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...
A shell is a piece of software that provides an interface for users of an operating system which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web...
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
In software development and domain engineering, a domain-specific language is a programming language or specification language dedicated to a particular problem domain, a particular problem representation technique, and/or a particular solution technique...
. Typical operations performed by shell scripts include file manipulation, program execution, and printing text.
Many shell script interpreters double as command line interface, such as the various Unix shells, Windows PowerShell
Windows PowerShell
Windows PowerShell is Microsoft's task automation framework, consisting of a command-line shell and associated scripting language built on top of, and integrated with the .NET Framework...
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
AppleScript is a scripting language created by Apple Inc. and built into Macintosh operating systems since System 7. The term "AppleScript" may refer to the scripting system itself, or to particular scripts that are written in the AppleScript language....
The Microsoft Windows Script Host is an automation technology for Microsoft Windows operating systems that provides scripting capabilities comparable to batch files, but with a greater range of supported features...
(WScript.exe), add scripting capability to computing environments without requiring a command line interface. Other examples of programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s primarily intended for shell scripting include DCL
DIGITAL Command Language
DCL, the DIGITAL Command Language, is the standard command languageadopted by most of the operating systems that were sold by the former Digital Equipment Corporation...
Job Control Language is a scripting language used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem....
.
Shortcuts
In its most basic form, a shell script can provide a convenient variation of a system command where special environment settings, command options, or post-processing is applied automatically, but in a way that allows the new script to still act as a fully normal Unix command.
In computing, ls is a command to list files in Unix and Unix-like operating systems. ls is specified by POSIX and the Single UNIX Specification.- History :An ls utility appeared in the original version of AT&T UNIX...
, the command to list files, giving it a shorter command name of l, which would be normally saved in a user's bin directory as /home/username/bin/l, and a default set of command options pre-supplied.
In computing, a shebang is the character sequence consisting of the characters number sign and exclamation point , when it occurs as the first two characters on the first line of a text file...
indicates what interpreter should be used to execute the rest of the script, and the second line makes a listing with options for file format indicators, columns, all files (none omitted), and a size in blocks. The LC_COLLATE=C sets the default collation order to not fold upper and lower case together, and the "$@" causes any parameters given to l to be passed through as parameters to ls, so that all of the normal options and other syntax known to ls can still be used.
The user would then be able to simply use l for the most commonly used short listing.
Another example of a shell script that could be used as a shortcut would be to print a list of all the files or directory[s] within a given directory.
! /bin/sh
clear
ls -l -a
In this case, the shell script would start with its normal starting line of #! /bin/sh. Following this, the script executes the command clear which clears terminal of all text before going to the next line. The following line provides the main function of the script. The ls -l -a command list the files and directories that are in the directory that the script is being run in. The ls
Ls
In computing, ls is a command to list files in Unix and Unix-like operating systems. ls is specified by POSIX and the Single UNIX Specification.- History :An ls utility appeared in the original version of AT&T UNIX...
command attributes could be changed to reflect the needs of the user.
Batch jobs
Shell scripts allow several commands that would be entered manually at a command line interface to be executed automatically, and without having to wait for a user to trigger each stage of the sequence. For example, in a directory with three C source code files, rather than manually running the four commands required to build the final program from them, one could instead create a C shell
C shell
The C shell is a Unix shell that was created by Bill Joy while a graduate student at University of California, Berkeley in the late 1970s. It has been distributed widely, beginning with the 2BSD release of the BSD Unix system that Joy began distributing in 1978...
script, here named build and kept in the directory with them, which would compile them automatically:
!/bin/csh
echo compiling...
cc -c foo.c
cc -c bar.c
cc -c qux.c
cc -o myprog foo.o bar.o qux.o
echo done.
The script would allow a user to save the file being edited, pause the editor, and then just run ./build to create the updated program, test it, and then return to the editor. Since the 1980s or so, however, scripts of this type have been replaced with utilities like make which are specialized for building programs.
Generalization
Simple batch jobs are not unusual for isolated tasks, but using shell loops, tests, and variables provides much more flexibility to users. A Bash (Unix shell) script to convert JPEG images to PNG images, where the image names are provided on the command line - possibly via wildcards - instead of each being listed within the script, can be created with this file, typically saved in a file like /home/username/bin/jpg2png
!/bin/bash
for jpg in "$@" ; do # use $jpg in place of each filename given, in turn
png="${jpg%.jpg}.png" # find the PNG version of the filename by replacing .jpg with .png
echo converting "$jpg" ... # output status info to the user running the script
if convert "$jpg" jpg.to.png ; then # use the convert program (common in Linux) to create the PNG in a temp file
mv jpg.to.png "$png" # if it worked, rename the temporary PNG image to the correct name
else # ...otherwise complain and exit from the script
echo 'error: failed output saved in "jpg.to.png".' 1>&2
exit 1
fi # the end of the "if" test construct
done # the end of the "for" loop
echo all conversions successful # tell the user the good news
exit 0
The jpg2png command can then be run on an entire directory full of JPEG images with just jpg2png *.jpg
Verisimilitude
A key feature of shell scripts is that the invocation of their interpreters is handled as a core operating system feature. So rather than a user's shell only being able to execute scripts in that shell's language, or a script only having its interpreter directive
Interpreter directive
An interpreter directive is a computer language construct that is used to control which interpreter parses and interprets the instructions in a computer program.- See also :* Shebang * Bourne-Again Shell* C Shell...
handled correctly if it was run from a shell (both of which were limitations in the early Bourne shell's handling of scripts), shell scripts are set up and executed by the OS itself. A modern shell script is not just on the same footing as system commands, but rather many system commands are actually shell scripts (or more generally, scripts, since some of them are not interpreted by a shell, but instead by Perl
Perl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, or some other language). This extends to returning exit codes like other system utilities to indicate success or failure, and allows them to be called as components of larger programs regardless of how those larger tools are implemented.
Like standard system commands, shell scripts classically omit any kind of filename extension unless intended to be read into a running shell through a special mechanism for this purpose (such as sh’s “. ”, or csh’s source).
In computer software a general-purpose programming language is a programming language designed to be used for writing software in a wide variety of application domains...
s, such as control-flow constructs, variables, comments
Comment (computer programming)
In computer programming, a comment is a programming language construct used to embed programmer-readable annotations in the source code of a computer program. Those annotations are potentially significant to programmers but typically ignorable to compilers and interpreters. Comments are usually...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
s, and so on. With these sorts of features available, it is possible to write reasonably sophisticated applications as shell scripts. However, they are still limited by the fact that most shell languages have little or no support for data typing systems, classes, threading, complex math, and other common full language features, and are also generally much slower than compiled code or interpreted languages written with speed as a performance goal.
Other scripting languages
Many powerful scripting languages have been introduced for tasks that are too large or complex to be comfortably handled with ordinary shell scripts, but for which the advantages of a script are desirable and the development overhead of a full-blown, compiled programming language would be disadvantageous. The specifics of what separates scripting languages from high-level programming language
High-level programming language
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...
s is a frequent source of debate. But generally speaking a scripting language is one which requires an interpreter.
Life cycle
Shell scripts often serve as an initial stage in software development, and are often subject to conversion later to a different underlying implementation, most commonly being converted to Perl
Perl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
An interpreter directive is a computer language construct that is used to control which interpreter parses and interprets the instructions in a computer program.- See also :* Shebang * Bourne-Again Shell* C Shell...
allows the implementation detail to be fully hidden inside the script, rather than being exposed as a filename extension, and provides for seamless reimplementation in different languages with no impact on end users.
Advantages and disadvantages
Often, writing a shell script is much quicker than writing the equivalent code in other programming languages. The many advantages include easy program or file selection, quick start, and interactive debugging. A shell script can be used to provide a sequencing and decision-making linkage around existing programs, and for moderately-sized scripts the absence of a compilation step is an advantage. Interpretive running makes it easy to write debugging code into a script and re-run it to detect and fix bugs. Non-expert users can use scripting to tailor the behavior of programs, and shell scripting provides some limited scope for multiprocessing.
On the other hand, shell scripting is prone to costly errors. Inadvertent typing errors such as rm
Rm (Unix)
rm is a basic UNIX command used to remove objects such as files, directories, device nodes, symbolic links, and so on from the filesystem...
-rf * / (instead of the intended rm -rf */) are folklore in the Unix community; a single extra space converts the command from one that deletes everything in the sub-directories to one which deletes everything - and also tries to delete everything in the root directory
Root directory
In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree — the starting point where all branches originate.-Metaphor:...
cp is a UNIX command used to copy a file. Files can be copied either to the same directory or to a completely different directory, possibly on a different file system or hard disk drive. If the file is copied to the same directory, the new file must have a different name to the original; in all...
mv is a Unix command that moves one or more files or directories from one place to another. Since it can "move" files from one filename to another, it is also used to rename files. Using mv requires the user to have write permission for the directories which the file will move between...
into dangerous weapons, and misuse of the > redirect can delete the contents of a file. This is made more problematic by the fact that many UNIX commands differ in name by only one letter: cp, cd, dd
Dd (Unix)
In computing, dd is a common Unix program whose primary purpose is the low-level copying and conversion of raw data. According to the manual page for Version 7 Unix, it will "convert and copy a file". It is used to copy a specified number of bytes or blocks, performing on-the-fly byte order...
df is a standard Unix computer program used to display the amount of available disk space for filesystems on which the invoking user has appropriate read access...
, etc.
Another significant disadvantage is the slow execution speed and the need to launch a new process for almost every shell command executed. When a script's job can be accomplished by setting up a pipeline in which efficient filter
Filter (software)
A filter is a computer program to process a data stream. Some operating systems such as Unix are rich with filter programs. Even Windows has some simple filters built into its command shell, most of which have significant enhancements relative to the similar filter commands that were available in...
commands perform most of the work, the slowdown is mitigated, but a complex script is typically several orders of magnitude slower than a conventional compiled program that performs an equivalent task.
There are also compatibility problems between different platforms. Larry Wall
Larry Wall
Larry Wall is a programmer and author, most widely known for his creation of the Perl programming language in 1987.-Education:Wall earned his bachelor's degree from Seattle Pacific University in 1976....
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
, famously wrote that "It is easier to port a shell than a shell script."
Similarly, more complex scripts can run into the limitations of the shell scripting language itself; the limits make it difficult to write quality code and extensions by various shells to ameliorate problems with the original shell language can make problems worse.
Many disadvantages of using some script languages are caused by design flaws within the language syntax or implementation, and are not necessarily imposed by the use of a text-based command line; there are a number of shells which use other shell programming languages or even full-fledged languages like Scsh
Scsh
Scsh is a POSIX API layered on top of the Scheme programming language in a manner to make the most of Scheme's capability for scripting. It is limited to 32-bit platforms but there is a development version against the latest scheme48 that works in 64bit mode.....
In programming, glue code is code that does not contribute any functionality towards meeting the program's requirements, but instead serves solely to "glue together" different parts of code that would not otherwise be compatible...
An interpreter directive is a computer language construct that is used to control which interpreter parses and interprets the instructions in a computer program.- See also :* Shebang * Bourne-Again Shell* C Shell...
In computing, a shebang is the character sequence consisting of the characters number sign and exclamation point , when it occurs as the first two characters on the first line of a text file...
A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...
Windows PowerShell is Microsoft's task automation framework, consisting of a command-line shell and associated scripting language built on top of, and integrated with the .NET Framework...
The Microsoft Windows Script Host is an automation technology for Microsoft Windows operating systems that provides scripting capabilities comparable to batch files, but with a greater range of supported features...
