SwiftCLI

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

import SwiftCLI

class GreetCommand: Command {
    let name = "greet"
    let person = Parameter()
    func execute() throws {
        print("Hello \(person.value)!")
    }
}

let greeter = CLI(name: "greeter")
greeter.commands = [GreetCommand()]
greeter.go()

~ > greeter greet world
Hello world!

With SwiftCLI, you get for free:

• Command routing
• Option parsing
• Help messages
• Usage statements
• Error messages when commands are used incorrectly
• Zsh completions

Table of Contents

• Installation
• Creating a CLI
• Commands
  • Parameters
    • Required parameters
    • Optional parameters
    • Collected parameters
  • Options
    • Flag options
    • Keyed options
    • Option groups
    • Global options
• Command groups
• Routing commands
  • Aliases
• Shell completions
• Special commands
• Input
• Customization
• Running your CLI
• Example

Installation

Ice Package Manager

> ice add jakeheis/Ice

Swift Package Manager

Add SwiftCLI as a dependency to your project:

dependencies: [
    .package(url: "https://github.com/jakeheis/SwiftCLI", from: "4.0.0")
]

4.0.0 has a number of breaking changes. Use 3.1.0 if you wish to use the new features without the breaking changes.

Creating a CLI

When creating a CLI, a name is required, and a version and description are both optional.

let myCli = CLI(name: "greeter", version: "1.0.0", description: "Greeter - your own personal greeter")

You set commands through the .commands property:

myCli.commands = [myCommand, myOtherCommand]

Finally, to actually start the CLI, you call one of the go methods. In a production app, go() or goAndExit() should be used. These methods use the arguments passed to your CLI on launch.

// Use go if you want program execution to continue afterwards
myCli.go() 

// Use exitAndGo if you want your program to terminate after CLI has finished
myCli.exitAndGo()

When you are creating and debugging your app, you can use debugGo(with:) which makes it easier to pass an argument string to your app during development.

myCli.debugGo(with: "greeter greet")

Commands

In order to create a command, you must implement the Command protocol. All that's required is to implement a name property and an execute function; the other properties of Command are optional (though a shortDescription is highly recommended). A simple hello world command could be created as such:

class GreetCommand: Command {

    let name = "greet"
    let shortDescription = "Says hello to the world"

    func execute() throws  {
        print("Hello world!")
    }

}

Parameters

A command can specify what parameters it accepts through certain instance variables. Using reflection, SwiftCLI will identify instance variables of type Parameter, OptionalParameter, CollectedParameter, and OptionalCollectedParameter. These instance variables should appear in the order that the command expects the user to pass the arguments:

class GreetCommand: Command {
    let name = "greet"
    let firstParam = Parameter()
    let secondParam = Parameter()
}

In this example, if the user runs greeter greet Jack Jill, firstParam will be updated to have the value Jack and secondParam will be updated to have the value Jill. The values of these parameters can be accessed in func execute() by calling firstParam.value, etc.

Required parameters

Required parameters take the form of the type Parameter. If the command is not passed enough arguments to satisfy all required parameters, the command will fail.

class GreetCommand: Command {
    let name = "greet"

    let person = Parameter()
    let greeting = Parameter()

    func execute() throws {
        print("\(greeting.value), \(person.value)!")
    }
}

~ > greeter greet Jack
Expected 2 arguments, but got 1.
~ > greeter greet Jack Hello
Hello, Jack!

Optional parameters

Optional parameters take the form of the type OptionalParameter. Optional parameters must come after all required parameters. If the user does not pass enough arguments to satisfy all optional parameters, the .value of these unsatisfied parameters will be nil.

class GreetCommand: Command {
    let name = "greet"

    let person = Parameter()
    let greeting = OptionalParameter()

    func execute() throws {
        let greet = greeting.value ?? "Hey there"
        print("\(greet), \(person.value)!")
    }
}

~ > greeter greet Jack
Hey there, Jack!
~ > greeter greet Jack Hello
Hello, Jack!

Collected parameters

Commands may have a single collected parameter, a CollectedParameter or a OptionalCollectedParameter. These parameters allow the user to pass any number of arguments, and these arguments will be collected into the value array of the collected parameter.

class GreetCommand: Command {
    let name = "greet"

    let people = CollectedParameter()

    func execute() throws {
        let peopleString = people.value.joined(separator: ", ")
        print("Hey there, \(peopleString)!")
    }
}

~ > greeter greet Jack
Hey there, Jack!
~ > greeter greet Jack Jill
Hey there, Jack, Jill!
~ > greeter greet Jack Jill Hill
Hey there, Jack, Jill, Hill!

Options

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 (e.g. git commit -a) or two dashes followed by the option name (e.g. git commit --all). Single letter options can be cascaded into a single dash followed by all the desired options: git commit -am "message" == git commit -a -m "message".

Options are specified as instance variables on the command class, just like parameters:

class ExampleCommand: Command {
    ...
    let flag = Flag("-a", "--all")
    let key = Key<Int>("-t", "--times")
    ...
}

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. They take the form of variables of the type Flag.

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

class GreetCommand: Command {

    ...

    let loudly = Flag("-l", "--loudly", description: "Say the greeting loudly")

    func execute() throws {
        if loudly.value {
             ...
        } else {
            ...
        }
    }

}

Keyed options

Keyed options are options that have an associated value. Using "git commit" as an example, "-m" would be a keyed option, as it has an associated value - the commit message. They take the form of variables of the generic type Key<T>, where T is the type of the option.

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

class GreetCommand: Command {

    ...

    let numberOfTimes = Key<Int>("-n", "--number-of-times", usage: "Say the greeting a certain number of times")

    func execute() throws {
        for i in 0..<(numberOfTimes ?? 1) {
            ...
        }
    }

}

Option groups

The relationship between multiple options can be specified through option groups. Option groups allow a command to specify that the user must pass at most one option of a group (passing more than one is an error), must pass exactly one option of a group (passing zero or more than one is an error), or must pass one or more options of a group (passing zero is an error).

To add option groups, a Command should implement the property optionGroups. For example, if the GreetCommand had a loudly flag and a whisper flag but didn't want the user to be able to pass both, an OptionGroup could be used:

class GreetCommand: Command {

    ...

    let loudly = Flag("-l", "--loudly", description: "Say the greeting loudly")
    let whisper = Flag("-w", "--whisper", description: "Whisper the greeting")
    
    var optionGroups: [OptionGroup] {
        let volume = OptionGroup(options: [loudly, whisper], restriction: .atMostOne)
        return [volume]
    }

    func execute() throws {
        if loudly.value {
             ...
        } else {
            ...
        }
    }

}

Global options

Global options can be used to specify that every command should have a certain option. This is how the -h flag is implemented for all commands. Simply add an option to CLI's .globalOptions array (and optionally extend Command to make the option easy to access in your commands):

private let verboseFlag = Flag("-v")
extension Command {
    var verbose: Flag {
        return verboseFlag
    }
}

myCli.globalOptions.append(verboseFlag)

With this, every command now has a verbose flag.

Usage of options

As seen in the above examples, Flag() and Key() both take an optional description parameter. A concise description of what the option does should be included here. This allows the UsageStatementGenerator to generate a fully informative usage statement for the command.

A command's usage statement 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                             Say the greeting loudly
-n, --number-of-times <value>            Say the greeting a certain number of times
-h, --help                               Show help information for this command

Command groups

Command groups provide a way for related commands to be nested under a certain namespace. Groups can themselves contain other groups.

class ConfigGroup: CommandGroup {
    let name = "config"
    let children = [GetCommand(), SetCommand()]
}
class GetCommand: Command {
    let name = "get"
    func execute() throws {}
}
class SetCommand: Command {
    let name = "set"
    func execute() throws {}
}

You can add a command group to your CLI's .commands array just as add a normal command:

greeter.commands = [ConfigGroup()]

> greeter config

Usage: greeter config <command> [options]

Commands:
  get
  set

> greeter config set
> greeter config get

Routing commands

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

func route(routables: [Routable], arguments: ArgumentList) -> RouteResult

SwiftCLI supplies a default implementation of Router with DefaultRouter. DefaultRouter finds commands based on the first passed argument (or, in the case of command groups, the first several arguments). For example, greeter greet would search for commands with the name of "greet".

SwiftCLI also supplies an implementation of Router called SingleCommandRouter which should be used if your command is only a single command. For example, if you were implementing the ln command, you would say CLI.router = SingleCommandRouter(command: LinkCommand()).

If a command is not found, CLI outputs a help message.

~ > greeter
Greeter - your own personal greeter

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

Aliases

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

CommandAliaser.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".

Shell completions

Zsh completions can be automatically generated for your CLI (bash completions coming soon).

let myCli = CLI(...)

let generator = ZshCompletionGenerator(cli: myCli)
generator.writeCompletions()

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

Version Command

The VersionCommand can be invoked with myapp version or myapp -v. The VersionCommand prints the version of the app given during init CLI(name:version:). If no version is given, the command is not available.

~ > greeter -v
Version: 1.0

Input

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 {}

Customization

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.

Given a call like

~> baker bake cake -qt frosting

the flow of the CLI is as such:

User calls "baker bake cake -qt frosting"
    Command: ?
    Parameters: ?
    Options: ?
    Arguments: Node(bake) -> Node(cake) -> Node(-qt) -> Node(frosting)
ArgumentListManipulators() (including CommandAliaser() and OptionSplitter()) manipulate the nodes
    Command: ?
    Parameters: ?
    Options: ?
    Arguments: Node(bake) -> Node(cake) -> Node(-q) -> Node(-t) -> Node(frosting)
Router() uses the argument nodes to find the appropriate command
    Command: bake
    Parameters: ?
    Options: ?
    Arguments: Node(cake) -> Node(-q) -> Node(-t) -> Node(frosting)
OptionRecognizer() recognizes the options present within the argument nodes
    Command: bake
    Parameters: ?
    Options: quietly, topped with frosting
    Arguments: Node(cake)
ParameterFiller() fills the parameters of the routed command with the remaining arguments
    Command: bake
    Parameters: cake
    Options: quietly, topped with frosting
    Arguments: (none)

All four of these steps can be customized:

public static var argumentListManipulators: [ArgumentListManipulator] = [CommandAliaser(), OptionSplitter()]

public static var router: Router = DefaultRouter()

public static var optionRecognizer: OptionRecognizer = DefaultOptionRecognizer()

public static var parameterFiller: ParameterFiller = DefaultParameterFiller()

The messages formed by SwiftCLI can also be customized:

public var helpMessageGenerator: HelpMessageGenerator = DefaultHelpMessageGenerator()

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

Running your CLI

Simply call swift run. In order to ensure your CLI gets the arguments passed on the command line, make sure to call CLI.go(), not CLI.debugGo(with: "").

Example

An example of a CLI developed with SwfitCLI can be found at https://github.com/jakeheis/Baker.