Commandline Swift Script - use 'xcrun -sdk macosx swiftc SimpleArraySum.swift'

SimpleArraySum.swift

import Foundation

// Setup Standard Output, Error
let stderr = NSFileHandle.fileHandleWithStandardError()
let stdout = NSFileHandle.fileHandleWithStandardOutput()

func writeToStd(handle: NSFileHandle, _ str: String)
{
    if let data = str.dataUsingEncoding(NSUTF8StringEncoding)
    {
        handle.writeData(data)
    }
}

// Check if > 1 input argument
if Process.arguments.count < 2
{
    // Expecting a string but didn't receive it
    writeToStd(stderr,"Expecting string argument!\n")
    writeToStd(stderr,"Usage: \(Process.arguments[0]) [list of numbers]\n")
    exit(EXIT_FAILURE)
}

// Create Int array from arguments
var numbers: [Int] = []

for arg in 1..<Process.arguments.count
{
    let input = Process.arguments[arg]
    if let num = Int(input)
    {
      numbers.append(num)
    }
    else
    {
      writeToStd(stderr,"Input required to be of type Int\n")
      exit(EXIT_FAILURE)
    }
}

// Sum and print to stdout
var sum = 0

for num in numbers
{
  sum += num
}

writeToStd(stdout, "\(sum)\n")
exit(EXIT_SUCCESS)

If you want to use swift as an alternative to shell scripts, then you don't really need that xcrun stuff. Only put the line below as the first line in the .swift file
#!/usr/bin/swift

and change the swift file to be executable
chmod o+x SimpleArraySum.swift

and you can run it directly
./SimpleArraySum.swift