NEW: Learn to build amazing SwiftUI apps for macOS with my new book! >>

How to make async command-line tools and scripts

Paul Hudson    @twostraws   

Updated for Xcode 13.3

If you’re writing a command-line tool, you can use async in conjunction with the @main attribute to launch your app into an async context immediately. To do this, first create the static main() method as you normally would with @main, then add async to it. You can optionally also add throws if you don’t intend to handle errors there.

For example, we could write a small command-line tool that fetches data from a URL and prints it out:

struct UserFetcher {
    static func main() async throws {
        let url = URL(string: "")!

        for try await line in url.lines {
            print("Received user: \(line)")

Download this as an Xcode project

Tip: Just like using the @main attribute with a synchronous main() method, you should not include a main.swift file in your command-line project.

Using async and @main together benefits from the full range of Swift concurrency features. Behind the scenes, Swift will automatically create a new task in which it runs your main() method, then terminate the program when that task finishes.

Although it doesn’t work in the current Xcode release, the goal is for Swift to support async calls in top-level code. This would mean you could use main.swift files and remove most of the code in the previous sample – you could just go ahead and make async calls outside of a function.

Hacking with Swift is sponsored by RevenueCat

SPONSORED Spend less time managing in-app purchase infrastructure so you can focus on building your app. RevenueCat gives everything you need to easily implement, manage, and analyze in-app purchases and subscriptions without managing servers or writing backend code.

Get Started

Sponsor Hacking with Swift and reach the world's largest Swift community!

Similar solutions…

Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 5.0/5

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.