A shell script is a computer program designed to be run by the 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, printing text. A script which sets up the environment, runs the program, does any necessary cleanup, etc. is called a wrapper. The term is used more to mean the automated mode of running an operating system shell; the typical Unix/Linux/POSIX-compliant installation includes the KornShell in several possible versions such as ksh88, Korn Shell'93 and others. The oldest shell still in common use is the Bourne shell; the C and Tcl shells have syntax quite similar to that of said programming languages, the Korn shells and Bash are developments of the Bourne shell, 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, grep, BASIC, Lisp, C and so forth contributed to the Perl programming language.
Other shells available on a machine or available for download and/or purchase include Almquist shell, PowerShell, Z shell, the Tenex C Shell, a Perl-like shell. Related programs such as shells based on Python, Ruby, C, Perl, Rexx &c in various forms are widely available. Another somewhat common shell is osh, 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 MKS Toolkit, Cygwin, UWIN, Interix and others make the above shells and Unix programming available on Windows systems, providing functionality all the way down to signals and other inter-process communication, system calls and APIs. The Hamilton C shell is a Windows shell, similar to the Unix C Shell. Microsoft distributes Windows Services for UNIX for use with its NT-based operating systems in particular, which have a POSIX environmental subsystem. 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 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 saved in a user's bin directory as /home/username/bin/l, a default set of command options pre-supplied. Here, the first line indicates which interpreter should execute the rest of the script, the second line makes a listing with options for file format indicators, all files, 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, 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 simply use l for the most 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 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. Shell scripts allow several commands that would be entered manually at a command-line interface to be executed automatically, 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 script, here named build 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, just run./build to create the updated program, test it, return to the editor.
Since the 1980s or so, scripts of this type have been replaced with utilities like make which are specialized for building programs. Simple batch jobs are not unusual for isolated tasks, but using shell loops and variables provides much more flexibility to users. A Bash 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 saved in a file like /home/username/bin/jpg2png The jpg2png command can be run on an entire directory full of JPEG images with just /home/username/
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, is used by the operating system to control the execution of the system using shell scripts. Users interact with a Unix shell using a terminal emulator. All Unix shells provide filename wildcarding, here documents, command substitution and control structures for condition-testing and iteration; the most generic sense of the term shell means any program. A shell hides the details of the underlying operating system and manages the technical details of the operating system kernel interface, the lowest-level, or "inner-most" component of most operating systems. In Unix-like operating systems, users have many choices of command-line interpreters for interactive sessions; when a user logs into the system interactively, a shell program is automatically executed for the duration of the session. The type of shell, which may be customized for each user, is stored in the user's profile, for example in the local passwd file or in a distributed configuration system such as NIS or LDAP.
On hosts with a windowing system, like macOS, some users may never use the shell directly. On Unix systems, the shell has been the implementation language of system startup scripts, including the program that starts a windowing system, configures networking, many other essential functions. However, some system vendors have replaced the traditional shell-based startup system with different approaches, such as systemd; the first Unix shell was the Thompson shell, sh, written by Ken Thompson at Bell Labs and distributed with Versions 1 through 6 of Unix, from 1971 to 1975. Though rudimentary by modern standards, it introduced many of the basic features common to all Unix shells, including piping, simple control structures using if and goto, filename wildcarding. Though not in current use, it is still available as part of some Ancient UNIX Systems, it was modeled after the Multics shell, developed in 1965 by American software engineer Glenda Schroeder. Schroeder's Multics shell was itself modeled after the RUNCOM program Louis Pouzin showed to the Multics Team.
The "rc" suffix on some Unix configuration files, is a remnant of the RUNCOM ancestry of Unix shells. The PWB shell or Mashey shell, sh, was an upward-compatible version of the Thompson shell, augmented by John Mashey and others and distributed with the Programmer's Workbench UNIX, circa 1975-1977, it focused on making shell programming practical in large shared computing centers. It added shell variables, user-executable shell scripts, interrupt-handling. Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, while/end/break/continue; as shell programming became widespread, these external commands were incorporated into the shell itself for performance. But the most distributed and influential of the early Unix shells were the Bourne shell and the C shell. Both shells have been used as the coding base and model for many derivative and work-alike shells with extended feature sets; the Bourne shell, sh, was a new Unix shell by Stephen Bourne at Bell Labs. Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the Unix shells, including here documents, command substitution, more generic variables and more extensive builtin control structures.
The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68. Traditionally, the Bourne shell program name is sh and its path in the Unix file system hierarchy is /bin/sh, but a number of compatible work-alikes are available with various improvements and additional features. On many systems, sh may be a symbolic link or hard link to one of these alternatives: Almquist shell: written as a BSD-licensed replacement for the Bourne Shell; the sh of FreeBSD, NetBSD are based on ash, enhanced to be POSIX conformant for the occasion. Bourne-Again shell: written as part of the GNU Project to provide a superset of Bourne Shell functionality; this shell can be found installed and is the default interactive shell for users on most Linux and macOS systems. Debian Almquist shell: a modern replacement for ash in Debian and Ubuntu Korn shell: written by David Korn based on the Bourne shell sources while working at Bell Labs Public domain Korn shell MirBSD Korn shell: a descendant of the OpenBSD /bin/ksh and pdksh, developed as part of MirOS BSD Z shell: a modern shell, backward compatible with bash Busybox: a set of Unix utilities for small and embedded systems, which includes 2 shells: ash, a derivative of the Almquist shell.
The POSIX standard specifies its standard shell as a strict subset of the Korn shell, an enhanced version of the Bourne shell. From a user's perspective the Bourne shell was recognized when active by its characteristic default command line prompt character, the dollar sign; the C shell, was modeled on the C programming language, including the control structures and the expression grammar. It was written by Bill Joy as a graduate student at University of California and was wide
Rc is the command line interpreter for Version 10 Unix and Plan 9 from Bell Labs operating systems. It resembles the Bourne shell, but its syntax is somewhat simpler, it was created by Tom Duff, better known for an unusual C programming language construct. A port of the original rc to Unix is part of Plan 9 from User Space. A rewrite of rc for Unix-like operating systems by Byron Rakitzis is available but includes some incompatible changes. Rc uses C-like control structures instead of the original Bourne shell's ALGOL-like structures, except that it uses an if not construct instead of else, has a Bourne-like for loop to iterate over lists. In rc, all variables are lists of strings, which eliminates the need for constructs like "$@". Es is an open source, command line interpreter developed by Rakitzis and Paul Haahr that uses a scripting language syntax influenced by the rc shell, it was based on code from Byron Rakitzis's clone of rc for UnixExtensible shell is intended to provide a functional programming language as a Unix shell.
The bulk of es development occurred in the early 1990s, after the shell was introduced at the Winter 1993 USENIX conference in San Diego, Official releases appear to have ceased after 0.9-beta-1 in 1997, es lacks features as compared to more popular shells, such as zsh and bash. The Bourne shell script: is expressed in rc as: Because if and if not are two different statements, they must be grouped in order to be used in certain situations. Rc supports more dynamic piping: a | b # pipe only standard error of a to b — equivalent to'3>&2 2>&1 >&3 | b' in Bourne shell a <>b # opens b as a's standard input and standard output a < < # becomes a rc - Plan 9 manual page. Plan 9 from User Space - Includes rc and other Plan 9 tools for Linux, Mac OS X and other Unix-like systems. Byron Rakitzis' rewrite for Unix es Official website
In software, a spell checker is a software feature that checks for misspellings in a text. Features are in software, such as a word processor, email client, electronic dictionary, or search engine. A basic spell checker carries out the following processes: It scans the text and extracts the words contained in it, it compares each word with a known list of spelled words. This might contain just a list of words, or it might contain additional information, such as hyphenation points or lexical and grammatical attributes. An additional step is a language-dependent algorithm for handling morphology. For a inflected language like English, the spell-checker will need to consider different forms of the same word, such as plurals, verbal forms and possessives. For many other languages, such as those featuring agglutination and more complex declension and conjugation, this part of the process is more complicated, it is unclear whether morphological analysis—allowing for many different forms of a word depending on its grammatical role—provides a significant benefit for English, though its benefits for synthetic languages such as German, Hungarian or Turkish are clear.
As an adjunct to these components, the program's user interface will allow users to approve or reject replacements and modify the program's operation. An alternative type of spell checker uses statistical information, such as n-grams, to recognize errors instead of correctly-spelled words; this approach requires a lot of effort to obtain sufficient statistical information. Key advantages include needing less runtime storage and the ability to correct errors in words that are not included in a dictionary. In some cases spell checkers use a fixed list of misspellings and suggestions for those misspellings. Clustering algorithms have been used for spell checking combined with phonetic information. In 1961, Les Earnest, who headed the research on this budding technology, saw it necessary to include the first spell checker that accessed a list of 10,000 acceptable words. Ralph Gorin, a graduate student under Earnest at the time, created the first true spelling checker program written as an applications program for general English text: SPELL for the DEC PDP-10 at Stanford University's Artificial Intelligence Laboratory, in February 1971.
Gorin wrote SPELL in assembly language, for faster action. Gorin made SPELL publicly accessible, as was done with most SAIL programs, it soon spread around the world via the new ARPAnet, about ten years before personal computers came into general use. SPELL, its algorithms and data structures inspired the Unix ispell program; the first spell checkers were available on mainframe computers in the late 1970s. A group of six linguists from Georgetown University developed the first spell-check system for the IBM corporation. Henry Kučera invented one for the VAX machines of Digital Equipment Corp in 1981; the first spell checkers for personal computers appeared in 1980, such as "WordCheck" for Commodore systems, released in late 1980 in time for advertisements to go to print in January 1981. Developers such as Maria Mariani and Random House rushed OEM packages or end-user products into the expanding software market for the PC but for Apple Macintosh, VAX, Unix. On the PCs, these spell checkers were standalone programs, many of which could be run in TSR mode from within word-processing packages on PCs with sufficient memory.
However, the market for standalone packages was short-lived, as by the mid-1980s developers of popular word-processing packages like WordStar and WordPerfect had incorporated spell checkers in their packages licensed from the above companies, who expanded support from just English to European and even Asian languages. However, this required increasing sophistication in the morphology routines of the software with regard to heavily-agglutinative languages like Hungarian and Finnish. Although the size of the word-processing market in a country like Iceland might not have justified the investment of implementing a spell checker, companies like WordPerfect nonetheless strove to localize their software for as many national markets as possible as part of their global marketing strategy. Firefox 2.0, a web browser, has spell check support for user-written content, such as when editing Wikitext, writing on many webmail sites and social networking websites. The web browsers Google Chrome and Opera, the email client Kmail and the instant messaging client Pidgin offer spell checking support, transparently using GNU Aspell and Hunspell as their engine.
Mac OS X now has spell check system-wide, extending the service to all bundled and third party applications. Some spell checkers have separate support for medical dictionaries to help prevent medical errors; the first spell checkers were "verifiers" instead of "correctors." They offered no suggestions for incorrectly spelled words. This was helpful for typos but it was not so helpful for logical or phonetic errors; the challenge the developers faced was the difficulty in offering useful suggestions for misspelled words. This requires applying pattern-matching algorithms, it might seem logical that where spell-checking dictionaries are concerned, "the bigger, the better," so that
Comparison of command shells
A command shell is a command line interface computer program to an operating system. Background execution allows a shell to run a command in background. POSIX shells and other Unix shells allow background execution by using the & character and the end of command, in PowerShell you can use Start-Process or Start-Job commands. Completion features assist the user in typing commands at the command line, by looking for and suggesting matching words for incomplete ones. Completion is requested by pressing the completion key. Command name completion is the completion of the name of a command. In most shells, a command can be a program in the command path, a builtin command, a function or alias. Path completion is the completion of the path to a file, absolute. Wildcard completion is a generalization of path completion, where an expression matches any number of files, using any supported syntax for file matching. Variable completion is the completion of the name of a variable name. Bash and fish have completion for all variable names.
PowerShell has completions for environment variable names, shell variable names and — from within user-defined functions — parameter names. Command argument completion is the completion of a specific command's arguments. There are two types of arguments and positional: Named arguments called options, are identified by their name or letter preceding a value, whereas positional arguments consist only of the value; some shells allow completion of argument names. Bash and fish offer parameter name completion through a definition external to the command, distributed in a separate completion definition file. For command parameter name/value completions, these shells assume path/filename completion if no completion is defined for the command. Completion can be set up to suggest completions by calling a shell function; the fish shell additionally supports parsing of man pages to extract parameter information that can be used to improve completions/suggestions. In PowerShell, all types of commands inherently expose data about the names and valid value ranges/lists for each argument.
This metadata is used by PowerShell to automatically support argument name and value completion for built-in commands/functions, user-defined commands/functions as well as for script files. Individual cmdlets can define dynamic completion of argument values where the completion values are computed dynamically on the running system. A user of a shell may find. If the shell supports command history the user can call the previous command into the line editor and edit it before issuing it again. Shells that support completion may be able to directly complete the command from the command history given a partial/initial part of the previous command. Most modern shells support command history. Shells which support command history in general support completion from history rather than just recalling commands from the history. In addition to the plain command text, PowerShell records execution start- and end time and execution status in the command history. Mandatory arguments/parameters are arguments/parameters which must be assigned a value upon invocation of the command, function or script file.
A shell that can determine ahead of invocation that there are missing mandatory values, can assist the interactive user by prompting for those values instead of letting the command fail. Having the shell prompt for missing values will allow the author of a script, command or function to mark a parameter as mandatory instead of creating script code to either prompt for the missing values or fail with a message. PowerShell allows commands and scripts to define arguments/parameters as mandatory; the shell determines prior to invocation if there is any mandatory arguments/parameters which have not been bound, will prompt the user for the value before actual invocation. With automatic suggestions the shell monitors while the interactive user is typing and displays context-relevant suggestions without interrupting the typing instead of the user explicitly requesting completion; the PowerShell Integrated Scripting Environment use the discoverable metadata to provide "intellisense" - i.e. suggestions that automatically pops up as the user types, in addition to when the user explicitly requests completion lists by pressing e.g. Tab ↹ A shell may record the locations the user has used as current locations and allow fast switching to any location/directory in the history.
One of the uses of the zsh directory stack is to record a directory history. In particular, the AUTO_PUSHD option and advanced cd arguments and completion are used for this purpose. PowerShell allows multiple named stacks to be used. Locations can be pushed onto/popped from a named stack. Any stack can become the current stack. Unlike most other shells, PowerShell's location concept allow location stacks to hold file system locations as well as other location types like e.g. Active Directory organizational units/groups, SQL Server databases/tables/objects, Internet Information Server applications/sites/virtual directories. 4DOS and Take Command Console record history of current directories and allows the user to switch to a directory in the history using a popup a window. A directory name can be used directly as a command which implicitly changes the current location to the directory; when a command line does not match a command or arguments directly, spell checking can automatically correct common typing mistakes (such as case sensitivity
An operating system is system software that manages computer hardware and software resources and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage and other resources. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is executed directly by the hardware and makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers; the dominant desktop operating system is Microsoft Windows with a market share of around 82.74%. MacOS by Apple Inc. is in second place, the varieties of Linux are collectively in third place. In the mobile sector, use in 2017 is up to 70% of Google's Android and according to third quarter 2016 data, Android on smartphones is dominant with 87.5 percent and a growth rate 10.3 percent per year, followed by Apple's iOS with 12.1 percent and a per year decrease in market share of 5.2 percent, while other operating systems amount to just 0.3 percent.
Linux distributions are dominant in supercomputing sectors. Other specialized classes of operating systems, such as embedded and real-time systems, exist for many applications. A single-tasking system can only run one program at a time, while a multi-tasking operating system allows more than one program to be running in concurrency; this is achieved by time-sharing, where the available processor time is divided between multiple processes. These processes are each interrupted in time slices by a task-scheduling subsystem of the operating system. Multi-tasking may be characterized in co-operative types. In preemptive multitasking, the operating system slices the CPU time and dedicates a slot to each of the programs. Unix-like operating systems, such as Solaris and Linux—as well as non-Unix-like, such as AmigaOS—support preemptive multitasking. Cooperative multitasking is achieved by relying on each process to provide time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking.
32-bit versions of both Windows NT and Win9x, used preemptive multi-tasking. Single-user operating systems have no facilities to distinguish users, but may allow multiple programs to run in tandem. A multi-user operating system extends the basic concept of multi-tasking with facilities that identify processes and resources, such as disk space, belonging to multiple users, the system permits multiple users to interact with the system at the same time. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage and other resources to multiple users. A distributed operating system manages a group of distinct computers and makes them appear to be a single computer; the development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine; when computers in a group work in cooperation, they form a distributed system.
In an OS, distributed and cloud computing context, templating refers to creating a single virtual machine image as a guest operating system saving it as a tool for multiple running virtual machines. The technique is used both in virtualization and cloud computing management, is common in large server warehouses. Embedded operating systems are designed to be used in embedded computer systems, they are designed to operate on small machines like PDAs with less autonomy. They are able to operate with a limited number of resources, they are compact and efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems. A real-time operating system is an operating system that guarantees to process events or data by a specific moment in time. A real-time operating system may be single- or multi-tasking, but when multitasking, it uses specialized scheduling algorithms so that a deterministic nature of behavior is achieved. An event-driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.
A library operating system is one in which the services that a typical operating system provides, such as networking, are provided in the form of libraries and composed with the application and configuration code to construct a unikernel: a specialized, single address space, machine image that can be deployed to cloud or embedded environments. Early computers were built to perform a series of single tasks, like a calculator. Basic operating system features were developed in the 1950s, such as resident monitor functions that could automatically run different programs in succession to speed up processing. Operating systems did not exist in their more complex forms until the early 1960s. Hardware features were added, that enabled use of runtime libraries and parallel processing; when personal computers became popular in the 1980s, operating systems were made for them similar in concept to those used on larger computers. In the 1940s, the earliest electronic digital systems had no operating systems.
Electronic systems of this time were programmed on rows of mechanical switches or by jumper wires on plug boards. These were special-purpose systems that, for example, generated ballistics tables for the military or controlled the pri
GitHub is a web-based hosting service for version control using Git. It is used for computer code, it offers all of the distributed version control and source code management functionality of Git as well as adding its own features. It provides access control and several collaboration features such as bug tracking, feature requests, task management, wikis for every project. GitHub offers plans for enterprise, team and free accounts which are used to host open-source software projects; as of January 2019, GitHub offers unlimited private repositories to all plans, including free accounts. As of June 2018, GitHub reports having over 28 million users and 57 million repositories, making it the largest host of source code in the world. GitHub was developed by Chris Wanstrath, P. J. Hyett, Tom Preston-Werner and Scott Chacon using Ruby on Rails, started in February 2008; the company, GitHub, Inc. is located in San Francisco. On February 24, 2009, GitHub team members announced, in a talk at Yahoo! headquarters, that within the first year of being online, GitHub had accumulated over 46,000 public repositories, 17,000 of which were formed in the previous month alone.
At that time, about 6,200 repositories had been forked at least. On July 5, 2009, GitHub announced. On July 27, 2009, in another talk delivered at Yahoo!, Preston-Werner announced that GitHub had grown to host 90,000 unique public repositories, 12,000 having been forked at least once, for a total of 135,000 repositories. On July 25, 2010, GitHub announced. On April 20, 2011, GitHub announced. On June 2, 2011, ReadWriteWeb reported that GitHub had surpassed SourceForge and Google Code in total number of commits for the period of January to May 2011. On July 9, 2012, Peter Levine, general partner at GitHub investor Andreessen Horowitz, stated that GitHub had been growing revenue at 300% annually since 2008 "profitably nearly the entire way". On January 16, 2013, GitHub announced it had passed the 3 million users mark and was hosting more than 5 million repositories. On December 23, 2013, GitHub announced. In June 2015, GitHub opened an office in Japan, its first office outside of the U. S. On July 29, 2015, GitHub announced it had raised $250 million in funding in a round led by Sequoia Capital.
The round valued the company at $2 billion. In 2016, GitHub was ranked No. 14 on the Forbes Cloud 100 list. On February 28, 2018, GitHub fell victim to the second largest distributed denial-of-service attack in history, with incoming traffic reaching a peak of about 1.35 terabits per second. On June 4, 2018, Microsoft announced it had reached an agreement to acquire GitHub for US$7.5 billion. The purchase closed on October 26, 2018. On June 19, 2018, GitHub expanded its GitHub Education by offering free education bundles to all schools. On June 4, 2018, Microsoft announced its intent to acquire GitHub for US$7.5 billion, the deal closed on Oct. 26, 2018. GitHub will continue to operate independently as a community and business. Under Microsoft, the service will be led by Xamarin's Nat Friedman, reporting to Scott Guthrie, executive vice president of Microsoft Cloud and AI. Current CEO Chris Wanstrath will be retained as a "technical fellow" reporting to Guthrie. Microsoft had become a significant user of GitHub, using it to host open source projects and development tools such as Chakra Core, PowerShell, Visual Studio Code, has backed other open source projects such as Linux, developed Git Virtual File System—a Git extension for managing large-scale repositories.
GitHub, Inc. was a flat organization with no middle managers. Employees can choose to work on projects. However, salaries are set by the chief executive. In 2014, GitHub, Inc. introduced a layer of middle management. GitHub.com was a start-up business, which in its first years provided enough revenue to be funded by its three founders and start taking on employees. In July 2012, four years after the company was founded, Andreessen Horowitz invested $100 million in venture capital. In July 2015 GitHub raised another $250 million of venture capital in a series B round. Investors were Andreessen Horowitz, Thrive Capital and other venture capital funds; as of August 2016, GitHub was making $140 million in Annual Recurring Revenue. GitHub's m