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Command Substitution
In computing, command substitution is a facility that allows a Command-line interpreter, command to be run and its output to be pasted back on the command line as arguments to another command. Command substitution first appeared in the Bourne shell, introduced with Version 7 Unix in 1979, and has remained a characteristic of all later Unix shells. The feature has since been adopted in other programming languages as well, including Perl, PHP, Ruby (programming language), Ruby and Microsoft's Powershell under Windows. It also appears in Microsoft's CMD.EXE in the FOR command and the ( ) command. Syntax and semantics Shells typically implement command substitution by creating a child process to run the first command with its stdout, standard output Pipeline (Unix), piped back to the shell, which reads that output, parsing it into words separated by whitespace character, whitespace. Because the shell can't know it has all the output from the child until the pipe closes or the child ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and software. Computing has scientific, engineering, mathematical, technological, and social aspects. Major computing disciplines include computer engineering, computer science, cybersecurity, data science, information systems, information technology, and software engineering. The term ''computing'' is also synonymous with counting and calculation, calculating. In earlier times, it was used in reference to the action performed by Mechanical computer, mechanical computing machines, and before that, to Computer (occupation), human computers. History The history of computing is longer than the history of computing hardware and includes the history of methods intended for pen and paper (or for chalk and slate) with or without the aid of tables. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
String (computer Science)
In computer programming, a string is traditionally a sequence of character (computing), characters, either as a literal (computer programming), literal constant or as some kind of Variable (computer science), variable. The latter may allow its elements to be Immutable object, mutated and the length changed, or it may be fixed (after creation). A string is often implemented as an array data structure of bytes (or word (computer architecture), words) that stores a sequence of elements, typically characters, using some character encoding. More general, ''string'' may also denote a sequence (or List (abstract data type), list) of data other than just characters. Depending on the programming language and precise data type used, a variable (programming), variable declared to be a string may either cause storage in memory to be statically allocated for a predetermined maximum length or employ dynamic allocation to allow it to hold a variable number of elements. When a string appears lit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wildcard Character
In software, a wildcard character is a kind of placeholder represented by a single character (computing), character, such as an asterisk (), which can be interpreted as a number of literal characters or an empty string. It is often used in file searches so the full name need not be typed. Telecommunication In telecommunications, a wildcard is a character that may be substituted for any of a defined subset of all possible characters. * In high-frequency (HF) radio automatic link establishment, the wildcard character may be substituted for any one of the 36 upper-case alphanumeric characters. * Whether the wildcard character represents a single character or a String (computer science), string of characters must be specified. Computing In computer (software) technology, a wildcard is a symbol used to replace or represent zero or more characters. Matching wildcards, Algorithms for matching wildcards have been developed in a number of recursion, recursive and non-recursive varietie ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, packaged as a Linux distribution (distro), which includes the kernel and supporting system software and library (computing), libraries—most of which are provided by third parties—to create a complete operating system, designed as a clone of Unix and released under the copyleft GPL license. List of Linux distributions, Thousands of Linux distributions exist, many based directly or indirectly on other distributions; popular Linux distributions include Debian, Fedora Linux, Linux Mint, Arch Linux, and Ubuntu, while commercial distributions include Red Hat Enterprise Linux, SUSE Linux Enterprise, and ChromeOS. Linux distributions are frequently used in server platforms. Many Linux distributions use the word "Linux" in their name, but the Free ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unix
Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley ( BSD), Microsoft (Xenix), Sun Microsystems ( SunOS/ Solaris), HP/ HPE ( HP-UX), and IBM ( AIX). The early versions of Unix—which are retrospectively referred to as " Research Unix"—ran on computers such as the PDP-11 and VAX; Unix was commonly used on minicomputers and mainframes from the 1970s onwards. It distinguished itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language (in 1973), which allows U ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
PowerShell
PowerShell is a shell program developed by Microsoft for task automation and configuration management. As is typical for a shell, it provides a command-line interpreter for interactive use and a script interpreter for automation via a language defined for it. Originally only for Windows, known as Windows PowerShell, it was made open-source and cross-platform on August 18, 2016, with the introduction of PowerShell Core. The former is built on the .NET Framework; the latter on .NET (previously .NET Core). PowerShell is bundled with current versions of Windows and can be installed on macOS and Linux. Since Windows 10 build 14971, PowerShell replaced Command Prompt as the default command shell exposed by File Explorer. In PowerShell, administrative tasks are generally performed via ''cmdlets'' (pronounced ''command-lets''), which are specialized .NET classes implementing a particular operation. These work by accessing data in different data stores, like the file system ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Variable (programming)
In computer programming, a variable is an abstract storage location paired with an associated symbol, symbolic name, which contains some known or unknown quantity of Data (computer science), data or Object (computer science), object referred to as a ''value (computer science), value''; or in simpler terms, a variable is a named container for a particular set of bits or Data type, type of data (like Integer (computer science), integer, Floating-point arithmetic, float, String (computer science), string, etc...). A variable can eventually be associated with or identified by a memory address. The variable name is the usual way to Reference (computer science), reference the stored value, in addition to referring to the variable itself, depending on the context. This separation of name and content allows the name to be used independently of the exact information it represents. The identifier in computer source code can be Name binding, bound to a Value (computer science), value during R ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
KornShell
KornShell (ksh) is a Unix shell which was developed by David Korn (computer scientist), David Korn at Bell Labs in the early 1980s and announced at USENIX Annual Technical Conference, USENIX on July 14, 1983. The initial development was based on Bourne shell source code. Other early contributors were Bell Labs developers Mike Veach and Pat Sullivan, who wrote the Emacs and Vi (text editor), vi-style line editing modes' code, respectively. KornShell is backward-compatible with the Bourne shell and includes many features of the C shell, inspired by the requests of Bell Labs users. Features KornShell complies with POSIX#POSIX.2, POSIX.2, Shell and Utilities, Command Interpreter (IEEE Std 1003.2-1992.) Major differences between KornShell and the traditional Bourne shell include: * Job control (Unix), job control, alias (command), command aliasing, and command history designed after the corresponding C shell features; job control was added to the Bourne Shell in 1989 * a choice ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Semantics
Semantics is the study of linguistic Meaning (philosophy), meaning. It examines what meaning is, how words get their meaning, and how the meaning of a complex expression depends on its parts. Part of this process involves the distinction between sense and reference. Sense is given by the ideas and concepts associated with an expression while reference is the object to which an expression points. Semantics contrasts with syntax, which studies the rules that dictate how to create grammatically correct sentences, and pragmatics, which investigates how people use language in communication. Lexical semantics is the branch of semantics that studies word meaning. It examines whether words have one or several meanings and in what lexical relations they stand to one another. Phrasal semantics studies the meaning of sentences by exploring the phenomenon of compositionality or how new meanings can be created by arranging words. Formal semantics (natural language), Formal semantics relies o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Syntax (programming Languages)
In computer science, the syntax of a computer language is the rules that define the combinations of symbols that are considered to be correctly structured Statement (computer science), statements or Expression (computer science), expressions in that language. This applies both to programming languages, where the document represents source code, and to markup languages, where the document represents data. The syntax of a language defines its surface form. Text-based user interface, Text-based computer languages are based on sequences of Character (computing), characters, while visual programming languages are based on the spatial layout and connections between symbols (which may be textual or graphical). Documents that are syntactically invalid are said to have a syntax error. When designing the syntax of a language, a designer might start by writing down examples of both legal and illegal String (computer science), strings, before trying to figure out the general rules from these ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Delimiter
A delimiter is a sequence of one or more Character (computing), characters for specifying the boundary between separate, independent regions in plain text, Expression (mathematics), mathematical expressions or other Data stream, data streams. An example of a delimiter is the comma character, which acts as a ''field delimiter'' in a sequence of comma-separated values. Another example of a delimiter is the time gap used to separate letters and words in the transmission of Morse code. In mathematics, delimiters are often used to specify the scope of an Operation (mathematics), operation, and can occur both as isolated symbols (e.g., Colon (punctuation), colon in "1 : 4") and as a pair of opposing-looking symbols (e.g., Angled bracket, angled brackets in \langle a, b \rangle). Delimiters represent one of various means of specifying boundaries in a data stream. String literal#Declarative notation, Declarative notation, for example, is an alternate method (without the use of delimiter ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
