Shell script

A shell script is a designed to be run by the, a. The various dialects of shell scripts are considered to be 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, logging, etc. is called a wrapper.

The term is also used more generally to mean the automated mode of running an operating system shell; in specific operating systems they are called other things such as batch files (MSDos-Win95 stream, ), command procedures (VMS), and shell scripts ( stream and third-party derivatives like —article is at ), and mainframe operating systems are associated with a number of terms.

The typical Unix/Linux/POSIX-compliant installation includes the  in several possible versions such as ksh88, Korn Shell '93 and others. The oldest shell still in common use is the ; Unix systems invariably also include the ,  , a  , a   for   connections, and a shell which is a main component of the  installation usually called  ;  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 language with elements of a number of others added as well. On the other hand, the various shells plus tools like, , , and , , and so forth contributed to the  programming language.

Other shells available on a machine or available for download and/or purchase include,  ,  ( , a particularly common enhanced KornShell), the  , a Perl-like shell. Related programs such as shells based on, , , , , , &c in various forms are also widely available. Another somewhat common shell is, whose manual page states it "is an enhanced, backward-compatible port of the standard command interpreter from Sixth Edition UNIX."

Windows-Unix interoperability software such as the, , , and others make the above shells and Unix programming available on Windows systems, providing functionality all the way down to signals and other , s and s. The  is a Windows shell that is very similar to the Unix C Shell. Microsoft distributes for use with its NT-based operating systems in particular, which have a POSIX.

Comments
Typically "comments begin with an octothorpe, or hash (#)... and continue until the end of the line. The shell ignores them", with one exception, as noted below.

Configurable choice of scripting language
If the first two bytes of the file consist of the # character followed by an exclamation mark (!) "this is a special type of comment called a '' or hash-bang. It tells the system which interpreter to use to execute the file." In Unix-like operating systems the characters following #! 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, 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.

Here, indicates 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 s with normal filenames as a side effect of ignoring punctuation in the names (dotfiles are usually only shown if an option like -a</tt> is used), and the "$@"</tt> causes any parameters given to l</tt> to pass through as parameters to ls, so that all of the normal options and other known to ls can still be used.

The user could then simply use l</tt> 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.

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 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, here named build</tt> and kept in the directory with them, which would compile them automatically:

The script would allow a user to save the file being edited, pause the editor, and then just run ./build</tt> 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 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 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</tt>

The jpg2png</tt> command can then be run on an entire directory full of JPEG images with just /home/username/bin/jpg2png *.jpg</tt>

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 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, , 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</tt>’s " ", or csh</tt>’s source</tt>).

Programming
Many modern shells also supply various features usually found only in more sophisticated s, such as control-flow constructs, variables,, arrays, 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 and  provide extra capabilities for shell programming;  can also be embedded in shell scripts as can other scripting languages like. Perl and Tcl come with graphics toolkits as well.

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 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, , or. The 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" 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  -rf * /</tt> (instead of the intended rm -rf */</tt>) 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. Similar problems can transform </tt> and </tt> into dangerous weapons, and misuse of the <tt>></tt> 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: <tt>cp</tt>, <tt></tt>, <tt></tt>, <tt></tt>, 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 in which efficient  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. , creator of, 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 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  (which uses ).

Shell scripting on other operating systems
Interoperability software such as, the , (which is available in the Microsoft Windows Services for UNIX), ,  (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 - branch, as well as earlier MKS Toolkit versions for OS/2. At least three DCL implementations for Windows type operating systems—in addition to, a multiple-use scripting language package which is used with the command shell, and  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 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, subsystem often called the MS-DOS subsystem uses the Command.com provided with these operating systems to run the aforementioned MS-DOS batch files.

The console alternatives, , , 's and  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, , and  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. is quite similar to MS-DOS, whilst 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, , various , , , and implementations, the  and its installed engines. On Unix and other -compliant systems, and  are used to extend the string and numeric processing ability of shell scripts. , 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, direct memory access, various types of and more, which are difficult or impossible in the main script, and so on. and 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.