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 Execution (computing), execute. Computer programs are one component of software, which also includes software documentation, documentation and oth ...
designed to be run by a
Unix shell, a
command-line interpreter. 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 scripting ...
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), 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 Win ...
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, 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 computers. Within the market of desktop and la ...
, these shells are typically present for backwards compatibility.
Capabilities
Comments
Comments 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 "$@"
Here, the first line uses a
shebang to indicate which interpreter should execute the rest of the script, and the second line makes a listing with options for file format indicators, columns, all files (none omitted), and a size in blocks. The
LC_COLLATE=C
sets the default collation order to not fold upper and lower case together, not intermix
dotfiles with normal filenames as a side effect of ignoring punctuation in the names (dotfiles are usually only shown if an option like
-a
is used), and the
"$@"
causes any parameters given to
l
to pass through as parameters to ls, so that all of the normal options and other
syntax
In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituenc ...
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 in ...
-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 languages, such as control-flow constructs, variables,
comments, 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 may ...
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, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
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 (
ksh
) in several possible versions such as ksh88, Korn Shell '93 and others.
* The
Bourne shell (
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 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 language with elements of a number of others added as well. On the other hand, the various shells plus tools like
awk,
sed,
grep, and
BASIC,
Lisp,
C and so forth contributed to the
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
programming language.
Other shells that may be available on a machine or for download and/or purchase include:
*
Almquist shell
Almquist shell (also known as A Shell, ash and sh) is a lightweight Unix shell originally written by Kenneth Almquist in the late 1980s. Initially a clone of the System V.4 variant of the Bourne shell, it replaced the original Bourne shell in th ...
(
ash
)
*
PowerShell (
msh
)
*
Z shell (
zsh
, a particularly common enhanced KornShell)
* The
Tenex C Shell
tcsh ( “tee-see-shell”, “tee-shell”, or as “tee see ess aitch”, tcsh) is a Unix shell based on and backward compatible with the C shell (csh).
Shell
It is essentially the C shell with programmable command-line completion, comm ...
(
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 ...
,
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, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
,
Pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
,
Rexx 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
The remote shell (rsh) is a command line computer program that can execute shell commands as another user, and on another computer across a computer network.
The remote system to which ''rsh'' connects runs the ''rsh'' daemon (rshd). The daem ...
(
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 u ...
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, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
,
Python, or
C. The
interpreter directive
An interpreter directive is a computer language construct, that on some systems is better described as an aspect of the system's executable file format, that is used to control which interpreter parses and interprets the instructions in a compu ...
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. 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, creator of
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
, 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 (a ''Scheme shell'') is computer software, a type of shell for an operating system. It is a Portable Operating System Interface ( POSIX) application programming interface (API) layered on the programming language Scheme, in a manner to mak ...
(which uses
Scheme A scheme is a systematic plan for the implementation of a certain idea.
Scheme or schemer may refer to:
Arts and entertainment
* ''The Scheme'' (TV series), a BBC Scotland documentary series
* The Scheme (band), an English pop band
* ''The Schem ...
).
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
MKS Toolkit is a software package produced and maintained by PTC that provides a Unix-like environment for scripting, connectivity and porting Unix and Linux software to Microsoft Windows. It was originally created for MS-DOS, and OS/2 versi ...
,
Interix
Interix was an optional, POSIX-conformant Unix subsystem for Windows NT operating systems. Interix was a component of Windows Services for UNIX, and a superset of the Microsoft POSIX subsystem. Like the POSIX subsystem, Interix was an environment ...
(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 o ...
-
Windows 95 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 in ...
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 ...
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,
4OS2,
FreeDOS,
Peter Norton
Peter Norton (born November 14, 1943) is an American programmer, software publisher, author, and philanthropist. He is best known for the computer programs and books that bear his name and portrait. Norton sold his software business to Symant ...
'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 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 Automation applica ...
, 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
KiXtart is a closed source free-format scripting language for Windows. It is described as a logon script processor and enhanced batch scripting language by the official website. Its name is a portmanteau of "kick start".
Overview
KiXtart is deve ...
,
QBasic, various
BASIC,
Rexx,
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
, 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 in ...
-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 and
VBScript
VBScript (''" Microsoft Visual Basic Scripting Edition"'') is an Active Scripting language developed by Microsoft that is modeled on Visual Basic. It allows Microsoft Windows system administrators to generate powerful tools for managing comput ...
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
An interpreter directive is a computer language construct, that on some systems is better described as an aspect of the system's executable file format, that is used to control which interpreter parses and interprets the instructions in a compu ...
*
Shebang symbol (#!)
*
Unix shells
*
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