Swift 2.1 OSx shell commands using NSTask work when run from xcode, but not when exported

Question

I wrote a simple OSx (10.11) application to execute shell commands when a button is pressed. It works when I run it from xcode, but when I export the application via "archive", one of the buttons no longer works. I don't get an error and I don't get any output either. I am using absolute paths so I don't understand why it works in xcode but not as an exported application, Nor do I understand why one button works and the other doesn't.

Here is the main function that I am using the make the shell commands

    func runCommand(path : String, args : [String]) -> (output: [String], error: [String], exitCode: Int32) {
    var output : [String] = []
    var error : [String] = []

    let task = NSTask()
    task.launchPath = path
    task.arguments = args

    let outpipe = NSPipe()
    task.standardOutput = outpipe
    let errpipe = NSPipe()
    task.standardError = errpipe

    task.launch()

    let outdata = outpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String.fromCString(UnsafePointer(outdata.bytes)) {
        string = string.stringByTrimmingCharactersInSet(NSCharacterSet.newlineCharacterSet())
        output = string.componentsSeparatedByString("\n")
    }

    let errdata = errpipe.fileHandleForReading.readDataToEndOfFile()
    if var string = String.fromCString(UnsafePointer(errdata.bytes)) {
        string = string.stringByTrimmingCharactersInSet(NSCharacterSet.newlineCharacterSet())
        error = string.componentsSeparatedByString("\n")
    }

    //task.waitUntilExit()
    let status = task.terminationStatus

    return (output, error, status)
}

and here is the button that works:

      // Check for configurator 2 app installation
    let (output, error, status) = self.runCommand("/bin/bash", args:  ["-c", "/bin/ls", "/Applications/Apple Configurator 2.app"])

and here is the button that doesn't:

        // Check if the phone is plugged in and paired
    let (output, error, status) = self.runCommand("/bin/bash", args: ["-c", "/usr/local/bin/cfgutil", "get", "isPaired"])

What is even more strange, I discovered (through sheer frustration) that if I repeatedly click the button that doesn't work, it will sometimes eventually work.

For a moment I thought maybe I could help you debug this, but you lost me at "I discovered [...] it will *sometimes* eventually work"... Are you sure about your observations? — Eric Aya, Mar 10 '16 at 20:51
I am sure about this. I put in an alert message that would show me the output of the commands when the click didn't work and there is no output (""), no error output either — AllenMoh, Mar 11 '16 at 23:21
Are all the `if var string` successfull? Did you try adding `else` branches? Since you have default values, it could be the reason for the silent failures. — Eric Aya, Mar 11 '16 at 23:37
Thanks, that was the problem. You should post the answer so you get credit — AllenMoh, Mar 15 '16 at 18:27

Eric Aya · Answer 1 · 2016-03-15T18:53:07.817

Your issue is the result of two things happening together:

you return default values
you don't specify alternative branches for the control flow

What happens is that it hides potential failures, and leads to code that is very hard to debug, as you experienced.

A possible solution with your existing code is to cover all possible ways, meaning providing else branches to your if var string = String.fromCString(UnsafePointer(errdata.bytes)) conditions, where you will handle errors.

score 0 · Answer 2 · answered Mar 15 '16 at 18:41

Thanks to Eric D. I simplified my code and now everything is working.

func runCommand(path : String, args : [String]) -> (output: NSString, error: NSString, exitCode: Int32) {
    let task = NSTask()
    task.launchPath = path
    task.arguments = args

    let outpipe = NSPipe()
    task.standardOutput = outpipe
    let errpipe = NSPipe()
    task.standardError = errpipe

    task.launch()

    let outdata = outpipe.fileHandleForReading.readDataToEndOfFile()
    let output = NSString(data: outdata, encoding:  NSUTF8StringEncoding)

    let errdata = errpipe.fileHandleForReading.readDataToEndOfFile()
    let error_output = NSString(data: errdata, encoding: NSUTF8StringEncoding)

    task.waitUntilExit()
    let status = task.terminationStatus

    return (output!, error_output!, status)
}

Swift 2.1 OSx shell commands using NSTask work when run from xcode, but not when exported

2 Answers2

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