Create Command Line Tools

As software developers, we tend to depend on command line, especially me. Command line interface (CLI) are on fire this current time. Python, Ruby, Erlang and Elixir provide us with awesome tools on command line.

So in this article we will attempt to creating a command line tools. And I have feeling that Elixir will do great in this area.

Setting Up The Application

Let’s start with a new project using mix.

$> mix new awesome_cli
$> cd awesome_cli

Open up lib/awesome_cli.ex and you’ll see something like this:

defmodule AwesomeCli do
  use Application.Behaviour

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    AwesomeCli.Supervisor.start_link
  end
end

Let's do me a favor to create hello world message in the project, will you?!

defmodule AwesomeCli do
  use Application.Behaviour

  # See http://elixir-lang.org/docs/stable/elixir/Application.html
  # for more information on OTP Applications
  def start(_type, _args) do
    AwesomeCli.Supervisor.start_link
  end

  def main(args) do
    IO.puts "Hello, world!"
  end
end

Now run mix escript.build to make it executeable.

$> mix escript.build

If you get error like: ** (Mix) Could not generate escript, please set :escript in your project configuration to a function that returns the escript configuration for our application. So let's do that by opening mix.exs` file.

defmodule AwesomeCli.Mixfile do
  use Mix.Project

  def project do
    [app: :awesome_cli,
    version: "0.0.1",
    elixir: "~> 1.0.4",
    escript: escript,
    deps: deps]
  end

  def escript do
    [main_module: AwesomeCli]
  end


  def application do
    [ applications: [],
      mod: { AwesomeCli, [] } ]
  end

  defp deps do
    []
  end
end

Let's rerun mix escript.build and mix will compile our awesome_cli.ex file and generate a file Elixir.AwesomeCli.beam in the _build/dev/lib/awesome_cli/ebin directory as well as one executable file called awesome_cli. Let's execute the file.

$> ./awesome_cli
Hello, world!

There you have it our first Elixir-powered executable application!

Parsing Argument(s)

Lucky us, Elixir has OptionParser for parsing CLI argument(s). We will use this module to create an awesome command line tools that will get an argument or two from user.

First thing first, we will create command line tools that will say hello to name we given. We will do something like: ./awesome_cli --name ElixirFriend.

Open up lib/awesome_cli.ex and add code below:

def main(args) do
  args |> parse_args
end

def parse_args(args) do
  {[name: name], _, _} = OptionParser.parse(args)

  IO.puts "Hello, #{name}! You're awesome!!"
end

We used |> operator to passing an argument to parse_args function. Then we used OptionParser.parse to parse the argument, take exactly one argument then print it out. Whe we run mix escript.build again then execute the app, we got something like this.

$> mix escript.build
$> ./awesome_cli --name ElixirFriend
Hello, ElixirFriend! You're awesome!!

Awesome, right?! Ok, now to make our cli more awesome, let's implement help message to guide user how to use this tool. Let's use case syntax to and pattern matching for this case.

def parse_args(args) do
  options = OptionParser.parse(args)

  case options do
    {[name: name], _, _} -> IO.puts "Hello, #{name}! You're awesome!!"
    {[help: true], _, _} -> IO.puts "This is help message"

  end
end

Rerun mix escript.build again and execute it with --help option.

$> ./awesome_cli --name ElixirFriend
Hello, ElixirFriend! You're awesome!!
$> ./awesome_cli --help
This is help message

Finishing Touch

Let's refactor the code for little bit. First, we will make parse_args just for parsing arguments and return something to be used in another function.

def main(args) do
  args |> parse_args |> do_process
end

def parse_args(args) do
  options = OptionParser.parse(args)

  case options do
    {[name: name], _, _} -> [name]
    {[help: true], _, _} -> :help
    _ -> :help

  end
end

def do_process([name]) do
  IO.puts "Hello, #{name}! You're awesome!!"
end

def do_process(:help) do
  IO.puts """
    Usage:
    ./awesome_cli --name [your name]

    Options:
    --help  Show this help message.

    Description:
    Prints out an awesome message.
  """

  System.halt(0)
end

For the last time, rerun mix escript.build then try to execute it.

$> mix escript.build
$> ./awesome_cli --name ElixirFriend
Hello, ElixirFriend! You're awesome!!
$> ./awesome_cli --help
Usage:
./awesome_cli --name [your name]

Options:
--help  Show this help message.

Description:
Prints out an awesome message.
$> ./awesome_cli
Usage:
./awesome_cli --name [your name]

Options:
--help  Show this help message.

Description:
Prints out an awesome message.

Conclusion

Today we are using Elixir's OptionParser for creating a simple command line tools. And with help from mix escript.build we able to generate the tools became executable. This example maybe simple enough but with this we can take conclusion that very easy and feel natural to create command line tools with Elixir.

References

• OptionParser Docs