Admiral.cr

A robust DSL for writing command line interfaces written in Crystal.

Installation | Usage | Examples | Contributing | In the Wild

Installation

Add the following to your application's shard.yml file.

dependencies:
  admiral:
    github: jwaldrip/admiral.cr

Usage

Creating a new CLI | Flags | Arguments | Sub Commands | Command Help | Command Version

Creating a new CLI

You can define a CLI by creating a new class that inherits from Admiral::Command. All your class needs to implement is a run method. Inside the run method will be the logic of your cli application. The following is a very basic CLI. You can run the command by invoking HelloWorld.run. By default this method will use ARGV, but you can also pass Array(String) or String.

# hello_world.cr
require "admiral"

class HelloWorld < Admiral::Command
  def run
    puts "Hello World"
  end
end

HelloWorld.run

$ crystal run ./hello_world.cr
Hello World

Flags

Flags can be added to the command. To define a flag use the define_flag macro.

Note: When defining flags, the underscore method name will translate to a hyphen on the command line. This can be overridden with the long: my_name option when defining the flag.

Simple Flags

Simple flags are denoted only by a name and will compile to returning a String | Nil.

# hello_world.cr
class HelloWorld < Admiral::Command
  define_flag planet

  def run
    puts "Hello #{flags.planet || "World"}"
  end
end

HelloWorld.run

$ crystal build ./hello_world.cr
$ ./hello_world
Hello World
$ ./hello_world --planet Alderaan
Hello Alderaan

Typed Flags

Flags can also be assigned a type. This will result in a properly typed value when calling flags.flag_name. By default flags are not required and will return a Union including the type and Nil.

# hello_world.cr
class HelloWorld < Admiral::Command
  define_flag number_of_greetings : UInt32, default: 1_u32, long: times

  def run
    flags.times.times do
      puts "Hello World"
    end
  end
end

HelloWorld.run

$ crystal build ./hello_world.cr
$ ./hello_world  --times 3
Hello World
Hello World
Hello World

Built in flag types

The following classes are assignable as flags by default:

• String
• Bool
• Float32
• Float64
• Int8
• Int16
• Int32
• Int64
• UInt8
• UInt16
• UInt32
• UInt64

Pro Tip
To make any Class or Struct assignable as a flag, define a .new(value : ::Admiral::StringValue) or
#initialize(value : ::Admiral::StringValue).

Enumerable Flags

Enumerable flags allow for multiple values to be passed on the command line. For example with a defined flag with Array(String) would return an array of String values when calling the flag.

# hello_world.cr
class HelloWorld < Admiral::Command
  define_flag citizens : Array(String), long: citizen

  def run
    flags.citizen.each do |citizen|
      puts "Hello #{citizen}, citizen of Earth!"
    end
  end
end

HelloWorld.run

$ crystal build ./hello_world.cr
$ ./hello_world  --citizen Jim --citizen Harry
Hello Jim, citizen of Earth!
Hello Harry, citizen of Earth!

Additional Flag Options

# hello_world.cr
class HelloWorld < Admiral::Command
  define_flag number_of_greetings : UInt32,
              description: "The number of times to greet the world",
              default: 1_u32,
              long: times,
              short: t,
              required: true

  def run
    flags.number_of_greetings.times do
      puts "Hello World"
    end
  end
end

HelloWorld.run

Arguments

Arguments can be added to the command. To define a argument use the define_argument macro.

Simple Arguments

Simple arguments are denoted only by a name and will compile to returning a String | Nil.

# hello.cr
class Hello < Admiral::Command
  define_argument planet

  def run
    puts "Hello #{arguments.planet || "World"}"
  end
end

HelloWorld.run

$ crystal build ./world.cr
$ ./hello
Hello World
$ ./hello Alderaan
Hello Alderaan

Typed Arguments

Arguments can also be assigned a type. This will result in a properly typed value when calling arguments.arg_name. By default arguments are not required and will return a Union including the type and Nil.

# hello_world.cr
class HelloWorld < Admiral::Command
  define_argument number_of_greetings : UInt32, default: 1_u32

  def run
    arguments.number_of_greetings.times do
      puts "Hello World"
    end
  end
end

HelloWorld.run

$ crystal build ./hello_world.cr
$ ./hello_world  3
Hello World
Hello World
Hello World

Built in argument types

The following classes are assignable as arguments by default:

• String
• Bool
• Float32
• Float64
• Int8
• Int16
• Int32
• Int64
• UInt8
• UInt16
• UInt32
• UInt64

Pro Tip
To make any Class or Struct assignable as a argument, define a .new(value : ::Admiral::StringValue) or
#initialize(value : ::Admiral::StringValue).

Additional Argument Options

# hello_world.cr
class HelloWorld < Admiral::Command
  define_argument number_of_greetings : UInt32,
                  description: "The number of times to greet the world",
                  default: 1_u32,
                  required: true

  def run
    arguments.number_of_greetings.times do
      puts "Hello World"
    end
  end
end

HelloWorld.run

Note:
Required arguments cannot be defined after optional arguments.

Sub Commands

Sub commands can be added to the command. To define a sub command use the register_sub_command macro. You also have the option to add a description for the auto-generated help.

# hello.cr
class Hello < Admiral::Command
  class Planetary < Admiral::Command
    def run
      puts "Hello World"
    end
  end

  class Municipality < Admiral::Command
    def run
      puts "Hello Denver"
    end
  end

  register_sub_command planet : Planetary
  register_sub_command city : Municipality

  def run
    puts help
  end
end

HelloWorld.run

$ crystal build ./hello.cr
$ ./hello planet
Hello World
$ ./hello city
Hello Denver

Command Help

Auto-generated Help

You can add a help command to your CLI by using the define_help macro. define_help also takes a description argument to give additional information about your command.

# hello.cr
class Hello < Admiral::Command
  define_help description: "A command that says hello"
  define_argument planet, default: "World"

  def run
    puts "Hello #{arguments.planet}"
  end
end

$ crystal build ./hello.cr
$ ./hello --help
Usage:
  ./hello [flags...] <planet> [arg...]

A command that says hello

Flags:
  --help

Arguments:
  planet (default: World)

Custom Help

You can also generate your own custom help text.

# hello.cr
class Hello < Admiral::Command
  define_help custom: "This is the help for my command"

  def run
  end
end

Command Version

Like most CLI applications, you can set a version flag.

# hello.cr
class Hello < Admiral::Command
  define_version "1.0.0"

  def run
  end
end

$ crystal build ./hello.cr
$ hello --version
1.0.0

Examples

Example CLIs can be found in ./examples

In the wild

Here are some tools using Admiral.cr in the wild. Have your own you would like to plug? Submit a pull request!

• commercialtribe/psykube

Todo

• [x] Basic Flags
• [x] Typed Flags
• [x] Boolean Flags
• [x] Enum Flags
• [x] Named Arguments
• [x] Positional Arguments
• [x] Sub Commands
• [x] Documentation
• [ ] Fully Tested
• [ ] Bash Completion
• [ ] Zsh Completion

Contributing

See CONTRIBUTING for details on how to contribute.