A shell script is a
computer program
A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components.
A computer progra ...
designed to be run by a
Unix shell
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system t ...
, a
command-line interpreter
A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive command (computing), commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invokin ...
. The various dialects of shell scripts are considered to be
scripting language
A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled.
A scripti ...
s. 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.
The term is also used more generally to mean the automated mode of running an operating system shell; each operating system uses a particular name for these functions including batch files (MSDos-Win95 stream,
OS/2
OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
), command procedures (VMS), and shell scripts (
Windows NT
Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system.
The first version of Wi ...
stream and third-party derivatives like
4NT—article is at
cmd.exe), and mainframe operating systems are associated with a number of terms.
Shells commonly present in Unix and Unix-like systems include the
Korn shell, the
Bourne shell
The Bourne shell (sh) is a shell command-line interpreter for computer operating systems.
The Bourne shell was the default shell for Version 7 Unix. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link ...
, and
GNU Bash. While a Unix operating system may have a different default shell, such as
Zsh on
macOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
, these shells are typically present for backwards compatibility.
Capabilities
Comments
Comments
Comment may refer to:
* Comment (linguistics) or rheme, that which is said about the topic (theme) of a sentence
* Bernard Comment (born 1960), Swiss writer and publisher
Computing
* Comment (computer programming), explanatory text or informa ...
are ignored by the shell. They typically begin with the hash symbol (
#
), and continue until the end of the line.
[Johnson, Chris (2009)]
''Pro Bash Programming: Scripting the Linux Shell'', Apress, Retrieved on September 27, 2019.
Configurable choice of scripting language
The
Shebang (Unix), shebang, or hash-bang, is a special kind of comment which the system uses to determine what interpreter to use to execute the file. The shebang must be the first line of the file, and start with "
#!
".
In Unix-like operating systems, the characters following the "
#!
" prefix are interpreted as a path to an executable program that will interpret the script.
Shortcuts
A shell script can provide a convenient variation of a system command where special environment settings, command options, or post-processing apply automatically, but in a way that allows the new script to still act as a fully normal
Unix command.
One example would be to create a version of
ls, 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.
#!/bin/sh
LC_COLLATE=C ls -FCas "$@"
Here, the first line uses a
shebang to indicate which interpreter should execute the rest of the script, and the second line makes a listing with options for file format indicators, columns, all files (none omitted), and a size in blocks. The
LC_COLLATE=C
sets the default collation order to not fold upper and lower case together, not intermix
dotfile
In computing, a hidden folder (sometimes hidden directory) or hidden file is a folder or file which filesystem utilities do not display by default when showing a directory listing. They are commonly used for storing user preferences or preservin ...
s with normal filenames as a side effect of ignoring punctuation in the names (dotfiles are usually only shown if an option like
-a
is used), and the
"$@"
causes any parameters given to
l
to pass through as parameters to ls, so that all of the normal options and other
syntax known to ls can still be used.
The user could then 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 and directories within a given directory.
#!/bin/sh
clear
ls -al
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 the terminal of all text before going to the next line. The following line provides the main function of the script. The
ls -al command lists the files and directories that are in the directory from which the script is being run. The
ls command attributes could be changed to reflect the needs of the user.
Note: If an implementation does not have the
clear command, try using the
clr command instead.
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 script for
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
-compliant shells, here named
build
and kept in the directory with them, which would compile them automatically:
#!/bin/sh
printf 'compiling...\n'
cc -c foo.c
cc -c bar.c
cc -c qux.c
cc -o myprog foo.o bar.o qux.o
printf 'done.\n'
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 POSIX sh 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/sh
for jpg; do # use $jpg in place of each filename given, in turn
png=$.png # construct the PNG version of the filename by replacing .jpg with .png
printf 'converting "%s" ...\n' "$jpg" # output status info to the user running the script
if convert "$jpg" jpg.to.png; then # use convert (provided by ImageMagick) 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
printf >&2 'jpg2png: error: failed output saved in "jpg.to.png".\n'
exit 1
fi # the end of the "if" test construct
done # the end of the "for" loop
printf 'all conversions successful\n' # tell the user the good news
The
jpg2png
command can then be run on an entire directory full of JPEG images with just
/home/''username''/bin/jpg2png *.jpg
Programming
Many modern shells also supply various features usually found only in more sophisticated
general-purpose programming language
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 is used within a specific area. For ex ...
s, such as control-flow constructs, variables,
comments
Comment may refer to:
* Comment (linguistics) or rheme, that which is said about the topic (theme) of a sentence
* Bernard Comment (born 1960), Swiss writer and publisher
Computing
* Comment (computer programming), explanatory text or informa ...
, arrays,
subroutine
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions ma ...
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.
The standard Unix tools
sed and
awk provide extra capabilities for shell programming;
Perl
Perl is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
can also be embedded in shell scripts as can other scripting languages like
Tcl. Perl and Tcl come with graphics toolkits as well.
Typical POSIX scripting languages
Scripting languages commonly found on UNIX, Linux, and POSIX-compliant operating system installations include:
*
KornShell
KornShell (ksh) is a Unix shell which was developed by David Korn at Bell Labs in the early 1980s and announced at USENIX on July 14, 1983. The initial development was based on Bourne shell source code. Other early contributors were Bell ...
(
ksh
) in several possible versions such as ksh88, Korn Shell '93 and others.
* The
Bourne shell
The Bourne shell (sh) is a shell command-line interpreter for computer operating systems.
The Bourne shell was the default shell for Version 7 Unix. Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link ...
(
sh
), one of the oldest shells still common in use
* The
C shell (
csh
)
*
GNU Bash (
bash
)
*
tclsh
, a shell which is a main component of the
Tcl/Tk programming language.
* The
wish
A wish is a hope or desire for something. In fiction, wishes can be used as plot devices. In folklore, opportunities for "making a wish" or for wishes to "come true" or "be granted" are themes that are sometimes used.
In fiction
In ficti ...
is a GUI-based Tcl/Tk shell.
The C and Tcl shells have syntax quite similar to that of said programming languages, and the Korn shells and Bash are developments of the Bourne shell, which is based on the
ALGOL
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by th ...
language with elements of a number of others added as well. On the other hand, the various shells plus tools like
awk,
sed,
grep
grep is a command-line utility for searching plain-text data sets for lines that match a regular expression. Its name comes from the ed command ''g/re/p'' (''globally search for a regular expression and print matching lines''), which has the sa ...
, and
BASIC
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
,
Lisp
A lisp is a speech impairment in which a person misarticulates sibilants (, , , , , , , ). These misarticulations often result in unclear speech.
Types
* A frontal lisp occurs when the tongue is placed anterior to the target. Interdental lispi ...
,
C and so forth contributed to the
Perl
Perl is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
programming language.
Other shells that may be available on a machine or for download and/or purchase include:
*
Almquist shell (
ash
)
*
PowerShell (
msh
)
*
Z shell (
zsh
, a particularly common enhanced KornShell)
* The
Tenex C Shell (
tcsh
).
Related programs such as shells based on
Python,
Ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapp ...
,
C,
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
,
Perl
Perl is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
,
Pascal,
Rexx
Rexx (Restructured Extended Executor) 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. ...
etc. in various forms are also widely available. Another somewhat common shell is
Old shell (
osh
), whose manual page states it "is an enhanced, backward-compatible port of the standard command interpreter from Sixth Edition UNIX."
So called remote shells such as
* a
Remote Shell (
rsh
)
* a
Secure Shell
The Secure Shell Protocol (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution.
SSH applications are based ...
(
ssh
)
are really just tools to run a more complex shell on a remote system and have no 'shell' like characteristics themselves.
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
In computer science, a high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language ''elements'', be easier to ...
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 is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
,
Python, or
C. The
interpreter directive 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.
While files with the ".sh"
file extension are usually a shell script of some kind, most shell scripts do not have any filename extension.
Advantages and disadvantages
Perhaps the biggest advantage of writing a shell script is that the commands and syntax are exactly the same as those directly entered at the command-line. The programmer does not have to switch to a totally different syntax, as they would if the script were written in a different language, or if a compiled language were used.
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 -rf * /
(instead of the intended
rm -rf */
) are folklore in the Unix community; a single extra space converts the command from one that deletes all subdirectories contained in the current directory, to one which deletes everything from the file system's
root directory
In a computer file system, and primarily used in the Unix and Unix-like operating systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the trunk of a tree, as the starting point where all branche ...
. Similar problems can transform
cp
and
mv
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
,
df
, 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 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 Arnold Wall (born September 27, 1954) is an American computer programmer and author. He created the Perl programming language.
Personal life
Wall grew up in Los Angeles and then Bremerton, Washington, before starting higher education at ...
, creator of
Perl
Perl is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
, 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 (which uses
Scheme).
Interoperability among scripting languages
Different scripting languages may share many common elements, largely due to being POSIX based, and some shells offer modes to emulate different shells. This allows a shell script written in one scripting language to be adapted into another.
One example of this is Bash, which offers the same grammar and syntax as the Bourne shell, and which also provides a POSIX-compliant mode. As such, most shell scripts written for the Bourne shell can be run in BASH, but the reverse may not be true since BASH has extensions which are not present in the Bourne shell. As such, these features are known as
bashisms.
Shell scripting on other operating systems
Interoperability software such as
Cygwin, the
MKS Toolkit,
Interix (which is available in the Microsoft Windows Services for UNIX),
Hamilton C shell,
UWIN (AT&T Unix for Windows) and others allow Unix shell programs to be run on machines running Windows NT and its successors, with some loss of functionality on the
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few oper ...
-
Windows 95
Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufactu ...
branch, as well as earlier MKS Toolkit versions for OS/2. At least three DCL implementations for Windows type operating systems—in addition to
XLNT, a multiple-use scripting language package which is used with the command shell,
Windows Script Host and
CGI programming—are available for these systems as well. Mac OS X and subsequent are Unix-like as well.
In addition to the aforementioned tools, some
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
and OS/2 functionality can be used with the corresponding environmental subsystems of the Windows NT operating system series up to Windows 2000 as well. A third,
16-bit
16-bit microcomputers are microcomputers that use 16-bit microprocessors.
A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mos ...
subsystem often called the MS-DOS subsystem uses the Command.com provided with these operating systems to run the aforementioned MS-DOS batch files.
[Windows NT 4 Workstation Resource Kit]
The console alternatives
4DOS
4DOS is a command-line interpreter by JP Software, designed to replace the default command interpreter COMMAND.COM in Microsoft DOS and Windows. It was written by Rex C. Conn and Tom Rawson and first released in 1989. Compared to the default ...
,
4OS2,
FreeDOS
FreeDOS (formerly Free-DOS and PD-DOS) is a free software operating system for IBM PC compatible computers. It intends to provide a complete MS-DOS-compatible environment for running legacy software and supporting embedded systems.
FreeDOS ca ...
,
Peter Norton's
NDOS and
4NT / Take Command which add functionality to the Windows NT-style cmd.exe, MS-DOS/Windows 95 batch files (run by Command.com), OS/2's cmd.exe, and 4NT respectively are similar to the shells that they enhance and are more integrated with the Windows Script Host, which comes with three pre-installed engines, VBScript,
JScript
JScript is Microsoft's legacy software, legacy dialect of the ECMAScript standard that is used in Microsoft's Internet Explorer 11 and older.
JScript is implemented as an Active Scripting engine. This means that it can be "plugged in" to OLE Aut ...
, and
VBA and to which numerous third-party engines can be added, with Rexx, Perl, Python, Ruby, and Tcl having pre-defined functions in 4NT and related programs.
PC DOS is quite similar to MS-DOS, whilst
DR DOS is more different. Earlier versions of Windows NT are able to run contemporary versions of 4OS2 by the OS/2 subsystem.
Scripting languages are, by definition, able to be extended; for example, a MS-DOS/Windows 95/98 and Windows NT type systems allows for shell/batch programs to call tools like
KiXtart,
QBasic, various
BASIC
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
,
Rexx
Rexx (Restructured Extended Executor) 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. ...
,
Perl
Perl is a family of two High-level programming language, high-level, General-purpose programming language, general-purpose, Interpreter (computing), interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it ...
, and
Python implementations, the
Windows Script Host and its installed engines. On Unix and other
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
-compliant systems,
awk and
sed are used to extend the string and numeric processing ability of shell scripts.
Tcl, Perl, Rexx, and Python have graphics toolkits and can be used to code functions and procedures for shell scripts which pose a speed bottleneck (C, Fortran, assembly language &c are much faster still) and to add functionality not available in the shell language such as sockets and other connectivity functions, heavy-duty text processing, working with numbers if the calling script does not have those abilities, self-writing and self-modifying code, techniques like
recursion
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathematic ...
, direct memory access, various types of
sorting and more, which are difficult or impossible in the main script, and so on.
Visual Basic for Applications
Visual Basic for Applications (VBA) is an implementation of Microsoft's event-driven programming language Visual Basic 6.0 built into most desktop Microsoft Office applications. Although based on pre-.NET Visual Basic, which is no longer supporte ...
and
VBScript can be used to control and communicate with such things as spreadsheets, databases, scriptable programs of all types, telecommunications software, development tools, graphics tools and other software which can be accessed through the
Component Object Model.
See also
*
Glue code
*
Interpreter directive
*
Shebang symbol (#!)
*
Unix shell
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system t ...
s
*
PowerShell
*
Windows Script Host
References
External links
''An Introduction To Shell Programming'' by Greg Goebel''UNIX / Linux shell scripting tutorial'' by Steve Parker''Shell Scripting Primer'' (Apple)''What to watch out for when writing portable shell scripts'' by Peter SeebachBeginners/BashScripting Ubuntu Linux
{{Programming languages
Scripting languages