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Cling

Based on spf13/cobra, Cling is built to be almost entirely modular, giving you absolute control over almost everything without the need for embedded macros - there isn't even a default help command or flag!

Contents

Installation

  1. Add the dependency to your shard.yml:
dependencies:
  cling:
    github: devnote-dev/cling
    version: ">= 3.0.0"
  1. Run shards install

Basic Usage

require "cling"

class MainCommand < Cling::Command
  def setup : Nil
    @name = "greet"
    @description = "Greets a person"
    add_argument "name", description: "the name of the person to greet", required: true
    add_option 'c', "caps", description: "greet with capitals"
    add_option 'h', "help", description: "sends help information"
  end

  def pre_run(arguments : Cling::Arguments, options : Cling::Options) : Bool
    if options.has? "help"
      puts help_template # generated using Cling::Formatter

      false
    else
      true
    end
  end

  def run(arguments : Cling::Arguments, options : Cling::Options) : Nil
    message = "Hello, #{arguments.get("name")}!"

    if options.has? "caps"
      puts message.upcase
    else
      puts message
    end
  end
end

main = MainCommand.new
main.execute ARGV

$ crystal greet.cr -h
Usage:
        greet <arguments> [options]

Arguments:
        name    the name of the person to greet (required)

Options:
        -c, --caps  greet with capitals
        -h, --help  sends help information

$ crystal greet.cr Dev
Hello, Dev!

$ crystal greet.cr -c Dev
HELLO, DEV!

Commands

By default, the Command class is initialized with almost no values. All information about the command must be defined in the setup method.

class MainCommand < Cling::Command
  def setup : Nil
    @name = "greet"
    @description = "Greets a person"
    # defines an argument
    add_argument "name", description: "the name of the person to greet", required: true
    # defines a flag option
    add_option 'c', "caps", description: "greet with capitals"
    add_option 'h', "help", description: "sends help information"
  end
end

[!NOTE] See command.cr for the full list of options.

Commands can also contain children, or subcommands:

require "cling"
# import our subcommand here
require "./welcome_command"

# using the `MainCommand` created earlier
main = MainCommand.new
main.add_command WelcomeCommand.new
# there is also the `add_commands` method for adding multiple
# subcommands at one time

# run the command
main.execute ARGV

$ crystal greet.cr -h
Usage:
        greet <arguments> [options]

Commands:
        welcome    sends a friendly welcome message

Arguments:
        name    the name of person to greet (required)

Options:
        -c, --caps  greet with capitals
        -h, --help  sends help information

$ crystal greet.cr welcome Dev
Welcome to the CLI world, Dev!

As well as being able to have subcommands, they can also inherit certain properties from the parent command:

# in welcome_command.cr ...
class WelcomeCommand < Cling::Command
  def setup : Nil
    # ...

    # this will inherit the header and footer properties
    @inherit_borders = true
    # this will NOT inherit the parent flag options
    @inherit_options = false
    # this will inherit the input, output and error IO streams
    @inherit_streams = true
  end
end

Arguments and Options

Arguments and flag options can be defined in the setup method of a command using the add_argument and add_option methods respectively.

class MainCommand < Cling::Command
  def setup : Nil
    add_argument "name",
      # sets a description for it
      description: "the name of the person to greet",
      # set it as a required or optional argument
      required: true,
      # allow multiple values for the argument
      multiple: false,
      # make it hidden from the help template
      hidden: false

    # define an option with a short flag using chars
    add_option 'c', "caps",
      # sets a description for it
      description: "greet with capitals",
      # set it as a required or optional flag
      required: false,
      # the type of option it is, can be:
      # :none to take no arguments
      # :single to take one argument
      # or :multiple to take multiple arguments
      type: :none,
      # optionally set a default value
      default: nil,
      # make it hidden from the help template
      hidden: false
  end
end

[!WARNING] You can only have one argument with the multiple option which will include all the remaining input values (or unknown arguments). See the example command for usage.

These arguments and options can then be accessed at execution time via the arguments and options parameters in the pre_run, run and post_run methods of a command:

class MainCommand < Cling::Command
  # ...

  def pre_run(arguments : Cling::Arguments, options : Cling::Options) : Bool # can also be `Nil`
    if arguments.get("name").as_s.blank?
      stderr.puts "Your name can't be blank!"

      false
    else
      true
    end
  end
end

The pre_run method is slightly different to the other run methods: it allows returning a boolean to the command executor, which will determine whether the command should continue running – false will stop the command, true will continue. Explicitly returning nil or not specifying a return type is the same as returning true; the command will continue to run.

If you try to access the value of an argument or option that isn't set, it will raise a ValueNotFound exception. To avoid this, use the get? method and check accordingly:

# ...

def run(arguments : Cling::Arguments, options : Cling::Options) : Nil
  caps = options.get?("caps").try(&.as_bool) || false
  stdout.puts caps # => false
end

[!NOTE] See argument.cr and option.cr for more information on parameter methods, and value.cr for value methods.

Customising

The help template is divided into the following sections:

[HEADER]

[DESCRIPTION]

[USAGE]
    <NAME> <USE | "[<arguments>]" "[<options>]">

[COMMANDS]
    [ALIASES] <NAME> <SUMMARY>

[ARGUMENTS]
    <NAME> <DESCRIPTION> ["(required)"]

[OPTIONS]
    [SHORT] <LONG> <DESCRIPTION> ["(required)"] ["(default: ...)"]

[FOOTER]

Sections in <> will always be present, and ones in [] are optional depending on whether they are defined. Because of Cling's modularity, this means that you could essentially have a blank help template (wouldn't recommend it though).

You can customise the following options for the help template formatter:

class Cling::Formatter::Options
  # The character to use for flag option delimiters (default is `-`).
  property option_delim : Char

  # Whether to show the `default` tag for options with default values (default is `true`).
  property show_defaults : Bool

  # Whether to show the `required` tag for required arguments/options (default is `true`).
  property show_required : Bool
end

And pass it to the command like so:

require "cling"

options = Cling::Formatter::Options.new option_delim: '+', show_defaults: false
# we can re-use this in multiple commands
formatter = Cling::Formatter.new options

class MainCommand < Cling::Command
  # ...

  def help_template : String
    formatter.generate self
  end
end

Alternatively, if you want a completely custom design, you can pass a string directly:

def help_template : String
  <<-TEXT
    My custom command help text!

    Use:
        greet <name> [-c | --caps] [-h | --help]
    TEXT
end

Extensions

Cling comes with a few useful extension methods for handling argument and option values:

require "cling"
require "cling/ext"

class StatCommand < Cling::MainCommand
  def setup : Nil
    super

    @name = "stat"
    @description = "Gets the stat information of a file"

    add_argument "path", description: "the path of the file to stat", required: true
  end

  def run(arguments : Cling::Arguments, options : Cling::Options) : Nil
    path = arguments.get("path").as_path

    if File.exists? path
      info = File.info path
      stdout.puts <<-INFO
        name:        #{path.basename}
        size:        #{info.size}
        directory:   #{info.directory?}
        symlink:     #{info.symlink?}
        permissions: #{info.permissions}
        INFO
    else
      stderr.puts "No file found at that path"
    end
  end
end

StatCommand.new.execute ARGV

$ crystal stat.cr ./shard.yml
name:        shard.yml
size:        272
directory:   false
symlink:     false
permissions: rwxrwxrwx (0o777)

[!NOTE] See ext.cr for the full list of extension methods.

Additionally, you can define your own extension methods on the Value struct like so:

require "cling"

module Cling
  struct Value
    def as_chars : Array(Char)
      @raw.to_s.chars
    end
  end
end

class StatCommand
  # ...

  def pre_run(arguments : Cling::Arguments, options : Cling::Options) : Nil
    puts arguments.get("path").as_chars
    # => ['.', '/', 's', 'h', 'a', 'r', 'd', '.', 'y', 'm', 'l']
  end
end

Motivation

Most Crystal CLI builders/DSLs are opinionated with limited customisation available. Cling aims to be entirely modular so that you have the freedom to change whatever you want without having to write tons of boilerplate or monkey-patch code. Macro-based CLI shards can also be quite restrictive as they are not scalable, meaning that you may eventually have to refactor your application to another CLI shard. This is not meant to discourage you from using macro-based CLI shards, they are still useful for short and simple applications with a general template, but if you are looking for something to handle larger applications with guaranteed stability and scalability, Cling is the library for you.

Projects using Cling

Information made available thanks to shards.info.

Contributing

  1. Fork it (https://github.com/devnote-dev/cling/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request

Contributors

This repository is managed under the Mozilla Public License v2.

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