从macOS 10.7开始，NSPipe上还有readabilityHandler属性，您可以使用它来设置新数据可用时的回调：

let task = NSTask()

task.launchPath = "/bin/sh"
task.arguments = ["-c", "echo 1 ; sleep 1 ; echo 2 ; sleep 1 ; echo 3 ; sleep 1 ; echo 4"]

let pipe = NSPipe()
task.standardOutput = pipe
let outHandle = pipe.fileHandleForReading

outHandle.readabilityHandler = { pipe in
    if let line = String(data: pipe.availableData, encoding: .utf8) {
        // Update your view with the new text here
        print("New ouput: \(line)")
    } else {
        print("Error decoding data: \(pipe.availableData)")
    }
}
    
task.launch()

我很惊讶没有人提到这一点，因为这是一个非常简单的。

• （有关Swift 3/4的更新，请参阅Patrick F.'s answer。）*

您可以使用通知从管道中异步读取。下面是一个简单的示例，演示了它是如何工作的，希望这能帮助您入门：

let task = NSTask()
task.launchPath = "/bin/sh"
task.arguments = ["-c", "echo 1 ; sleep 1 ; echo 2 ; sleep 1 ; echo 3 ; sleep 1 ; echo 4"]

let pipe = NSPipe()
task.standardOutput = pipe
let outHandle = pipe.fileHandleForReading
outHandle.waitForDataInBackgroundAndNotify()

var obs1 : NSObjectProtocol!
obs1 = NSNotificationCenter.defaultCenter().addObserverForName(NSFileHandleDataAvailableNotification,
    object: outHandle, queue: nil) {  notification -> Void in
        let data = outHandle.availableData
        if data.length > 0 {
            if let str = NSString(data: data, encoding: NSUTF8StringEncoding) {
                print("got output: \(str)")
            }
            outHandle.waitForDataInBackgroundAndNotify()
        } else {
            print("EOF on stdout from process")
            NSNotificationCenter.defaultCenter().removeObserver(obs1)
        }
}

var obs2 : NSObjectProtocol!
obs2 = NSNotificationCenter.defaultCenter().addObserverForName(NSTaskDidTerminateNotification,
    object: task, queue: nil) { notification -> Void in
        print("terminated")
        NSNotificationCenter.defaultCenter().removeObserver(obs2)
}

task.launch()

代替print("got output: \(str)")，您可以将接收到的字符串附加到文本视图中。
上面的代码假设runloop是活动的（这是默认可可应用程序中的情况）。

这是Martin在最新版本Swift上的回答的更新版本。

let task = Process()
    task.launchPath = "/bin/sh"
    task.arguments = ["-c", "echo 1 ; sleep 1 ; echo 2 ; sleep 1 ; echo 3 ; sleep 1 ; echo 4"]

    let pipe = Pipe()
    task.standardOutput = pipe
    let outHandle = pipe.fileHandleForReading
    outHandle.waitForDataInBackgroundAndNotify()

    var obs1 : NSObjectProtocol!
    obs1 = NotificationCenter.default.addObserver(forName: NSNotification.Name.NSFileHandleDataAvailable,
       object: outHandle, queue: nil) {  notification -> Void in
        let data = outHandle.availableData
        if data.count > 0 {
            if let str = NSString(data: data, encoding: String.Encoding.utf8.rawValue) {
                print("got output: \(str)")
            }
            outHandle.waitForDataInBackgroundAndNotify()
        } else {
            print("EOF on stdout from process")
            NotificationCenter.default.removeObserver(obs1)
        }
    }

    var obs2 : NSObjectProtocol!
    obs2 = NotificationCenter.default.addObserver(forName: Process.didTerminateNotification,
               object: task, queue: nil) { notification -> Void in
                print("terminated")
                NotificationCenter.default.removeObserver(obs2)
        }
    task.launch()

我有一个答案，我相信它比通知方法更干净，基于readabilityHandler。在Swift 5中，它是：

class ProcessViewController: NSViewController {

     var executeCommandProcess: Process!

     func executeProcess() {

     DispatchQueue.global().async {

           self.executeCommandProcess = Process()
           let pipe = Pipe()

           self.executeCommandProcess.standardOutput = pipe
           self.executeCommandProcess.launchPath = ""
           self.executeCommandProcess.arguments = []
           var bigOutputString: String = ""

           pipe.fileHandleForReading.readabilityHandler = { (fileHandle) -> Void in
               let availableData = fileHandle.availableData
               let newOutput = String.init(data: availableData, encoding: .utf8)
               bigOutputString.append(newOutput!)
               print("\(newOutput!)")
               // Display the new output appropriately in a NSTextView for example

           }

           self.executeCommandProcess.launch()
           self.executeCommandProcess.waitUntilExit()

           DispatchQueue.main.async {
                // End of the Process, give feedback to the user.
           }

     }
   }

}

请注意，Process必须是一个属性，因为在上面的例子中，假设命令是在后台执行的，如果它是一个局部变量，那么进程将立即被释放。感谢您的关注。