README.md

Kingpin - A Go (golang) command line and flag parser

Features

• POSIX-style short flag combining.
• Parsed, type-safe flags.
• Parsed, type-safe positional arguments.
• Support for required flags and required positional arguments
• Callbacks per command, flag and argument.
• Help output that isn't as ugly as sin.

Versions

Kingpin uses gopkgin.in for versioning.

Changes

• 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 to PlaceHolder.
  • Removed MetaVarFromDefault. Kingpin now uses heuristics to determine what to display.

Simple Example

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.v1"
)

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)
}

Complex Example

Kingpin can also produce complex command-line applications with global flags, subcommands, and per-subcommand flags, like this:

$ chat
usage: chat [<flags>] <command> [<flags>] [<args> ...]

A command-line chat application.

Flags:
  --debug              enable debug mode
  --help               Show help.
  --server=127.0.0.1   server address

Commands:
  help <command>
    Show help for a command.

  post [<flags>] <channel>
    Post a message to a channel.

  register <nick> <name>
    Register a new user.

$ 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"
  "gopkg.in/alecthomas/kingpin.v1"
)

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":
    println(*registerNick)

  // Post message
  case "post":
    if *postImage != nil {
    }
    text := strings.Join(*postText, " ")
  }
}

Reference Documentation

Custom Parsers

Kingpin supports both flag and positional argument parsers for converting to Go types. For example, some included parsers are Int(), Float(), Duration() and ExistingFile().

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 = new(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'))

Default Values

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 a string, which is parsed by the value itself, so it must be compliant with the format expected.

Place-holders in Help

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()

Consuming all remaining arguments

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

Kingpin supports this by having Value provide a IsCumulative() bool function. If this function exists and returns true, the value parser will be called repeatedly for every remaining argument.

Examples of this are the Strings() and StringMap() values.

To implement the above example 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."))