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====== General Purpose Scripting Languages ======

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==== Introduction & Overview ====

  * History of, rationale for development and use of dynamic, interpreted (scripting) languages
  * General characteristics of dynamic languages
  * Advantages / disadvantages of dynamic languages
  * Application of dynamic languages

===== "Scripting" Languages or Technologies =====

  * Some current open source languages and technologies
    * Perl
    * Python
    * PHP
    * Ruby
    * JavaScript
    * Lua

  * Significant market demand for "Scripting" language skills
===== History and Rationale =====

  * Display [[https://wikiless.tiekoetter.com/wiki/Computer_terminal|terminals]] running interactive shells developed in the 1960-70s
    * Examples: [[http://www.columbia.edu/cu/computinghistory/2260.html|IBM 2260]], [[http://www.columbia.edu/cu/computinghistory/h2000.html|Hazeltine 2000]], [[http://www.columbia.edu/cu/computinghistory/vt52.html|DEC VT52]]
    * Led to demand for scripts to relieve human operators of tedious, repetitive typing of commands
      * Led to shell scripting languages and DOS batch files
      * Eventually, general programming language features were added:
        * variables and structures (loops, conditionals)
      * The REXX scripting language started out as a scripting language on IBM mainframes.
        * [[https://cssegit.monmouth.edu/jchung/cs371/blob/master/examples/d.cmd|Sample REXX code]] (written around 1995)

  * Certain applications made scripting languages available to allow the creation of extensions and macros
    * emacs and its use of Lisp
    * Microsoft Office applications and their use of Visual Basic for Applications

===== History and Rationale (text processing) =====

  * Processing text-based records one of the oldest uses of scripting languages
    * Led to development of several standard UNIX tools: awk, sed, grep
    * Perl was developed initially to address limitations in the standard text processing UNIX tools.
    * //Regular expressions// are an important part of the text processing tasks performed by scripting languages.

===== History and Rationale =====

  * The growth of the World Wide Web (//server-side scripting//)
    * Demand for dynamic (non-static) content
    * Led to the Common Gateway Interface (CGI) being provided by web servers, e.g. Apache
      * CGI programs required heavy text parsing, and dynamic languages like Perl were suited to that task
    * CGI had limitations (slow, needing to launch a new process for each Web request)
      * Led to different approaches to dynamic content delivery (Web scripting)
        * mod_perl, mod_python, ASP, PHP, JSP, Zope (Python), Ruby on Rails
    * Modern web browsers typically provide scripting languages to allow scripting of dynamic-type behaviors on web pages
      * ECMAScript (JavaScript), CSS and increasingly in HTML itself (HTML 5)
      * AJAX (use of JavaScript and XML to increase responsiveness of Web apps)

===== General Purpose Dynamic Languages =====

  * Also called [[http://wikiless.tiekoetter.com/wiki/Dynamic_programming_language|"dynamic programming languages"]]
  * Languages that start as scripting languages can gain enough general programming features that they become suitable for general application development
    * Perl, Tcl, Lua
  * Some are developed from scratch with general usage in mind.
    * Python, Ruby
  * Still are often called "scripting languages"

===== Typical Characteristics =====

(of dynamic programming languages, including "scripting" languages)

  * Interpreted, rather than compiled
    * i.e., not directly executed but interpreted line-by-line by a language interpreter program which dynamically translates the instructions into machine-executable instructions "on the fly"
    * means that programs written in dynamic languages will execute much more slowly than compiled languages like C/C++ or more slowly than intermediate (byte-code compiled) languages like Java/C#
    * limits their use for very large applications
      * Though large applications are still written in dynamic languages, especially in Python
        * including media players, games, bittorrent clients
      * Components of or plug-ins to large applications are written in scripting languages.

===== Typical Characteristics (dynamic typing) =====

  * Dynamically or "weakly" typed
    * vs. statically or "strongly" typed (typical of compiled languages)
  * Introduce variables or data structures as needed without the need to "declare" variables
  * Variables start with generic type, often string
    * "Everything is a string." applies to some scripting languages.
      * Shell scripting languages like bash, Tcl, Perl
    * "Everything is an object." applies to other scripting languages.
      * Python, Ruby

===== Advantages/Disadvantages =====

(claimed)

  * Allow accomplishing relatively simple tasks with a smaller amount of code
  * Larger projects can be completed in a shorter amount of time
    * vs. compiled language
    * Assuming same programmer aptitude
    * Increased "programmer productivity"
    * Changes to code are quicker to test
    * Code is smaller, easier to understand

  * Execution is slow, requires greater CPU/memory overhead
  * Lack of strong typing can lead to unforeseen errors, errors which strong typing protects against

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