() is used if provided, then the value provided by Default() if provided, then finally the capitalised flag name is used.
Here are some examples of flags with various permutations:
--name=NAME // Flag(...).String()
--name="Harry" // Flag(...).Default("Harry").String()
--name=FULL-NAME // flag(...).PlaceHolder("FULL-NAME").Default("Harry").String()
A common command-line idiom is to use all remaining arguments for some purpose. eg. The following command accepts an arbitrary number of IP addresses as positional arguments:
./cmd ping 10.1.1.1 192.168.1.1
Such arguments are similar to repeatable flags, but for
arguments. Therefore they use the same IsCumulative() bool
function on the
underlying Value
, so the built-in Value
s for which the Set()
function
can be called several times will consume multiple arguments.
To implement the above example with a custom Value
, we might do something
like this:
type ipList []net.IP
func (i *ipList) Set(value string) error {
if ip := net.ParseIP(value); ip == nil {
return fmt.Errorf("'%s' is not an IP address", value)
} else {
*i = append(*i, ip)
return nil
}
}
func (i *ipList) String() string {
return ""
}
func (i *ipList) IsCumulative() bool {
return true
}
func IPList(s Settings) (target *[]net.IP) {
target = new([]net.IP)
s.SetValue((*ipList)(target))
return
}
And use it like so:
ips := IPList(kingpin.Arg("ips", "IP addresses to ping."))
By default, all flags and commands/subcommands generate completions internally.
Out of the box, CLI tools using kingpin should be able to take advantage
of completion hinting for flags and commands. By specifying
--completion-bash
as the first argument, your CLI tool will show
possible subcommands. By ending your argv with --
, hints for flags
will be shown.
To allow your end users to take advantage you must package a
/etc/bash_completion.d
script with your distribution (or the equivalent
for your target platform/shell). An alternative is to instruct your end
user to source a script from their bash_profile
(or equivalent).
Fortunately Kingpin makes it easy to generate or source a script for use
with end users shells. ./yourtool --completion-script-bash
and
./yourtool --completion-script-zsh
will generate these scripts for you.
Installation by Package
For the best user experience, you should bundle your pre-created
completion script with your CLI tool and install it inside
/etc/bash_completion.d
(or equivalent). A good suggestion is to add
this as an automated step to your build pipeline, in the implementation
is improved for bug fixed.
Installation by bash_profile
Alternatively, instruct your users to add an additional statement to
their bash_profile
(or equivalent):
eval "$(your-cli-tool --completion-script-bash)"
Or for ZSH
eval "$(your-cli-tool --completion-script-zsh)"
To provide more flexibility, a completion option API has been exposed for flags to allow user defined completion options, to extend completions further than just EnumVar/Enum.
Provide Static Options
When using an Enum
or EnumVar
, users are limited to only the options
given. Maybe we wish to hint possible options to the user, but also
allow them to provide their own custom option. HintOptions
gives
this functionality to flags.
app := kingpin.New("completion", "My application with bash completion.")
app.Flag("port", "Provide a port to connect to").
Required().
HintOptions("80", "443", "8080").
IntVar(&c.port)
Provide Dynamic Options Consider the case that you needed to read a local database or a file to provide suggestions. You can dynamically generate the options
func listHosts(args []string) []string {
// Provide a dynamic list of hosts from a hosts file or otherwise
// for bash completion. In this example we simply return static slice.
// You could use this functionality to reach into a hosts file to provide
// completion for a list of known hosts.
return []string{"sshhost.example", "webhost.example", "ftphost.example"}
}
app := kingpin.New("completion", "My application with bash completion.")
app.Flag("flag-1", "").HintAction(listHosts).String()
EnumVar/Enum
When using Enum
or EnumVar
, any provided options will be automatically
used for bash autocompletion. However, if you wish to provide a subset or
different options, you can use HintOptions
or HintAction
which will override
the default completion options for Enum
/EnumVar
.
Examples
You can see an in depth example of the completion API within
examples/completion/main.go
kingpin.CommandLine.HelpFlag.Short('h')
Kingpin v2 supports templatised help using the text/template library (actually, a fork).
You can specify the template to use with the Application.UsageTemplate() function.
There are four included templates: kingpin.DefaultUsageTemplate
is the default,
kingpin.CompactUsageTemplate
provides a more compact representation for more complex command-line structures,
kingpin.SeparateOptionalFlagsUsageTemplate
looks like the default template, but splits required
and optional command flags into separate lists, and kingpin.ManPageTemplate
is used to generate man pages.
See the above templates for examples of usage, and the the function UsageForContextWithTemplate() method for details on the context.
$ go run ./examples/curl/curl.go --help
usage: curl [<flags>] <command> [<args> ...]
An example implementation of curl.
Flags:
--help Show help.
-t, --timeout=5s Set connection timeout.
-H, --headers=HEADER=VALUE
Add HTTP headers to the request.
Commands:
help [<command>...]
Show help.
get url <url>
Retrieve a URL.
get file <file>
Retrieve a file.
post [<flags>] <url>
POST a resource.
$ go run ./examples/curl/curl.go --help
usage: curl [<flags>] <command> [<args> ...]
An example implementation of curl.
Flags:
--help Show help.
-t, --timeout=5s Set connection timeout.
-H, --headers=HEADER=VALUE
Add HTTP headers to the request.
Commands:
help [<command>...]
get [<flags>]
url <url>
file <file>
post [<flags>] <url>