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Script (computer programming)

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Script (computer programming)

A scripting language or script language is a programming language that supports scripts, programs written for a special run-time environment that can interpret (rather than compile) and automate the execution of tasks which could alternatively be executed one-by-one by a human operator. Environments that can be automated through scripting include software applications, web pages within a web browser, the shells of operating systems (OS), and embedded systems. A scripting language can be viewed as a domain-specific language for a particular environment; in the case of scripting an application, this is also known as an extension language. Scripting languages are also sometimes referred to as very high-level programming languages, as they operate at a high level of abstraction.

The term "scripting language" is also used loosely to refer to dynamic high-level general-purpose language, such as Perl,[1] Tcl, and Python,[2] with the term "script" often used for small programs (up to a few thousand lines of code) in such languages, or in domain-specific languages such as the text-processing languages sed and AWK. Some of these languages were originally developed for use within a particular environment, and later developed into portable domain-specific or general-purpose languages. Conversely, many general-purpose languages have dialects that are used as scripting languages. This article discusses scripting languages in the narrow sense of languages for a specific environment; dynamic, general-purpose, and high-level languages are discussed at those articles.

The spectrum of scripting languages ranges from very small and highly domain-specific languages to general-purpose programming languages used for scripting. Standard examples of scripting languages for specific environments include: bash, for the Unix or Unix-like operating systems; ECMAScript (JavaScript), for web browsers; and Visual Basic for Applications, for Microsoft Office applications. Lua is a language designed and widely used as an extension language. Python is a general-purpose language that is also commonly used as an extension language, while ECMAScript is still primarily a scripting language for web browsers, but is also used as a general-purpose language. The Emacs Lisp dialect of Lisp (for the Emacs editor) and the Visual Basic for Applications dialect of Visual Basic are examples of scripting language dialects of general-purpose languages. Some game systems, notably the Trainz franchise of Railroad simulators have been extensively extended in functionality by scripting extensions.

Characteristics

In principle any language can be used as a scripting language, given libraries or bindings for a specific environment. Formally speaking, "scripting" is a property of the primary implementations and uses of a language, hence the ambiguity about whether a language "is" a scripting language for languages with multiple implementations. However, many languages are not very suited for use as scripting languages and are rarely if ever used as such.

Typically scripting languages are intended to be very fast to pick up and author programs in. This generally implies relatively simple syntax and semantics. For example, it is uncommon to use Java as a scripting language due to the lengthy syntax and restrictive rules about which classes exist in which files – contrast to Python, where it is possible to briefly define some functions in a file. A scripting language is usually interpreted from source code or bytecode.[3] By contrast, the software environment the scripts are written for is typically written in a compiled language and distributed in machine code form. Scripting languages may be designed for use by end users of a program – end-user development – or may be only for internal use by developers, so they can write portions of the program in the scripting language.

Scripts are often created or modified by the person executing them,[4] though they are also often distributed, such as when large portions of games are written in a scripting language. In many implementations a script or portions of one may be executed interactively on a command line.

History

Early mainframe computers (in the 1950s) were non-interactive, instead using batch processing. IBM's Job Control Language (JCL) is the archetype of languages used to control batch processing.[5]

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. Calvin Mooers in his TRAC language is generally credited with inventing command substitution, the ability to embed commands in scripts that when interpreted insert a character string into the script.[6] Multics calls these active functions.[7] Louis Pouzin wrote an early processor for command scripts called RUNCOM for CTSS around 1964. Stuart Madnick at MIT wrote a scripting language for IBM's CP/CMS in 1966. He originally called this processor COMMAND, later named EXEC.[8] Multics included an offshoot of CTSS RUNCOM, also called RUNCOM.[9]

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.

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 human user would. They do this by simulating the actions of a user. These languages are typically used to automate user actions. 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 GUI application; but, in practice their use is limited because their use needs support from the application and from the 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.

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 may be technically equivalent to an application-specific extension language but when an application embeds a "common" language, the user gets the advantage of being able to transfer skills from application to application. A more generic alternative is simply to provide a library (often a C library) which a general-purpose language can use to control the application, without modifying the language for the specific domain.

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

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.

Ch is another C compatible scripting option for the industry to embed into C/C++ application programs.

Glue language

Main article: Glue language

Scripting is often contrasted with system programming, as in Ousterhout's dichotomy or "programming in the large and programming in the small". In this view, scripting is particularly glue code, connecting system components, and a language specialized for this purpose is a glue language. Pipelines and shell scripting are archetypal examples of glue languages, and Perl was initially developed to fill this same role. The characterization of glue languages as scripting languages is ambiguous, however, as if a substantial amount of logic is part of the "glue" code, it is better characterized as simply another software component.

See also

References

External links

  • Patterns for Scripted Applications

Template:Programming language

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