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Straightforward, type-safe argument parsing for Swift

Swift Argument Parser


Begin by declaring a type that defines the information that you need to collect from the command line. Decorate each stored property with one of ArgumentParser's property wrappers, declare conformance to ParsableCommand, and implement your command's logic in the run() method.

import ArgumentParser

struct Repeat: ParsableCommand {
    @Flag(help: "Include a counter with each repetition.")
    var includeCounter = false

    @Option(name: .shortAndLong, help: "The number of times to repeat 'phrase'.")
    var count: Int?

    @Argument(help: "The phrase to repeat.")
    var phrase: String

    mutating func run() throws {
        let repeatCount = count ?? .max

        for i in 1...repeatCount {
            if includeCounter {
                print("\(i): \(phrase)")
            } else {


You kick off execution by calling your type's static main() method. The ArgumentParser library parses the command-line arguments, instantiates your command type, and then either executes your run() method or exits with a useful message.

ArgumentParser uses your properties' names and type information, along with the details you provide using property wrappers, to supply useful error messages and detailed help:

$ repeat hello --count 3
$ repeat --count 3
Error: Missing expected argument 'phrase'.
Usage: repeat [--count <count>] [--include-counter] <phrase>
  See 'repeat --help' for more information.
$ repeat --help
USAGE: repeat [--count <count>] [--include-counter] <phrase>

  <phrase>                The phrase to repeat.

  --include-counter       Include a counter with each repetition.
  -c, --count <count>     The number of times to repeat 'phrase'.
  -h, --help              Show help for this command.

For more information and documentation about all supported options, see the Documentation folder at the root of the repository.


This repository includes a few examples of using the library:

  • repeat is the example shown above.
  • roll is a simple utility implemented as a straight-line script.
  • math is an annotated example of using nested commands and subcommands.

You can also see examples of ArgumentParser adoption among Swift project tools:

  • indexstore-db is a simple utility with two commands.
  • swift-format uses some advanced features, like custom option values and hidden flags.

Adding ArgumentParser as a Dependency

To use the ArgumentParser library in a SwiftPM project, add the following line to the dependencies in your Package.swift file:

.package(url: "https://github.com/apple/swift-argument-parser", from: "0.3.0"),

Because ArgumentParser is under active development, source-stability is only guaranteed within minor versions (e.g. between 0.0.3 and 0.0.4). If you don't want potentially source-breaking package updates, use this dependency specification instead:

.package(url: "https://github.com/apple/swift-argument-parser", .upToNextMinor(from: "0.3.0")),

Finally, include "ArgumentParser" as a dependency for your executable target:

let package = Package(
    // name, platforms, products, etc.
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser", from: "0.3.0"),
        // other dependencies
    targets: [
        .target(name: "<command-line-tool>", dependencies: [
            .product(name: "ArgumentParser", package: "swift-argument-parser"),
        // other targets


Stars: 2001
Last commit: 3 days ago


ArgumentParser 0.3.2 - 2021-01-16T05:47:24


  • Changes made to a command's properties in its validate method are now persisted.
  • The exit code defined by error types that conform to CustomNSError are now honored.
  • Improved error message when declaring a command type with an unadorned mutable property. (See #256 for more.)
  • Migrated from CRT to MSVCRT for Windows platforms.
  • Fixes and improvements for building with CMake for Windows and Apple Silicon.
  • Documentation improvements.