A powerful framework for developing CLIs in Swift

cli, command-line, framework, option-parser, swift



A powerful framework that can be used to develop a CLI, from the simplest to the most complex, in Swift.

//  main.swift
//  Example
//  Created by Jake Heiser on 7/31/14.
//  Copyright (c) 2014 jakeheis. All rights reserved.

import Foundation

CLI.setup(name: "greeter")
CLI.registerChainableCommand(name: "greet")
    .withExecutionBlock { (arguments) in
        print("Hey there!")
~ > greeter greet
Hey there!

Upgrading to SwiftCLI 2.0?

Check out the migration guide!



With Swift Package Manager

Add SwiftCLI as a dependency to your project:

dependencies: [
    .Package(url: "", majorVersion: 2, minor: 0)

With Xcode

See below

Creating a CLI


In the call to CLI.setup(), a name must be passed, and a version and a description are both optional.

CLI.setup(name: "greeter", version: "1.0", description: "Greeter - your own personal greeter")

Registering commands

CLI.register(command: myCommand)
CLI.register(commands: [myCommand, myOtherCommand])

Calling go

In any production app, go() should be used. This method uses the arguments passed to it on launch.


When you are creating and debugging your app, debugGo(with:) is the better choice. Xcode does make it possible to pass arguments to a command line app on launch by editing the app's scheme, but this can be a pain. debugGo(with:) makes it easier to pass an argument string to your app during development.

CLI.debugGo(with: "greeter greet")


There are three ways to create a command. You should decide which way to create your command based on how complex the command will be. In order to highlight the differences between the different command creation methods, the same command "greet" will be implemented each way.

Implement CommandType

This is usually the best choice for a command. Any command that involves a non-trivial amount of execution or option-handling code should be created with this method. A command subclass provides a structured way to develop a complex command, keeping it organized and easy to read.

class GreetCommand: Command {

    let name = "greet"
    let shortDescription = "Greets the given person"
    let signature = "<person>"

    func execute(arguments: CommandArguments) throws  {
        let person = arguments.requiredArgument("person")
        print("Hey there, \(person)!")


Create a ChainableCommand

This is the most lightweight option. You should only create this kind of command if the command is very simple and doesn't involve a lot of execution or option-handling code. It has all the same capabilities as a subclass of Command does, but it can quickly become bloated and hard to understand if there is a large amount of code involved.

let greetCommand = ChainableCommand(name: "greet")
    .withShortDescription("Greets the given person")
    .withExecutionBlock { (arguments) in
        let person = arguments.requiredArgument("person")
        print("Hey there, \(person)!")

CLI also offers a shortcut method to register a ChainableCommand:

CLI.registerChainableCommand(commandName: "greet")

Create a LightweightCommand

This type of command is very similar to ChainableCommand. In fact, all ChainableCommand does is provide an alternative interface to its superclass, LightweightCommand. As with ChainableCommands, this type of command should only be used when the command is relatively simple.

let greetCommand = LightweightCommand(commandName: "greet")
greetCommand.shortDescription = "Greets the given person"
greetCommand.signature = "<person>"
greetCommand.executionBlock = { (arguments) in
    let person = arguments.requiredArgument("person")
    print("Hey there, \(person)!")


Each command must have a command signature describing its expected/permitted arguments. The command signature is used to map the array of user-passed arguments into a keyed dictionary. When a command is being executed, it is passed this dictionary of arguments, with the command signature segments used as keys and the user-passed arguments as values.

For example, a signature of <person> <greeting> and a call of greeter greet Jack Hello would result in the arguments dictionary ["greeting": "Hello", "person": "Jack"].

To set a command's signature:

  • Implement CommandType: var signature: String { get }
  • ChainableCommand: .withSignature("")
  • LightweightCommand: cmd.signature = ""

Required parameters

Required parameters are surrounded by a less-than and a greater-than sign: <requiredParameter> If the command is not passed enough arguments to satisfy all required parameters, it will fail.

~ > # Greet command with a signature of "<person> <greeting>"
~ > greeter greet Jack
Expected 2 arguments, but got 1.
~ > greeter greet Jack Hello
Hello, Jack!

Optional parameters

Optional parameters are surrounded by a less-than and a greater-than sign, and a set of brackets: [<optionalParameter>] Optional parameters must come after all required parameters.

~ > # Greet command with a signature of "<person> [<greeting>]"
~ > greeter greet Jack
Hey there, Jack!
~ > greeter greet Jack Hello
Hello, Jack!

Collection operator

The collection operator is an ellipses placed at the end of a command signature to signify that the last parameter can take an indefinite number of arguments. It must come at the very end of a command signature, after all required parameters and optional parameters.

~ > # Greet command with a signature of "<person> ..."
~ > greeter greet Jack
Hey there, Jack!
~ > greeter greet Jack Jill
Hey there, Jack and Jill!
~ > greeter greet Jack Jill Hill
Hey there, Jack, Jill, and Hill!

The collection operator results in all the last arguments being grouped into an array and passed to the parameter immediately before it (required or optional).

With one argument: greeter greet Jack -> ["person": ["Jack"]]

With multiple arguments: greeter greet Jack Jill Hill -> ["person": ["Jack", "Jill", "Hill"]]

Accessing arguments

During execution, a command has access to an instance of CommandArguments that contains the passed arguments which have been keyed using the command signature. Arguments can be accessed with subscripts or the typesafe shortcuts CommandArguments includes:

func execute(arguments: CommandArguments) throws  {
    // Given command signature --- <name>
    let name = arguments.requiredArgument("name") // of type String

    // Given command signature --- [<name>]
    let name = arguments.optionalArgument("name") // of type String?

    // Given command signature --- <names> ...
    let names = arguments.requiredCollectedArgument("names") // of type [String]

    // Given command signature --- [<names>] ...
    let names = arguments.optionalCollectedArgument("names") // of type [String]?


Commands have support for two types of options: flag options and keyed options. Both types of options can either be denoted by a dash followed by a single letter git commit -a or two dashes followed by the option name git commit --ammend. Single letter options can be cascaded into a single dash followed by all the desired options: git commit -am == git commit -a -m.

ChainableCommand and `LightweightCommand have built in support for option handling, but if you want your custom command class to have this capability, you must implement OptionCommandType instead of CommandType.

Flag options

Flag options are simple options that act as boolean switches. For example, if you were to implement "git commit", "-a" would be a flag option.

To configure a command for flag options:

  • Implement OptionCommandType:
func setupOptions(options: OptionRegistry) {
    options.add(flags: [], usage: "") {

  • ChainableCommand:
.withOptionsSetup ({ (options) in
    options.add(flags: [], usage: "") {

  • LightweightCommand:
cmd.optionsSetupBlock = { (options) in
    options.add(flags: [], usage: "") {


The GreetCommand could be modified to take a "loudly" flag:

class GreetCommand: OptionCommand {

    private var loudly = false


    func setupOptions(options: OptionRegistry) {
        options.add(flags: ["-l", "--loudly"], usage: "Makes the the greeting be said loudly") {
            self.loudly = true


Keyed options

Keyed options are options that have an associated value. Using "git commit" as an example again, "-m" would be a keyed option, as it has an associated value - the commit message.

To configure a command for keyed options:

  • Implement OptionCommandType:
func setupOptions(options: OptionRegistry) {
    options.add(keys: [], usage: "", valueSignature: "") { (value) in

  • ChainableCommand:
.withOptionsSetup ({ (options) in
    options.add(keys: [], usage: "", valueSignature: "") { (value) in

  • LightweightCommand:
cmd.optionsSetupBlock = { (options) in
    options.add(keys: [], usage: "", valueSignature: "") { (value) in


The GreetCommand could be modified to take a "number of times" option:

class GreetCommand: OptionCommand {

    private var numberOfTimes = 1


    func setupOptions(options: OptionRegistry) {
        options.add(keys: ["-n", "--number-of-times"], usage: "Makes the greeter greet a certain number of times", valueSignature: "times") { (value) in
            if let times = Int(value) {
                self.numberOfTimes = times


Unrecognized options

By default, if a command is passed any options it does not handle through add(flags:) or add(keys:), or their respective equivalents in ChainableCommand and LightweightCommand, the command will fail. This behavior can be changed to allow unrecognized options:

  • Implement OptionCommandType: var failOnUnrecognizedOptions: Bool { return false }
  • ChainableCommand: .withFailOnUnrecognizedOptions(false)
  • LightweightCommand: cmd.failOnUnrecognizedOptions = false

Usage of options

As seen in the above examples, add(flags:) and add(keys:) both take a usage parameter. A concise description of what the option does should be included here. This allows the command's usageStatement() to be computed.

A command's usageStatement() is shown in two situations:

  • The user passed an option that the command does not support -- greeter greet -z
  • The command's help was invoked -- greeter greet -h
~ > greeter greet -h
Usage: greeter greet <person> [options]

-l, --loudly                             Makes the the greeting be said loudly
-n, --number-of-times <times>            Makes the greeter greet a certain number of times
-h, --help                               Show help information for this command

The valueSignature argument in the add(keys:) family of methods is displayed like a parameter following the key: --my-key <valueSignature>.

Routing commands

Command routing is done by an object implementing Router, which is just one simple method:

func route(commands: [Command], aliases: [String: String], arguments: RawArguments) -> Command?

SwiftCLI supplies a default implementation of Router with DefaultRouter. DefaultRouter finds commands based on the first passed argument. For example, greeter greet would search for commmands with the commandName of "greet".

If a command is not found, DefaultRouter falls back to its fallbackCommand if given one. Otherwise, it outputs a help message.

~ > greeter
Greeter - your own personal greeter

Available commands: 
- greet                Greets the given person
- help                 Prints this help information

A custom fallback command can be specified by calling CLI.router = DefaultRouter(fallbackCommand: customDefault).


Aliases can be made through the call CLI.alias(from:to:). Router will take these aliases into account while routing to the matching command. For example, if this call is made:

CLI.alias(from: "-c", to: "command")

And the user makes the call myapp -c, the router will search for a command with the name "command" because of the alias, not a command with the name "-c".

Special commands

CLI has two special commands: helpCommand and versionCommand.

Help Command

The HelpCommand can be invoked with myapp help or myapp -h. The HelpCommand first prints the app description (if any was given during CLI.setup()). It then iterates through all available commands, printing their name and their short description.

~ > greeter help
Greeter - your own personal greeter

Available commands: 
- greet                Greets the given person
- help                 Prints this help information

A custom HelpCommand can be used by calling CLI.helpCommand = customHelp.

Version Command

The VersionCommand can be invoked with myapp version or myapp -v. The VersionCommand prints the version of the app given during CLI.setup().

~ > greeter -v
Version: 1.0

A custom VersionCommand can be used by calling CLI.versionComand = customVersion.


The Input class wraps the handling of input from stdin. Several methods are available:

// Simple input:
public static func awaitInput(message: String?) -> String {}
public static func awaitInt(message: String?) -> Int {}
public static func awaitYesNoInput(message: String = "Confirm?") -> Bool {}

// Complex input (if the simple input methods are not sufficient):
public static func awaitInputWithValidation(message: String?, validation: (input: String) -> Bool) -> String {}
public static func awaitInputWithConversion<T>(message: String?, conversion: (input: String) -> T?) -> T {}

Additionally, the Input class makes data piped to the CLI (echo "piped string" | myCLI command") easily available:

if let pipedData = Input.pipedData {
    print("Something was piped! " + pipedData)

See the RecipeCommand in the example project for a demonstration of all this input functionality.


SwiftCLI was designed with sensible defaults but also the ability to be customized at every level. CLI has six properties that can be changed from the default implementations to customized implementations:

// Convert an array of strings to RawArguments
public static var rawArgumentParser: RawArgumentParser = DefaultRawArgumentParser()

// Find the specified command using RawArguments
public static var router: Router = DefaultRouter()

// Convert RawArguments to CommandArguments using a CommandSignature
public static var commandArgumentParser: CommandArgumentParser = DefaultCommandArgumentParser()

// Recognize options in RawArguments
public static var optionParser: OptionParser = DefaultOptionParser()

// Generate a usage statement for the given command
public static var usageStatementGenerator: UsageStatementGenerator = DefaultUsageStatementGenerator()

// Generate a misused options message for the given command with the given incorrect options
public static var misusedOptionsMessageGenerator: MisusedOptionsMessageGenerator = DefaultMisusedOptionsMessageGenerator()

See the individual files of each of these protocols in order to see how to provide a custom implementation.

Running your CLI

Within Xcode

There are two methods to pass in arguments to your CLI within Xcode, explained below. After the arguments are set up using one of these methods, you just need to Build and Run, and your app will execute and print its ouput in Xcode's Console.

CLI debugGo(with:)

As discussed before, this is the easiest way to pass arguments to the CLI. Just replace the CLI.go() call with CLI.debugGo(with: ""). This is only appropriate for development, as when this method is called, the CLI disregards any arguments passed in on launch.

Xcode Scheme

This is not recommended, as the above option is simpler, but it is included for completions's sake. First click on your app's scheme, then "Edit Scheme...". Go to the "Run" section, then the "Arguments" tab. You can then add arguments where it says "Arguments Passed On Launch".

Make sure to use CLI.go() with this method, not CLI.debugGo(with: "").

In Terminal

To actually make your CLI accessible and executable outside of Xcode, you need to add a symbolic link somewhere in your $PATH to the executable product Xcode outputs. The easiest way to do this is to click on your project in Xcode, then your executable target, then Build Phases. Add a new Run Script with this command:

lowercase_name=`echo $PRODUCT_NAME | tr '[A-Z]' '[a-z]'`

if [ ! -f $new_path ]; then ln -s "$BUILT_PRODUCTS_DIR/$PRODUCT_NAME" "$new_path";fi

If you would rather have the symbolic link be placed in a different directory on your $PATH, change /usr/local/bin to your directory of choice. Also, if you would like the app to be executed with a different name then the product name, change the lowercase_name on the first line to your custom name.

You then need to Build your app once inside of Xcode. From then on, you should be able to access your CLI in your terminal.

Again, be sure to use CLI.go() with this method, not CLI.debugGoWithArgumentString("").

Xcode Installation

In your project directory, run:

git submodule add
git submodule update --init

Then drag the SwiftCLI/Sources folder into your Xcode project.


An example of a CLI developed with SwfitCLI can be found at