STPrivilegedTask *privilegedTask = [[STPrivilegedTask alloc] init];
[privilegedTask setLaunchPath:@"/usr/bin/touch"];
NSArray *args = [NSArray arrayWithObject:@"/etc/my_test_file"];
[privilegedTask setArguments:args];

// this is optional, defaults to /
// [privilegedTask setCurrentDirectoryPath:[[NSBundle mainBundle] resourcePath]];

// launch it, user is prompted for password
OSStatus err = [privilegedTask launch];
if (err != errAuthorizationSuccess) {
	if (err == errAuthorizationCanceled) {
	    NSLog(@"User cancelled");
	} else {
	    NSLog(@"Something went wrong");
	}
} else {
	// task successfully launched
}

// ... launch task

[privilegedTask waitUntilExit];

// Read output file handle for data
NSFileHandle *readHandle = [privilegedTask outputFileHandle];
NSData *outputData = [readHandle readDataToEndOfFile];
NSString *outputString = [[NSString alloc] initWithData:outputData encoding:NSUTF8StringEncoding];

// ... launch task

NSFileHandle *readHandle = [privilegedTask outputFileHandle];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(getOutputData:) name:NSFileHandleReadCompletionNotification object:readHandle];
[readHandle readInBackgroundAndNotify];

// ...

- (void)getOutputData:(NSNotification *)aNotification {
    //get data from notification
    NSData *data = [[aNotification userInfo] objectForKey:NSFileHandleNotificationDataItem];
    
    //make sure there's actual data
    if ([data length]) {
        // do something with the data
        
        // go read more data in the background
        [[aNotification object] readInBackgroundAndNotify];
    } else {
        // do something else
    }
}

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(privilegedTaskFinished:) name:STPrivilegedTaskDidTerminateNotification object:nil];

- (void)privilegedTaskFinished:(NSNotification *)aNotification {
	// do something
}