Multiple NSTasks Causing Inconsistent Output

Question

I am working on a Swift app that interacts with external binaries (via NSTask) and returns the output as an array of strings, for each line.

When calling the executeCommand function multiple times, commands start failing to return expected data.

In the example Playground below I'm polling diskutil for volume names.

In the Debug Area the first response I get is:

Not applicable (no file system)
Macintosh HD
Unable to obtain volume name
Unable to obtain volume name
Unable to obtain volume name

Simply re-running the Playground results in:

Not applicable (no file system)
Unable to obtain volume name <-- What happened to Macintosh HD?
Unable to obtain volume name
Unable to obtain volume name

Unable to obtain volume name

    import Cocoa

func executeCommand(launchPath: String, arguments: [String], additionalDelay: Int=0) -> [String] {

    let outputPipe = NSPipe()
    var outputArray = [String]()

    let task = NSTask()
    task.launchPath = launchPath
    task.arguments = arguments
    task.standardOutput = outputPipe

    outputPipe.fileHandleForReading.waitForDataInBackgroundAndNotify()

    NSNotificationCenter.defaultCenter().addObserverForName(NSFileHandleDataAvailableNotification, object: outputPipe.fileHandleForReading , queue: nil) {
        notification in

        let output = outputPipe.fileHandleForReading.availableData
        let outputString = String(data: output, encoding: NSUTF8StringEncoding) ?? ""

        outputArray = outputString.componentsSeparatedByCharactersInSet(.newlineCharacterSet())

        outputPipe.fileHandleForReading.waitForDataInBackgroundAndNotify()

    }

    task.launch()
    sleep(UInt32(additionalDelay)) // Additional delay
    task.waitUntilExit()

    // Remove last empty line from output array.
    if outputArray.count > 0 {
        outputArray.removeLast()
    }

    return outputArray

}


// Example usage



// Output volume name from provided disk identifier
func volumeName(identifier: String) -> String {
    let volumeNameCommand = "/usr/sbin/diskutil info \(identifier) | awk '/Volume Name:/' | sed 's/Volume Name://g'| sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//'"
    let volumeNameOutput = executeCommand("/bin/bash", arguments: ["-c", volumeNameCommand])

    var volumeName = "Unable to obtain volume name"

    if volumeNameOutput.count > 0 {
        volumeName = volumeNameOutput[0]
    }

    return volumeName
}

let identifiers = ["/dev/disk0","/dev/disk1","/dev/disk2","/dev/disk3","/dev/disk4"]

for identifier in identifiers {
    print(volumeName(identifier))
}

I really need consistent results, but don't understand where I'm going wrong.

Any help would be greatly appreciated!

score 0 · Accepted Answer · answered May 02 '16 at 02:41

I believe the asynchronous execution of tasks was causing the inconsistent results. Below is a new function working as expected:

func executeCommand(launchPath: String, arguments: [String]) -> [String] {

    let task: NSTask = NSTask()
    let pipe: NSPipe = NSPipe()

    task.launchPath = launchPath
    task.arguments = arguments
    task.standardOutput = pipe
    task.launch()
    task.waitUntilExit()

    let handle = pipe.fileHandleForReading
    let data = handle.readDataToEndOfFile()
    let outputString = String(data: data, encoding: NSUTF8StringEncoding) ?? ""

    var outputArray = [String]()
    outputArray = outputString.componentsSeparatedByCharactersInSet(.newlineCharacterSet())
    return outputArray

}

Multiple NSTasks Causing Inconsistent Output

1 Answers1