cli v2 manual

Migrating From Older Releases
Getting Started
Examples

Migrating From Older Releases

There are a small set of breaking changes between v1 and v2. Converting is relatively straightforward and typically takes less than an hour. Specific steps are included in Migration Guide: v1 to 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 (
  "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...]

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

GLOBAL OPTIONS
    --help, -h  show help (default: false)

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 https://pkg.go.dev/github.com/urfave/cli/v2

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 snippet 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 main

import (
  "fmt"
  "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)
}

Required Flags

You can make a flag required by setting the Required field to true. If a user does not provide a required flag, they will be shown an error message.

Take for example this app that requires the lang flag:

package main

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

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

  app.Flags = []cli.Flag {
    &cli.StringFlag{
      Name: "lang",
      Value: "english",
      Usage: "language for the greeting",
      Required: true,
    },
  }

  app.Action = func(c *cli.Context) error {
    var output string
    if c.String("lang") == "spanish" {
      output = "Hola"
    } else {
      output = "Hello"
    }
    fmt.Println(output)
    return nil
  }

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

If the app is run without the lang flag, the user will see the following message

Required flag "lang" not set

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 (
  "log"
  "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):

Command line flag value from user
Environment variable (if specified)
Configuration file (if specified)
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",
        Usage: "is it in the soup?",
      },
    },
    Action: func(ctx *cli.Context) error {
      if !ctx.Bool("ginger-crouton") {
        return cli.Exit("Ginger croutons are 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", Aliases: []string{"s"}},
        &cli.BoolFlag{Name: "option", Aliases: []string{"o"}},
        &cli.StringFlag{Name: "message", Aliases: []string{"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 to true. By default, this setting will allow auto-completion for an app's subcommands, but you can write your own completion methods for the App or its subcommands as well.

Default auto-completion

package main
import (
	"fmt"
	"log"
	"os"
	"github.com/urfave/cli/v2"
)
func main() {
	app := cli.NewApp()
	app.EnableBashCompletion = true
	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)
	}
}

Custom auto-completion

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

To enable auto-completion for the current shell session, a bash script, autocomplete/bash_autocomplete is included in this repo.

To use autocomplete/bash_autocomplete set an environment variable named PROG to the name of your program and then source the autocomplete/bash_autocomplete file.

For example, if your cli program is called myprogram:

PROG=myprogram source path/to/cli/autocomplete/bash_autocomplete

Auto-completion is now enabled for the current shell, but will not persist into a new shell.

Distribution and Persistent Autocompletion

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 or restart your shell to activate the auto-completion.

sudo cp path/to/autocomplete/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 and set $PROG within their bash configuration file, adding these lines:

PROG=<myprogram>
source path/to/cli/autocomplete/bash_autocomplete

Keep in mind that if they are enabling auto-completion for more than one program, they will need to set PROG and source autocomplete/bash_autocomplete for each program, like so:

PROG=<program1>
source path/to/cli/autocomplete/bash_autocomplete
PROG=<program2>
source path/to/cli/autocomplete/bash_autocomplete

Customization

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

package main

import (
  "log"
  "os"

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

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

ZSH Support

Auto-completion for ZSH is also supported using the autocomplete/zsh_autocomplete file included in this repo. Two environment variables are used, PROG and _CLI_ZSH_AUTOCOMPLETE_HACK. Set PROG to the program name as before, set _CLI_ZSH_AUTOCOMPLETE_HACK to 1, and then source path/to/autocomplete/zsh_autocomplete. Adding the following lines to your ZSH configuration file (usually .zshrc) will allow the auto-completion to persist across new shells:

PROG=<myprogram>
_CLI_ZSH_AUTOCOMPLETE_HACK=1
source  path/to/autocomplete/zsh_autocomplete

ZSH default auto-complete example

ZSH custom auto-complete example

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"
  "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 (
  "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 (
  "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"
  "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)
}

Timestamp Flag

Using the timestamp flag is simple. Please refer to time.Parse to get possible formats.

package main

import (
  "fmt"
  "log"
  "os"

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

func main() {
  app := &cli.App{
    Flags: []cli.Flag {
      &cli.TimestampFlag{Name: "meeting", Layout: "2006-01-02T15:04:05"},
    },
    Action: func(c *cli.Context) error {
      fmt.Printf("%s", c.Timestamp("meeting").String())
      return nil
    },
  }

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

In this example the flag could be used like this :

myapp --meeting 2019-08-12T15:04:05

Side note: quotes may be necessary around the date depending on your layout (if you have spaces for instance)

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.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!",
        BashComplete: 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"},
    },
    EnableBashCompletion: true,
    HideHelp: false,
    HideVersion: false,
    BashComplete: 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)

      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
}

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