In computing, a script is a relatively short and simple set of instructions that typically automate an otherwise manual process. The act of writing a script is called scripting. Scripting language or script language describes a programming language that it is used for a script.[1]
Although any language could be used for scripting, in practice languages tend to either be used for scripting or for application development. But, some are used for both.
Overview
A scripting language can be a general purpose language or a domain-specific language for a particular environment. When embedded in an application, it may be called an extension language.
A scripting language is sometimes referred to as very high-level programming language if it operates at a high level of abstraction, or as a control language, particularly for job control languages on mainframes.
The term scripting language is sometimes used in a wider sense, to refer to dynamichigh-level programming languages in general. Some are strictly interpreted languages, while others use a form of compilation. In this context, the term script refers to a small program in such a language; typically, contained in a single file, and no larger than a few thousand lines of code.
The scope of scripting languages ranges from small to large, and from highly domain-specific language to general-purpose programming languages. A language may start as small and highly domain-specific and later develop into a portable and general-purpose language; conversely, a general-purpose language may later develop special domain-specific dialects.
JavaScript (later: ECMAScript), originally limited to running in a web browser to dynamically modify a web page; later enhanced into a widely portable, general-purpose programming language
Script is a subjective characterization that generally includes the following attributes.
Short & simple
A script is generally relatively short and simple. As there is no limit on size or complexity, script is subjective. A few lines of code without branching is probably considered a script. A codebase of multiple files, that performs sophisticated user or hardware interface or complicated algorithms or multiprogramming is probably not considered a script.
Automates
A script usually automates a task that would otherwise be performed by a person in a more manual way.
Limited language
A language that is primarily intended for scripting generally has limited capabilities compared to a general-purpose language. A scripting language my lack the functionality to write complex applications.
Starts at the top
Typically, a script starts executing at the first line of code whereas an application typically starts at a special point in the code called the entry point.
For example, Java is not script-like since an application starts at the function named main which need not be at the top of the code. The following code starts at main, then calls printHelloWorld which prints "Hello World".
Scripts are often created or modified by the person executing them,[5] but they are also often distributed, such as when large portions of games are written in a scripting language, notably the Google Chrome T-rex game.
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.[7]Multics calls these active functions.[8]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.[9] Multics included an offshoot of CTSS RUNCOM, also called RUNCOM.[10]EXEC was eventually replaced by EXEC 2 and REXX.
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
Scripting languages can be categorized into several different types, with a considerable degree of overlap among the types.
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 glue code, connecting software 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. Web development can be considered a use of glue languages, interfacing between a database and web server. But if a substantial amount of logic is written in script, it is better characterized as simply another software component, not "glue".
Glue languages are especially useful for writing and maintaining:
smaller programs than those that are better implemented in a compiled language;[12]
"wrapper" programs for executables, like a batch file that moves or manipulates files and does other things with the operating system before or after running an application like a word processor, spreadsheet, data base, assembler, compiler, etc.;[13]
Macro languages exposed to operating system or application components can serve as glue languages. These include Visual Basic for Applications, WordBasic, LotusScript, CorelScript, Hummingbird Basic, QuickScript, Rexx, SaxBasic, and WinWrap Basic. Other tools like AWK can also be considered glue languages, as can any language implemented by a Windows Script Host engine (VBScript, JScript and VBA by default in Windows and third-party engines including implementations of Rexx, Perl, Tcl, Python, XSLT, Ruby, Modern Pascal, Delphi, and C). A majority of applications can access and use operating system components via the object models or its own functions.
Other devices like programmable calculators may also have glue languages; the operating systems of PDAs such as Windows CE may have available native or third-party macro tools that glue applications together, in addition to implementations of common glue languages—including Windows NT, DOS, and some Unix shells, Rexx, Modern Pascal, PHP, and Perl. Depending upon the OS version, WSH and the default script engines (VBScript and JScript) are available.
Programmable calculators can be programmed in glue languages in three ways. For example, the Texas InstrumentsTI-92, by factory default can be programmed with a command script language. Inclusion of the scripting and glue language Lua in the TI-NSpire series of calculators could be seen as a successor to this. The primary on-board high-level programming languages of most graphing calculators (most often Basic variants, sometimes Lisp derivatives, and more uncommonly, C derivatives) in many cases can glue together calculator functions—such as graphs, lists, matrices, etc. Third-party implementations of more comprehensive Basic version that may be closer to variants listed as glue languages in this article are available—and attempts to implement Perl, Rexx, or various operating system shells on the TI and HP graphing calculators are also mentioned. PC-based C cross-compilers for some of the TI and HP machines used with tools that convert between C and Perl, Rexx, AWK, and shell scripts to Perl, Modern Pascal, VBScript to and from Perl make it possible to write a program in a glue language for eventual implementation (as a compiled program) on the calculator.[citation needed]
Editor languages
A number of text editors support macros written either using a macro language built into the editor, e.g., The SemWare Editor (TSE), vi improved (VIM), or using an external implementation, e.g., XEDIT, or both, e.g., KEDIT. Sometimes text editors and edit macros are used under the covers to provide other applications, e.g., FILELIST and RDRLIST in CMS .
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[15] (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, as well as tapping or pressing on a touch-activated screen.
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.
When the GUI provides the appropriate interfaces, as in the IBM Workplace Shell, a generic scripting language, e.g. OREXX, can be used for writing GUI scripts.
Application-specific languages
Application specific languages can be split in many different categories, i.e. standalone based app languages (executable) or internal application specific languages (postscript, xml, gscript as some of the widely distributed scripts, respectively implemented by Adobe, MS and Google) among others 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.
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) that 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 Python, Perl, and Ruby. On the other hand, Rexx was originally created as a job control language, but is widely used as an extension language as well as a general-purpose language. Perl is a general-purpose language, but had the Oraperl (1990) dialect, consisting of a Perl 4 binary with Oracle Call Interface compiled in. This has however since been replaced by a library (Perl Module), DBD::Oracle.[16][17]
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 Maya Embedded Language, or Blender which uses 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.
In computer programming, a macro is a rule or pattern that specifies how a certain input should be mapped to a replacement output. Applying a macro to an input is known as macro expansion. The input and output may be a sequence of lexical tokens or characters, or a syntax tree. Character macros are supported in software applications to make it easy to invoke common command sequences. Token and tree macros are supported in some programming languages to enable code reuse or to extend the language, sometimes for domain-specific languages.
A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text. A script which sets up the environment, runs the program, and does any necessary cleanup or logging, is called a wrapper.
VBScript is a deprecated programming language for scripting on Microsoft Windows using Component Object Model (COM) based on classic Visual Basic and Active Scripting.
let foo = 42; // foo is now a number
The Microsoft Windows Script Host (WSH) is an automation technology for Microsoft Windows operating systems that provides scripting abilities comparable to batch files, but with a wider range of supported features. This tool was first provided on Windows 95 after Build 950a on the installation discs as an optional installation configurable and installable by means of the Control Panel, and then a standard component of Windows 98 and subsequent and Windows NT 4.0 Build 1381 and by means of Service Pack 4. The WSH is also a means of automation for Internet Explorer via the installed WSH engines from IE Version 3.0 onwards; at this time VBScript became means of automation for Microsoft Outlook 97. The WSH is also an optional install provided with a VBScript and JScript engine for Windows CE 3.0 and following and some third-party engines including Rexx and other forms of Basic are also available.
A graphing calculator is a handheld computer that is capable of plotting graphs, solving simultaneous equations, and performing other tasks with variables. Most popular graphing calculators are programmable calculators, allowing the user to create customized programs, typically for scientific, engineering or education applications. They have large screens that display several lines of text and calculations.
In some programming languages, eval, short for the English evaluate, is a function which evaluates a string as though it were an expression in the language, and returns a result; in others, it executes multiple lines of code as though they had been included instead of the line including the eval. The input to eval is not necessarily a string; it may be structured representation of code, such as an abstract syntax tree, or of special type such as code. The analog for a statement is exec, which executes a string as if it were a statement; in some languages, such as Python, both are present, while in other languages only one of either eval or exec is.
In computer software, a general-purpose programming language (GPL) is a programming language for building software in a wide variety of application domains. Conversely, a domain-specific programming language (DSL) is used within a specific area. For example, Python is a GPL, while SQL is a DSL for querying relational databases.
Expect is an extension to the Tcl scripting language written by Don Libes. The program automates interactions with programs that expose a text terminal interface. Expect, originally written in 1990 for the Unix platform, has since become available for Microsoft Windows and other systems.
Active Scripting is the technology used in Windows to implement component-based scripting support. It is based on OLE Automation and allows installation of additional scripting engines in the form of COM modules.
Programmable calculators are calculators that can automatically carry out a sequence of operations under control of a stored program. Most are Turing complete, and, as such, are theoretically general-purpose computers. However, their user interfaces and programming environments are specifically tailored to make performing small-scale numerical computations convenient, rather than general-purpose use.
In Microsoft Windows applications programming, OLE Automation is an inter-process communication mechanism created by Microsoft. It is based on a subset of Component Object Model (COM) that was intended for use by scripting languages – originally Visual Basic – but now is used by several languages on Windows. All automation objects are required to implement the IDispatch interface. It provides an infrastructure whereby applications called automation controllers can access and manipulate shared automation objects that are exported by other applications. It supersedes Dynamic Data Exchange (DDE), an older mechanism for applications to control one another. As with DDE, in OLE Automation the automation controller is the "client" and the application exporting the automation objects is the "server".
Tk is a cross-platform widget toolkit that provides a library of basic elements of GUI widgets for building a graphical user interface (GUI) in many programming languages. It is free and open-source software released under a BSD-style software license.
Rexx is a programming language that can be interpreted or compiled. It was developed at IBM by Mike Cowlishaw. It is a structured, high-level programming language designed for ease of learning and reading. Proprietary and open source Rexx interpreters exist for a wide range of computing platforms; compilers exist for IBM mainframe computers.
A command-line interface (CLI) is a means of interacting with a computer program by inputting lines of text called command-lines. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user-friendly alternative to the non-interactive interface available with punched cards.
The following outline is provided as an overview of and topical guide to the Perl programming language:
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