Eavesdropping on Swift’s Print Statements

func openConsolePipe() {
//open a new Pipe to consume the messages on STDOUT and STDERR
inputPipe = Pipe()

//open another Pipe to output messages back to STDOUT
outputPipe = Pipe()

guard let inputPipe = inputPipe, let outputPipe = outputPipe else {

let pipeReadHandle = inputPipe.fileHandleForReading

//from documentation
//dup2() makes newfd (new file descriptor) be the copy of oldfd (old file descriptor), closing newfd first if necessary.

//here we are copying the STDOUT file descriptor into our output pipe's file descriptor
//this is so we can write the strings back to STDOUT, so it can show up on the xcode console
dup2(STDOUT_FILENO, outputPipe.fileHandleForWriting.fileDescriptor)

//In this case, the newFileDescriptor is the pipe's file descriptor and the old file descriptor is STDOUT_FILENO and STDERR_FILENO

dup2(inputPipe.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
dup2(inputPipe.fileHandleForWriting.fileDescriptor, STDERR_FILENO)

//listen in to the readHandle notification
NotificationCenter.default.addObserver(self, selector: #selector(self.handlePipeNotification), name: FileHandle.readCompletionNotification, object: pipeReadHandle)

//state that you want to be notified of any data coming across the pipe
func handlePipeNotification(notification: Notification) {
//note you have to continuously call this when you get a message
//see this from documentation:
//Note that this method does not cause a continuous stream of notifications to be sent. If you wish to keep getting notified, you’ll also need to call readInBackgroundAndNotify() in your observer method.

if let data = notification.userInfo[NSFileHandleNotificationDataItem] as? Data,
let str = String(data: data, encoding: String.Encoding.ascii) {

//write the data back into the output pipe. the output pipe's write file descriptor points to STDOUT. this allows the logs to show up on the xcode console

// `str` here is the log/contents of the print statement
//if you would like to route your print statements to the UI: make
//sure to subscribe to this notification in your VC and update the UITextView.
//Or if you wanted to send your print statements to the server, then
//you could do this in your notification handler in the app delegate.
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

// if not simulator, open console pipe
#if !((arch(i386) || arch(x86_64)) && os(iOS))

return true




Love podcasts or audiobooks? Learn on the go with our new app.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
Saad Ismail

Saad Ismail

More from Medium

Bees And Bomb Loading Animation Using SwiftUI

apple/swift-evolution 上關於 guard 的有趣討論

Dev Log (Kanji Aid #1)

Day 3 of 100 Days of SwiftUI