Scripting language

"Scripting" redirects here. For other uses, see Script.

A scripting language, script language, or extension language is a programming language that allows control of one or more applications. "Scripts" are distinct from the core code of the application, as they are usually written in a different language and are often created or at least modified by the end-user.^[1] Scripts are often interpreted from source code or bytecode, whereas the application is typically first compiled to native machine code.^[2]

Early script languages were often called batch languages or job control languages. Such early scripting languages were created to shorten the traditional edit-compile-link-run process.

History

Early mainframe computers (in the 1950s) were non-interactive, instead using batch processing. IBM's Job Control Language (JCL) is the archetype of language used to control batch processing.

The first interactive shells were developed in the 1960s to enable remote operation of the first time-sharing systems, and these used shell scripts, which controlled running computer programs within a computer program, the shell.

Languages such as Tcl and Lua were specifically designed as general purpose scripting languages that could be embedded in any application. Other languages such as Visual Basic for Applications (VBA) provided strong integration with the automation facilities of an underlying system. Embedding of such general purpose scripting languages instead of developing a new language for each application also had obvious benefits, relieving the application developer of the need to code a language translator from scratch and allowing the user to apply skills learned elsewhere.

Some software incorporates several different scripting languages. Modern web browsers typically provide a language for writing extensions to the browser itself, and several standard embedded languages for controlling the browser, including JavaScript (a dialect of ECMAScript) or XUL.

Types of scripting languages

Job control languages and shells

Main article: Shell script

A major class of scripting languages has grown out of the automation of job control, which relates to starting and controlling the behavior of system programs. (In this sense, one might think of shells as being descendants of IBM's JCL, or Job Control Language, which was used for exactly this purpose.) Many of these languages' interpreters double as command-line interpreters such as the Unix shell or the MS-DOS COMMAND.COM. Others, such as AppleScript offer the use of English-like commands to build scripts. This combined with Mac OS X's Cocoa frameworks allows user to build entire applications using AppleScript & Cocoa objects.

GUI scripting

With the advent of graphical user interfaces a specialized kind of scripting language emerged for controlling a computer. These languages interact with the same graphic windows, menus, buttons, and so on that a system generates. They do this by simulating the actions of a human user. These languages are typically used to automate user actions or configure a standard state. Such languages are also called "macros" when control is through simulated key presses or mouse clicks.

These languages could in principle be used to control any application running on a GUI-based computer; but, in practice, the support for such languages typically depends on the application and operating system. There are a few exceptions to this limitation. Some GUI scripting languages are based on recognizing graphical objects from their display screen pixels. These GUI scripting languages do not depend on support from the operating system or application.

Application-specific languages

Many large application programs include an idiomatic scripting language tailored to the needs of the application user. Likewise, many computer game systems use a custom scripting language to express the programmed actions of non-player characters and the game environment. Languages of this sort are designed for a single application; and, while they may superficially resemble a specific general-purpose language (e.g. QuakeC, modeled after C), they have custom features that distinguish them. Emacs Lisp, while a fully formed and capable dialect of Lisp, contains many special features that make it most useful for extending the editing functions of Emacs. An application-specific scripting language can be viewed as a domain-specific programming language specialized to a single application.

Web browsers

Main article: Client-side scripting

Web browsers are applications for displaying web pages. Scripts can be run by web browsers to change the appearance or behaviour of a web page, for example, to change the content to be specific to the current user. A host of special-purpose languages have been developed to control the operation of web browsers. These include JavaScript; VBScript by Microsoft, which only works in Internet Explorer; XUL by the Mozilla project, which only works in Firefox; and XSLT, a presentation language that transforms XML content into a new form. Techniques involving the combination of XML and JavaScript scripting to improve the user's impression of responsiveness have become significant enough to acquire a name: AJAX. Client-side scripts are sent by the server "as-is" and are run by the client's computer. An example of client side scripting is a JavaScript alert box popping up when the user clicks a button on the web page.

Text processing languages

The processing of text-based records is one of the oldest uses of scripting languages. Scripts written for the Unix tools AWK, sed, and grep automate tasks that involve text-based configuration and log files. Of high importance here is the regular expression, a language developed for the formal description of the lexical structure of text, and used by all of these tools.

Perl was originally designed to overcome limitations of these tools and has grown to be one of the most widespread general purpose languages.

General-purpose dynamic languages

See also: Dynamic programming language

Some languages, such as Perl, began as scripting languages^[3] but were developed into programming languages suitable for broader purposes. Other similar languages – frequently interpreted, memory-managed, or dynamic – have been described as "scripting languages" for these similarities, even if they are more commonly used for applications programming. They are usually not called "scripting languages" by their own users.

Extension/embeddable languages

A number of languages have been designed for the purpose of replacing application-specific scripting languages by being embeddable in application programs. The application programmer (working in C or another systems language) includes "hooks" where the scripting language can control the application. These languages serve the same purpose as application-specific extension languages but with the advantage of allowing some transfer of skills from application to application. JavaScript began as and primarily still is a language for scripting inside web browsers; however, the standardization of the language as ECMAScript has made it popular as a general purpose embeddable language. In particular, the Mozilla implementation SpiderMonkey is embedded in several environments such as the Yahoo! Widget Engine. Other applications embedding ECMAScript implementations include the Adobe products Adobe Flash (ActionScript) and Adobe Acrobat (for scripting PDF files).

Tcl was created as an extension language but has come to be used more frequently as a general purpose language in roles similar to Python, Perl, and Ruby.

Other complex and task-oriented applications may incorporate and expose an embedded programming language to allow their users more control and give them more functionality than can be available through a user interface, no matter how sophisticated. For example, Autodesk Maya 3D authoring tools embed the MEL scripting language, or Blender which has Python to fill this role.

Some other types of applications that need faster feature addition or tweak-and-run cycles (e.g. game engines) also use an embedded language. During the development, this allows them to prototype features faster and tweak more freely, without the need for the user to have intimate knowledge of the inner workings of the application or to rebuild it after each tweak (which can take a significant amount of time.) The scripting languages used for this purpose range from the more common and more famous Lua and Python to lesser-known ones such as AngelScript and Squirrel.

Market analysis

According to a global survey performed by Evans Data in 2008 [1], the most widespread scripting language is JavaScript. The second most widespread is PHP. Perl is the third most widespread scripting language, but in North America it enjoys significantly more popularity.^[4]

See also

• Architecture description language
• Build automation
• Domain-specific language
• Glue code
• Interpreted language
• Interpreter directive
• List of programming languages
• Macro and preprocessor languages
• Ousterhout's dichotomy
• Programming in the large and programming in the small
• Shebang (Unix)
• Shell (computing)
• System programming
• Web template languages

References

1. ^ IEEE Computer, 2008, In praise of scripting, Ronald Loui author
2. ^ Brown, Vicki. ""Scripting Languages"". http://www.mactech.com/articles/mactech/Vol.15/15.09/ScriptingLanguages/index.html. Retrieved 2009-07-22. 
3. ^ Sheppard, Doug (2000-10-16). "Beginner's Introduction to Perl". dev.perl.org. http://www.perl.com/pub/2000/10/begperl1.html. Retrieved 2011-01-08. 
4. ^ DIE, JavaScript, Ruby, Perl, Python, and Tcl Today: The State of the Scripting Universe - CIO.com

External links

• Patterns for Scripted Applications
• A study of the Script-Oriented Programming (SOP) suitability of selected languages — from The Scriptometer
• A Slightly Skeptical View on Scripting Languages by Dr. Nikolai Bezroukov
• Rob van der Woude's Scripting Pages — Administrative scripting related information (includes examples)
• Are Scripting Languages Any Good? A Validation of Perl, Python, Rexx, and Tcl against C, C++, and Java (PDF) — 2003 study
• Use of VBScript in QTP automation
• Scripting on the Java platform — JavaWorld
• "Programming is Hard - Let's Go Scripting" by Larry Wall - Perl.com transcript of his State of the Onion speech.