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, and then declare conformance to ParsableCommand and add the @main attribute. (Note, for async renditions of run, conform to AsyncParsableCommand rather than ParsableCommand.) Finally, implement your command's logic in the run() method.

import ArgumentParser

@main
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? = nil

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

    mutating func run() throws {
        let repeatCount = count ?? 2

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

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

ARGUMENTS:
  <phrase>                The phrase to repeat.

OPTIONS:
  --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 guides, articles, and API documentation see the library's documentation on the Web or in Xcode.

• ArgumentParser documentation
• Getting Started with ArgumentParser
• ParsableCommand documentation
• AsyncParsableCommand documentation

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.
• count-lines uses async/await code in its implementation.

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

• swift-format uses some advanced features, like custom option values and hidden flags.
• swift-package-manager includes a deep command hierarchy and extensive use of option groups.

The Swift Argument Parser package is source-stable; version numbers follow semantic versioning. Source-breaking changes to public API can only land in a new major version.

The public API of version 1.0.0 of the swift-argument-parser package consists of non-underscored declarations that are marked public in the ArgumentParser module. Interfaces that aren't part of the public API may continue to change in any release, including the exact wording and formatting of the autogenerated help and error messages, as well as the package’s examples, tests, utilities, and documentation.

Future minor versions of the package may introduce changes to these rules as needed.

We want this package to quickly embrace Swift language and toolchain improvements that are relevant to its mandate. Accordingly, from time to time, we expect that new versions of this package will require clients to upgrade to a more recent Swift toolchain release. Requiring a new Swift release will only require a minor version bump.

To use the ArgumentParser library in a SwiftPM project, add it to the dependencies for your package and your command-line executable target:

let package = Package(
    // name, platforms, products, etc.
    dependencies: [
        // other dependencies
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.3.0"),
    ],
    targets: [
        .executableTarget(name: "<command-line-tool>", dependencies: [
            // other dependencies
            .product(name: "ArgumentParser", package: "swift-argument-parser"),
        ]),
        // other targets
    ]
)

The minimum Swift version supported by swift-argument-parser releases are detailed below: