cli

cli is a simple, fast, and fun package for building command line apps in Go. The goal is to enable developers to write fast and distributable command line applications in an expressive way.

• Overview
• Installation
  • Supported platforms
  • Using the v2 branch
  • Using v1 releases
• Getting Started
• Examples
  • Arguments
  • Flags
    • Placeholder Values
    • Alternate Names
    • Ordering
    • Values from the Environment
    • Values from files
    • Values from alternate input sources (YAML, TOML, and others) <<<<<<< HEAD
    • Default Values for help output
  • Subcommands
  • Subcommands categories
  • Exit code
  • Shell Completion =======
    • Precedence
  • Subcommands
  • Subcommands categories
  • Exit code
  • Combining short options
  • Bash Completion

master + Enabling + Distribution + Customization

• Generated Help Text
  • Customization
• Version Flag
  • Customization
• Full API Example

• Contribution Guidelines

Overview

Command line apps are usually so tiny that there is absolutely no reason why your code should not be self-documenting. Things like generating help text and parsing command flags/options should not hinder productivity when writing a command line app.

This is where cli comes into play. cli makes command line programming fun, organized, and expressive!

Installation

Make sure you have a working Go environment. Go version 1.10+ is supported. See the install instructions for Go.

To install cli, simply run:

$ go get github.com/urfave/cli/v2

Make sure your PATH includes the $GOPATH/bin directory so your commands can be easily used:

export PATH=$PATH:$GOPATH/bin

Supported platforms

cli is tested against multiple versions of Go on Linux, and against the latest released version of Go on OS X and Windows. For full details, see ./.travis.yml and ./appveyor.yml.

Using the v2 branch

Warning: The v2 branch is currently unreleased and considered unstable.

There is currently a long-lived branch named v2 that is intended to land as the new master branch once development there has settled down. The current master branch (mirrored as v1) is being manually merged into v2 on an irregular human-based schedule, but generally if one wants to "upgrade" to v2 now and accept the volatility (read: "awesomeness") that comes along with that, please use:

$ go get github.com/urfave/cli/v2

...
import (
  "github.com/urfave/cli/v2" // imports as package "cli"
)
...

<<<<<<< HEAD NOTE: There is a migrator (python) script available to aid with the transition from the v1 to v2 API.

Pinning to the v1 releases

Similarly to the section above describing use of the v2 branch, if one wants to avoid any unexpected compatibility pains once v2 becomes master, then pinning to v1 is an acceptable option, e.g.:

Using v1 releases

master

$ go get github.com/urfave/cli

...
import (
  "github.com/urfave/cli/v2"
)
...

Getting Started

One of the philosophies behind cli is that an API should be playful and full of discovery. So a cli app can be as little as one line of code in main().

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  (&cli.App{}).Run(os.Args)
}

This app will run and show help text, but is not very useful. Let's give an action to execute and some help documentation:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Name: "boom",
    Usage: "make an explosive entrance",
    Action: func(c *cli.Context) error {
      fmt.Println("boom! I say!")
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Running this already gives you a ton of functionality, plus support for things like subcommands and flags, which are covered below.

Examples

Being a programmer can be a lonely job. Thankfully by the power of automation that is not the case! Let's create a greeter app to fend off our demons of loneliness!

Start by creating a directory named greet, and within it, add a file, greet.go with the following code in it:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Name: "greet",
    Usage: "fight the loneliness!",
    Action: func(c *cli.Context) error {
      fmt.Println("Hello friend!")
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Install our command to the $GOPATH/bin directory:

$ go install

Finally run our new command:

$ greet
Hello friend!

cli also generates neat help text:

$ greet help
NAME:
    greet - fight the loneliness!

USAGE:
    greet [global options] command [command options] [arguments...]

VERSION:
    0.0.0

COMMANDS:
    help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS
    --version Shows version information

Arguments

You can lookup arguments by calling the Args function on cli.Context, e.g.:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Action: func(c *cli.Context) error {
      fmt.Printf("Hello %q", c.Args().Get(0))
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Flags

Setting and querying flags is simple.

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag {
      &cli.StringFlag{
        Name: "lang",
        Value: "english",
        Usage: "language for the greeting",
      },
    },
    Action: func(c *cli.Context) error {
      name := "Nefertiti"
      if c.NArg() > 0 {
        name = c.Args().Get(0)
      }
      if c.String("lang") == "spanish" {
        fmt.Println("Hola", name)
      } else {
        fmt.Println("Hello", name)
      }
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

You can also set a destination variable for a flag, to which the content will be scanned.

package main

import (
  "log"
  "os"
  "fmt"

  "github.com/urfave/cli/v2"
)

func main() {
  var language string

  app := &cli.App{
    Flags: []cli.Flag {
      &cli.StringFlag{
        Name:        "lang",
        Value:       "english",
        Usage:       "language for the greeting",
        Destination: &language,
      },
    },
    Action: func(c *cli.Context) error {
      name := "someone"
      if c.NArg() > 0 {
        name = c.Args().Get(0)
      }
      if language == "spanish" {
        fmt.Println("Hola", name)
      } else {
        fmt.Println("Hello", name)
      }
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

See full list of flags at http://godoc.org/github.com/urfave/cli

Placeholder Values

Sometimes it's useful to specify a flag's value within the usage string itself. Such placeholders are indicated with back quotes.

For example this:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.StringFlag{
        Name:    "config",
        Aliases: []string{"c"},
        Usage:   "Load configuration from `FILE`",
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Will result in help output like:

--config FILE, -c FILE   Load configuration from FILE

Note that only the first placeholder is used. Subsequent back-quoted words will be left as-is.

Alternate Names

You can set alternate (or short) names for flags by providing a comma-delimited list for the Name. e.g.

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag {
      &cli.StringFlag{
        Name:    "lang",
        Aliases: []string{"l"},
        Value:   "english",
        Usage:   "language for the greeting",
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

That flag can then be set with --lang spanish or -l spanish. Note that giving two different forms of the same flag in the same command invocation is an error.

Ordering

Flags for the application and commands are shown in the order they are defined. However, it's possible to sort them from outside this library by using FlagsByName or CommandsByName with sort.

For example this:

package main

import (
  "log"
  "os"
  "sort"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.StringFlag{
        Name:  "lang, l",
        Value: "english",
        Usage: "Language for the greeting",
      },
      &cli.StringFlag{
        Name:  "config, c",
        Usage: "Load configuration from `FILE`",
      },
    },
    Commands: []*cli.Command{
      {
        Name:    "complete",
        Aliases: []string{"c"},
        Usage:   "complete a task on the list",
        Action:  func(c *cli.Context) error {
          return nil
        },
      },
      {
        Name:    "add",
        Aliases: []string{"a"},
        Usage:   "add a task to the list",
        Action:  func(c *cli.Context) error {
          return nil
        },
      },
    },
  }

  sort.Sort(cli.FlagsByName(app.Flags))
  sort.Sort(cli.CommandsByName(app.Commands))

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Will result in help output like:

--config FILE, -c FILE  Load configuration from FILE
--lang value, -l value  Language for the greeting (default: "english")

Values from the Environment

You can also have the default value set from the environment via EnvVars. e.g.

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag {
      &cli.StringFlag{
        Name:    "lang",
        Aliases: []string{"l"},
        Value:   "english",
        Usage:   "language for the greeting",
        EnvVars: []string{"APP_LANG"},
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

If EnvVars contains more than one string, the first environment variable that resolves is used as the default.

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.StringFlag{
        Name:    "lang",
        Aliases: []string{"l"},
        Value:   "english",
        Usage:   "language for the greeting",
        EnvVars: []string{"LEGACY_COMPAT_LANG", "APP_LANG", "LANG"},
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Values from files

You can also have the default value set from file via FilePath. e.g.

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := cli.NewApp()

  app.Flags = []cli.Flag {
    cli.StringFlag{
      Name: "password, p",
      Usage: "password for the mysql database",
      FilePath: "/etc/mysql/password",
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Note that default values set from file (e.g. FilePath) take precedence over default values set from the environment (e.g. EnvVar).

Values from alternate input sources (YAML, TOML, and others)

There is a separate package altsrc that adds support for getting flag values from other file input sources.

Currently supported input source formats:

• YAML
• JSON
• TOML

In order to get values for a flag from an alternate input source the following code would be added to wrap an existing cli.Flag like below:

  altsrc.NewIntFlag(&cli.IntFlag{Name: "test"})

Initialization must also occur for these flags. Below is an example initializing getting data from a yaml file below.

  command.Before = altsrc.InitInputSourceWithContext(command.Flags, NewYamlSourceFromFlagFunc("load"))

The code above will use the "load" string as a flag name to get the file name of a yaml file from the cli.Context. It will then use that file name to initialize the yaml input source for any flags that are defined on that command. As a note the "load" flag used would also have to be defined on the command flags in order for this code snipped to work.

Currently only YAML, JSON, and TOML files are supported but developers can add support for other input sources by implementing the altsrc.InputSourceContext for their given sources.

Here is a more complete sample of a command using YAML support:

package notmain

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
  "github.com/urfave/cli/v2/altsrc"
)

func main() {
  flags := []cli.Flag{
    altsrc.NewIntFlag(&cli.IntFlag{Name: "test"}),
    &cli.StringFlag{Name: "load"},
  }

  app := &cli.App{
    Action: func(c *cli.Context) error {
      fmt.Println("yaml ist rad")
      return nil
    },
    Before: altsrc.InitInputSourceWithContext(flags, altsrc.NewYamlSourceFromFlagFunc("load")),
    Flags: flags,
  }

  app.Run(os.Args)
}

Default Values for help output

Sometimes it's useful to specify a flag's default help-text value within the flag declaration. This can be useful if the default value for a flag is a computed value. The default value can be set via the DefaultText struct field.

For example this:

package main

import (
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.IntFlag{
        Name:    "port",
        Usage:   "Use a randomized port",
        Value: 0,
        DefaultText: "random",
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Will result in help output like:

--port value  Use a randomized port (default: random)

Precedence

The precedence for flag value sources is as follows (highest to lowest):

0. Command line flag value from user
1. Environment variable (if specified)
2. Configuration file (if specified)
3. Default defined on the flag

Subcommands

Subcommands can be defined for a more git-like command line app.

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Commands: []*cli.Command{
      {
        Name:    "add",
        Aliases: []string{"a"},
        Usage:   "add a task to the list",
        Action:  func(c *cli.Context) error {
          fmt.Println("added task: ", c.Args().First())
          return nil
        },
      },
      {
        Name:    "complete",
        Aliases: []string{"c"},
        Usage:   "complete a task on the list",
        Action:  func(c *cli.Context) error {
          fmt.Println("completed task: ", c.Args().First())
          return nil
        },
      },
      {
        Name:        "template",
        Aliases:     []string{"t"},
        Usage:       "options for task templates",
        Subcommands: []*cli.Command{
          {
            Name:  "add",
            Usage: "add a new template",
            Action: func(c *cli.Context) error {
              fmt.Println("new task template: ", c.Args().First())
              return nil
            },
          },
          {
            Name:  "remove",
            Usage: "remove an existing template",
            Action: func(c *cli.Context) error {
              fmt.Println("removed task template: ", c.Args().First())
              return nil
            },
          },
        },
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Subcommands categories

For additional organization in apps that have many subcommands, you can associate a category for each command to group them together in the help output.

E.g.

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Commands: []*cli.Command{
      {
        Name: "noop",
      },
      {
        Name:     "add",
        Category: "template",
      },
      {
        Name:     "remove",
        Category: "template",
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Will include:

COMMANDS:
  noop

  Template actions:
    add
    remove

Exit code

Calling App.Run will not automatically call os.Exit, which means that by default the exit code will "fall through" to being 0. An explicit exit code may be set by returning a non-nil error that fulfills cli.ExitCoder, or a cli.MultiError that includes an error that fulfills cli.ExitCoder, e.g.:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{
    Flags: []cli.Flag{
      &cli.BoolFlag{
        Name:  "ginger-crouton",
        Value: true,
        Usage: "is it in the soup?",
      },
    },
    Action: func(ctx *cli.Context) error {
      if !ctx.Bool("ginger-crouton") {
        return cli.Exit("it is not in the soup", 86)
      }
      return nil
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Combining short options

Traditional use of options using their shortnames look like this:

$ cmd -s -o -m "Some message"

Suppose you want users to be able to combine options with their shortnames. This can be done using the UseShortOptionHandling bool in your app configuration, or for individual commands by attaching it to the command configuration. For example:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  app := &cli.App{}
  app.UseShortOptionHandling = true
  app.Commands = []*cli.Command{
    {
      Name:  "short",
      Usage: "complete a task on the list",
      Flags: []cli.Flag{
        cli.BoolFlag{Name: "serve, s"},
        cli.BoolFlag{Name: "option, o"},
        cli.StringFlag{Name: "message, m"},
      },
      Action: func(c *cli.Context) error {
        fmt.Println("serve:", c.Bool("serve"))
        fmt.Println("option:", c.Bool("option"))
        fmt.Println("message:", c.String("message"))
        return nil
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

If your program has any number of bool flags such as serve and option, and optionally one non-bool flag message, with the short options of -s, -o, and -m respectively, setting UseShortOptionHandling will also support the following syntax:

$ cmd -som "Some message"

If you enable UseShortOptionHandling, then you must not use any flags that have a single leading - or this will result in failures. For example, -option can no longer be used. Flags with two leading dashes (such as --options) are still valid.

Bash Completion

You can enable completion commands by setting the EnableBashCompletion flag on the App object. By default, this setting will only auto-complete to show an app's subcommands, but you can write your own completion methods for the App or its subcommands.

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  tasks := []string{"cook", "clean", "laundry", "eat", "sleep", "code"}

  app := &cli.App{
    EnableBashCompletion: true,
    Commands: []*cli.Command{
      {
        Name:    "complete",
        Aliases: []string{"c"},
        Usage:   "complete a task on the list",
        Action: func(c *cli.Context) error {
           fmt.Println("completed task: ", c.Args().First())
           return nil
        },
        BashComplete: func(c *cli.Context) {
          // This will complete if no args are passed
          if c.NArg() > 0 {
            return
          }
          for _, t := range tasks {
            fmt.Println(t)
          }
        },
      },
    },
  }

  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Enabling

Source the autocomplete/bash_autocomplete file in your .bashrc file while setting the PROG variable to the name of your program:

PROG=myprogram source /.../cli/autocomplete/bash_autocomplete

Distribution

Copy autocomplete/bash_autocomplete into /etc/bash_completion.d/ and rename it to the name of the program you wish to add autocomplete support for (or automatically install it there if you are distributing a package). Don't forget to source the file to make it active in the current shell.

sudo cp src/bash_autocomplete /etc/bash_completion.d/<myprogram>
source /etc/bash_completion.d/<myprogram>

Alternatively, you can just document that users should source the generic autocomplete/bash_autocomplete in their bash configuration with $PROG set to the name of their program (as above).

Customization

The default shell completion flag (--generate-completion) is defined as cli.GenerateCompletionFlag, and may be redefined if desired, e.g.:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  cli.GenerateCompletionFlag = &cli.BoolFlag{
    Name:   "compgen",
    Hidden: true,
  }

  app := &cli.App{
    EnableShellCompletion: true,
    Commands: []*cli.Command{
      {
        Name: "wat",
      },
    },
  }
  err := app.Run(os.Args)
  if err != nil {
    log.Fatal(err)
  }
}

Generated Help Text

The default help flag (-h/--help) is defined as cli.HelpFlag and is checked by the cli internals in order to print generated help text for the app, command, or subcommand, and break execution.

Customization

All of the help text generation may be customized, and at multiple levels. The templates are exposed as variables AppHelpTemplate, CommandHelpTemplate, and SubcommandHelpTemplate which may be reassigned or augmented, and full override is possible by assigning a compatible func to the cli.HelpPrinter variable, e.g.:

package main

import (
  "fmt"
  "log"
  "io"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  // EXAMPLE: Append to an existing template
  cli.AppHelpTemplate = fmt.Sprintf(`%s

WEBSITE: http://awesometown.example.com

SUPPORT: support@awesometown.example.com

`, cli.AppHelpTemplate)

  // EXAMPLE: Override a template
  cli.AppHelpTemplate = `NAME:
   {{.Name}} - {{.Usage}}
USAGE:
   {{.HelpName}} {{if .VisibleFlags}}[global options]{{end}}{{if .Commands}} command [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}
   {{if len .Authors}}
AUTHOR:
   {{range .Authors}}{{ . }}{{end}}
   {{end}}{{if .Commands}}
COMMANDS:
{{range .Commands}}{{if not .HideHelp}}   {{join .Names ", "}}{{ "\t"}}{{.Usage}}{{ "\n" }}{{end}}{{end}}{{end}}{{if .VisibleFlags}}
GLOBAL OPTIONS:
   {{range .VisibleFlags}}{{.}}
   {{end}}{{end}}{{if .Copyright }}
COPYRIGHT:
   {{.Copyright}}
   {{end}}{{if .Version}}
VERSION:
   {{.Version}}
   {{end}}
`

  // EXAMPLE: Replace the `HelpPrinter` func
  cli.HelpPrinter = func(w io.Writer, templ string, data interface{}) {
    fmt.Println("Ha HA.  I pwnd the help!!1")
  }
  
  (&cli.App{}).Run(os.Args)
}

The default flag may be customized to something other than -h/--help by setting cli.HelpFlag, e.g.:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  cli.HelpFlag = &cli.BoolFlag{
    Name: "haaaaalp", Aliases: []string{"halp"},
    Usage: "HALP",
    EnvVars: []string{"SHOW_HALP", "HALPPLZ"},
  }
  
  (&cli.App{}).Run(os.Args)
}

Version Flag

The default version flag (-v/--version) is defined as cli.VersionFlag, which is checked by the cli internals in order to print the App.Version via cli.VersionPrinter and break execution.

Customization

The default flag may be customized to something other than -v/--version by setting cli.VersionFlag, e.g.:

package main

import (
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

func main() {
  cli.VersionFlag = &cli.BoolFlag{
    Name: "print-version", Aliases: []string{"V"},
    Usage: "print only the version",
  }
  
  app := &cli.App{
    Name: "partay",
    Version: "v19.99.0",
  }
  app.Run(os.Args)
}

Alternatively, the version printer at cli.VersionPrinter may be overridden, e.g.:

package main

import (
  "fmt"
  "log"
  "os"

  "github.com/urfave/cli/v2"
)

var (
  Revision = "fafafaf"
)

func main() {
  cli.VersionPrinter = func(c *cli.Context) {
    fmt.Printf("version=%s revision=%s\n", c.App.Version, Revision)
  }
  
  app := &cli.App{
    Name: "partay",
    Version: "v19.99.0",
  }
  app.Run(os.Args)
}

Full API Example

Notice: This is a contrived (functioning) example meant strictly for API demonstration purposes. Use of one's imagination is encouraged.

package main

import (
  "errors"
  "flag"
  "fmt"
  "io"
  "io/ioutil"
  "os"
  "time"

  "github.com/urfave/cli/v2"
)

func init() {
  cli.AppHelpTemplate += "\nCUSTOMIZED: you bet ur muffins\n"
  cli.CommandHelpTemplate += "\nYMMV\n"
  cli.SubcommandHelpTemplate += "\nor something\n"

  cli.HelpFlag = &cli.BoolFlag{Name: "halp"}
  cli.GenerateCompletionFlag = &cli.BoolFlag{Name: "compgen", Hidden: true}
  cli.VersionFlag = &cli.BoolFlag{Name: "print-version", Aliases: []string{"V"}}

  cli.HelpPrinter = func(w io.Writer, templ string, data interface{}) {
    fmt.Fprintf(w, "best of luck to you\n")
  }
  cli.VersionPrinter = func(c *cli.Context) {
    fmt.Fprintf(c.App.Writer, "version=%s\n", c.App.Version)
  }
  cli.OsExiter = func(c int) {
    fmt.Fprintf(cli.ErrWriter, "refusing to exit %d\n", c)
  }
  cli.ErrWriter = ioutil.Discard
  cli.FlagStringer = func(fl cli.Flag) string {
    return fmt.Sprintf("\t\t%s", fl.Names()[0])
  }
}

type hexWriter struct{}

func (w *hexWriter) Write(p []byte) (int, error) {
  for _, b := range p {
    fmt.Printf("%x", b)
  }
  fmt.Printf("\n")

  return len(p), nil
}

type genericType struct {
  s string
}

func (g *genericType) Set(value string) error {
  g.s = value
  return nil
}

func (g *genericType) String() string {
  return g.s
}

func main() {
  app := &cli.App{
    Name: "kənˈtrīv",
    Version: "v19.99.0",
    Compiled: time.Now(),
    Authors: []*cli.Author{
      &cli.Author{
        Name:  "Example Human",
        Email: "human@example.com",
      },
    },
    Copyright: "(c) 1999 Serious Enterprise",
    HelpName: "contrive",
    Usage: "demonstrate available API",
    UsageText: "contrive - demonstrating the available API",
    ArgsUsage: "[args and such]",
    Commands: []*cli.Command{
      &cli.Command{
        Name:        "doo",
        Aliases:     []string{"do"},
        Category:    "motion",
        Usage:       "do the doo",
        UsageText:   "doo - does the dooing",
        Description: "no really, there is a lot of dooing to be done",
        ArgsUsage:   "[arrgh]",
        Flags: []cli.Flag{
          &cli.BoolFlag{Name: "forever", Aliases: []string{"forevvarr"}},
        },
        Subcommands: []*cli.Command{
          &cli.Command{
            Name:   "wop",
            Action: wopAction,
          },
        },
        SkipFlagParsing: false,
        HideHelp:        false,
        Hidden:          false,
        HelpName:        "doo!",
        ShellComplete: func(c *cli.Context) {
          fmt.Fprintf(c.App.Writer, "--better\n")
        },
        Before: func(c *cli.Context) error {
          fmt.Fprintf(c.App.Writer, "brace for impact\n")
          return nil
        },
        After: func(c *cli.Context) error {
          fmt.Fprintf(c.App.Writer, "did we lose anyone?\n")
          return nil
        },
        Action: func(c *cli.Context) error {
          c.Command.FullName()
          c.Command.HasName("wop")
          c.Command.Names()
          c.Command.VisibleFlags()
          fmt.Fprintf(c.App.Writer, "dodododododoodododddooooododododooo\n")
          if c.Bool("forever") {
            c.Command.Run(c)
          }
          return nil
        },
        OnUsageError: func(c *cli.Context, err error, isSubcommand bool) error {
          fmt.Fprintf(c.App.Writer, "for shame\n")
          return err
        },
      },
    },
    Flags: []cli.Flag{
      &cli.BoolFlag{Name: "fancy"},
      &cli.BoolFlag{Value: true, Name: "fancier"},
      &cli.DurationFlag{Name: "howlong", Aliases: []string{"H"}, Value: time.Second * 3},
      &cli.Float64Flag{Name: "howmuch"},
      &cli.GenericFlag{Name: "wat", Value: &genericType{}},
      &cli.Int64Flag{Name: "longdistance"},
      &cli.Int64SliceFlag{Name: "intervals"},
      &cli.IntFlag{Name: "distance"},
      &cli.IntSliceFlag{Name: "times"},
      &cli.StringFlag{Name: "dance-move", Aliases: []string{"d"}},
      &cli.StringSliceFlag{Name: "names", Aliases: []string{"N"}},
      &cli.UintFlag{Name: "age"},
      &cli.Uint64Flag{Name: "bigage"},
    },
    EnableShellCompletion: true,
    HideHelp: false,
    HideVersion: false,
    ShellComplete: func(c *cli.Context) {
      fmt.Fprintf(c.App.Writer, "lipstick\nkiss\nme\nlipstick\nringo\n")
    },
    Before: func(c *cli.Context) error {
      fmt.Fprintf(c.App.Writer, "HEEEERE GOES\n")
      return nil
    },
    After: func(c *cli.Context) error {
      fmt.Fprintf(c.App.Writer, "Phew!\n")
      return nil
    },
    CommandNotFound: func(c *cli.Context, command string) {
      fmt.Fprintf(c.App.Writer, "Thar be no %q here.\n", command)
    },
    OnUsageError: func(c *cli.Context, err error, isSubcommand bool) error {
      if isSubcommand {
        return err
      }

      fmt.Fprintf(c.App.Writer, "WRONG: %#v\n", err)
      return nil
    },
    Action: func(c *cli.Context) error {
      cli.DefaultAppComplete(c)
      cli.HandleExitCoder(errors.New("not an exit coder, though"))
      cli.ShowAppHelp(c)
      cli.ShowCommandCompletions(c, "nope")
      cli.ShowCommandHelp(c, "also-nope")
      cli.ShowCompletions(c)
      cli.ShowSubcommandHelp(c)
      cli.ShowVersion(c)

      categories := c.App.Categories
      categories.AddCommand("sounds", &cli.Command{
        Name: "bloop",
      })

      for _, category := range c.App.Categories.Categories() {
        fmt.Fprintf(c.App.Writer, "%s\n", category.Name)
        fmt.Fprintf(c.App.Writer, "%#v\n", category.VisibleCommands())
        fmt.Fprintf(c.App.Writer, "%#v\n", category.VisibleCommands())
      }

      fmt.Printf("%#v\n", c.App.Command("doo"))
      if c.Bool("infinite") {
        c.App.Run([]string{"app", "doo", "wop"})
      }

      if c.Bool("forevar") {
        c.App.RunAsSubcommand(c)
      }
      c.App.Setup()
      fmt.Printf("%#v\n", c.App.VisibleCategories())
      fmt.Printf("%#v\n", c.App.VisibleCommands())
      fmt.Printf("%#v\n", c.App.VisibleFlags())

      fmt.Printf("%#v\n", c.Args().First())
      if c.Args().Len() > 0 {
        fmt.Printf("%#v\n", c.Args().Get(1))
      }
      fmt.Printf("%#v\n", c.Args().Present())
      fmt.Printf("%#v\n", c.Args().Tail())

      set := flag.NewFlagSet("contrive", 0)
      nc := cli.NewContext(c.App, set, c)

      fmt.Printf("%#v\n", nc.Args())
      fmt.Printf("%#v\n", nc.Bool("nope"))
      fmt.Printf("%#v\n", !nc.Bool("nerp"))
      fmt.Printf("%#v\n", nc.Duration("howlong"))
      fmt.Printf("%#v\n", nc.Float64("hay"))
      fmt.Printf("%#v\n", nc.Generic("bloop"))
      fmt.Printf("%#v\n", nc.Int64("bonk"))
      fmt.Printf("%#v\n", nc.Int64Slice("burnks"))
      fmt.Printf("%#v\n", nc.Int("bips"))
      fmt.Printf("%#v\n", nc.IntSlice("blups"))
      fmt.Printf("%#v\n", nc.String("snurt"))
      fmt.Printf("%#v\n", nc.StringSlice("snurkles"))
      fmt.Printf("%#v\n", nc.Uint("flub"))
      fmt.Printf("%#v\n", nc.Uint64("florb"))

      fmt.Printf("%#v\n", nc.FlagNames())
      fmt.Printf("%#v\n", nc.IsSet("wat"))
      fmt.Printf("%#v\n", nc.Set("wat", "nope"))
      fmt.Printf("%#v\n", nc.NArg())
      fmt.Printf("%#v\n", nc.NumFlags())
      fmt.Printf("%#v\n", nc.Lineage()[1])

      nc.Set("wat", "also-nope")

      ec := cli.Exit("ohwell", 86)
      fmt.Fprintf(c.App.Writer, "%d", ec.ExitCode())
      fmt.Printf("made it!\n")
      return ec
    },
    Metadata: map[string]interface{}{
      "layers":          "many",
      "explicable":      false,
      "whatever-values": 19.99,
    },
  }

  if os.Getenv("HEXY") != "" {
    app.Writer = &hexWriter{}
    app.ErrWriter = &hexWriter{}
  }
  
  app.Run(os.Args)
}

func wopAction(c *cli.Context) error {
  fmt.Fprintf(c.App.Writer, ":wave: over here, eh\n")
  return nil
}

Contribution Guidelines

See ./CONTRIBUTING.md