readLine+yn.swift

extension String? {
    /// This property is true on encountering one of "Y", "y", "T", "t", or a digit 1-9—the method ignores any trailing characters.
    /// This property is false if the receiver doesn’t begin with a valid decimal text representation of a number.
    ///
    /// The property assumes a decimal representation and skips whitespace at the beginning of the string. 
    /// It also skips initial whitespace characters, or optional -/+ sign followed by zeroes.
    var yN: Bool { if let self { NSString(string: self).boolValue } else { false } }
    /// This property is true when it is empty (or nil) or when yN is true
    var Yn: Bool { (self?.isEmpty ?? true) || yN }
}

usage.swift

#!/usr/bin/env swift

import Foundation

print("Are you sure? [y/N]")
guard readLine().yN else { exit(0) }