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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 {
        stdout <<< "Hello \(person.value)!"

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


Ice Package Manager

> ice add jakeheis/SwiftCLI

Swift Package Manager

Add SwiftCLI as a dependency to your project:

dependencies: [
    .package(url: "https://github.com/jakeheis/SwiftCLI", from: "5.0.0")


github "jakeheis/SwiftCLI" ~> 5.2.2


pod 'SwiftCLI', '~> 5.2.2'

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

// Use goAndExit if you want your program to terminate after CLI has finished

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")


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  {
        stdout <<< "Hello world!"



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 {
        stdout <<< "\(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"
        stdout <<< "\(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: ", ")
        stdout <<< "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!


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", description: "Say the greeting a certain number of times")

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


A related option type is VariadicKey, which allows the user to pass the same key multiples times with different values. For example, with a key declaration like:

class GreetCommand: Command {
    let locations = VariadicKey<String>("-l", "--location", description: "Say the greeting in a certain location")

the user can write greeter greet -l Chicago -l NYC, and locations.value will then be set to ["Chicago", "NYC"].

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 = .atMostOne(loudly, whipser)
        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


With this, every command now has a verbose flag.

By default, every command will have a -h flag which prints help information. You can turn this off by setting the CLI helpFlag to nil:

myCli.helpFlag = nil

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 HelpMessageGenerator 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]


> greeter config set
> greeter config get

Shell completions

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

let myCli = CLI(...)

let generator = ZshCompletionGenerator(cli: myCli)

Completions will be automatically generated for command names and options. Parameter completion mode can be specified:

let noCompletions = Parameter(completion: .none)

let aFile = Parameter(completion: .filename)

let aValue = Parameter(completion: .values([
    ("optionA", "the first available option"),
    ("optionB", "the second available option")

let aFunction = Parameter(completion: .function("_my_custom_func"))

The default parameter completion mode is .filename. If you specify a custom function with .function, that function must be supplied when creating the completion generator:

class MyCommand {
    let pids = Parameter(completion: .function("_list_processes"))

let myCLI = CLI(...)
myCLI.commands [MyCommand()]
let generator = ZshCompletionGenerator(cli: myCli, functions: [
    "_list_processes": """
        local pids
        pids=( $(ps -o pid=) )
        _describe '' pids

Built-in commands

CLI has two built-in 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

Usage: greeter <command> [options]

Greeter - your own personal greeter

  greet        Greets the given person
  help         Prints this help information

If you don't want this command to be automatically included, set the helpCommand property to nil:

myCLI.helpCommand = nil

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

If you don't want this command to be automatically included, set the versionCommand property to nil:

myCLI.versionCommand = nil


The Input class makes it easy to read input from stdin. Several methods are available:

let str = Input.readLine()
let int = Input.readInt()
let double = Input.readDouble()
let bool = Input.readBool()

All read methods have four optional parameters:

  • prompt: the message to print before accepting input (e.g. "Input: ")
  • secure: if true, the input is hidden as the user types
  • validation: a closure which defines whether the input is valid, or if the user should be reprompted
  • errorResponse: a closure which is executed when the user enters input which is not valid

For example, you could write:

let percentage = Input.readDouble(
    prompt: "Percentage:",
    validation: { $0 >= 0 && $0 <= 100 },
    errorResponse: { (input) in
        stderr <<< "'\(input)' is invalid; must be a number between 0 and 100"

which would result in interaction such as:

Percentage: asdf
'asdf' is invalid; must be a number between 0 and 100
Percentage: 104
'104' is invalid; must be a number between 0 and 100
Percentage: 43.6

External tasks

SwiftCLI makes it easy to execute external tasks:

// Execute a command and print output:
try run("echo", "hello")
try run(bash: "while true; do echo hi && sleep 1; done")

// Execute a command and capture the output:
let currentDirectory = try capture("pwd").stdout
let sorted = try capture(bash: "cat Package.swift | sort").stdout

You can also use the Task class for more custom behavior:

let input = PipeStream()
let output = PipeStream()
let task = Task(executable: "sort", currentDirectory: "~/Ice", stdout: output, stdin: input)

input <<< "beta"
input <<< "alpha"

output.readAll() // will be alpha\nbeta\n

See Sources/SwiftCLI/Task.swift for full documentation on Task.

Single command CLIs

If your CLI only contains a single command, you may want to execute the command simply by calling cli, rather than cli command. In this case, you can create your CLI as such:

class Ln: Command {
    let name = "ln"
    func execute() throws { ... }

let ln = CLI(singleCommand: Ln())

In this case, if the user writes ln myFile newLocation, rather than searching for a command with the name "myFile", SwiftCLI will execute the Ln command and pass on "myFile` as the first argument to that command.

Keep in mind that when creating a single command CLI, you lose the default VersionCommand. This means that cli -v will not work automatically, and that if you want to print your CLI version you will need to manually implement a Flag("-v") on your single command.


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


The Parser steps through arguments to find the corresponding command, update its parameter values, and recognizes options. Parser has two stages, the first driven by its Router and the second by its ParameterFiller. SwiftCLI supplies default implementations of these two stages with DefaultRouter and DefaultParameterFiller. DefaultRouter finds commands based on the first passed argument (or, in the case of command groups, the first several arguments), and DefaultParameterFiller uses the remaining arguments which don't start with a dash to satisfy the command's parameters.

SwiftCLI also supplies an implementation of Router called SingleCommandRouter which is automatically used if you create your CLI using CLI(singleCommand: myCmd). For example, if you were implementing the ln command, you could manually write myCLI.parser = DefaultParser(router: SingleCommandRouter(command: LinkCommand()). This router will then always return the same command and will leave all arguments to the ParameterFiller. If a user wrote cli my.txt, the DefaultRouter would look for a command named my.txt which takes no arguments, while SingleCommandRouter would treat 'my.txt' as an argument to the single command.

You can implement Router or ParameterFiller on your own types and update your CLI's property to use them:

myCLI.parser = Parser(router: MyRouter(), parameterFiller: MyParameterFiller())


Aliases can be made through the the aliases property on CLI. DefaultRouter will take these aliases into account while routing to the matching command. For example, if you write:

myCLI.aliases["-c"] = "command"

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

By default, "-h" is aliased to "help" and "-v" to "version", but you can remove these if they're not wanted:

myCLI.aliases["-h"] = nil


ArgumentListManipulators act before the Parser begins. They take in the arguments as given by the user and can change them slightly. By default, the only argument list manipulator used is OptionSplitter which splits options like -am into -a -m.

You can implement ArgumentListManipulator on your own type and update CLI's property:

public var argumentListManipulators: [ArgumentListManipulator] = [OptionSplitter()]


The messages formed by SwiftCLI can also be customized:

public var helpMessageGenerator: HelpMessageGenerator = DefaultHelpMessageGenerator()

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: "").


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