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27-461: Active network architecture is composed of execution environments (similar to a unix shell that can execute active packets), a node operating system capable of supporting one or more execution environments. It also consists of active hardware, capable of routing or switching as well as executing code within active packets. This differs from the traditional network architecture which seeks robustness and stability by attempting to remove complexity and

54-675: 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 to control the execution of the system using shell scripts . Users typically interact with a Unix shell using a terminal emulator ; however, direct operation via serial hardware connections or Secure Shell are common for server systems. All Unix shells provide filename wildcarding , piping , here documents , command substitution , variables and control structures for condition-testing and iteration . Generally,

81-408: A shell is a program that executes other programs in response to text commands. A sophisticated shell can also change the environment in which other programs execute by passing named variables , a parameter list, or an input source. In Unix-like operating systems, users typically have many choices of command-line interpreters for interactive sessions. When a user logs into the system interactively,

108-477: A symbolic link or hard link to TENEX C shell (tcsh), an improved version of Joy's original version. Although the interactive features of csh have been copied to most other shells, the language structure has not been widely copied. The only work-alike is Hamilton C shell , written by Nicole Hamilton, first distributed on OS/2 in 1988 and on Windows since 1992. Shells read configuration files in various circumstances. These files usually contain commands for

135-443: A wildcard character is a kind of placeholder represented by a single 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. In telecommunications , a wildcard is a character that may be substituted for any of a defined subset of all possible characters. In computer ( software ) technology,

162-416: A date stamp, wildcards can be used to match date ranges, such as 202411*.mp4 to select video recordings from November 2024, to facilitate file operations such as copying and moving. In Unix-like and DOS operating systems, the question mark ? matches exactly one character. In DOS, if the question mark is placed at the end of the word, it will also match missing (zero) trailing characters; for example,

189-420: A leading caret ^ negates the set and matches only a character not within the list. In Microsoft Access , the asterisk sign * matches zero or more characters, the question mark ? matches a single character, the number sign # matches a single digit (0–9), and square brackets can be used for sets or ranges of characters to match. In regular expressions , the period ( . , also called "dot")

216-480: A shell program is automatically executed for the duration of the session. The type of shell, which may be customized for each user, is typically stored in the user's profile, for example in the local passwd file or in a distributed configuration system such as NIS or LDAP ; however, the user may execute any other available shell interactively. On operating systems with a windowing system , such as macOS and desktop Linux distributions , some users may never use

243-410: A single physical switch. Control Plane Technologies (CPT) was founded to commercialize this concept. Active network research addresses the nature of how best to incorporate extremely dynamic capability within networks. In order to do this, active network research must address the problem of optimally allocating computation versus communication within communication networks. A similar problem related to

270-535: A wildcard is a symbol used to replace or represent zero or more characters. Algorithms for matching wildcards have been developed in a number of recursive and non-recursive varieties. When specifying file names (or paths) in CP/M , DOS , Microsoft Windows , and Unix-like operating systems , the asterisk character ( * , also called "star") matches zero or more characters. For example, doc* matches doc and document but not dodo . If files are named with

297-587: Is /bin/sh . But a number of compatible work-alikes are also available with various improvements and additional features. On many systems, sh may be a symbolic link or hard link to one of these alternatives: 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 immediately recognized when active by its characteristic default command line prompt character,

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324-721: 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, especially in large shared computing centers. It added shell variables (precursors of environment variables , including the search path mechanism that evolved into $ PATH), user-executable shell scripts, and interrupt-handling. Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, and while/end/break/continue. As shell programming became widespread, these external commands were incorporated into

351-441: The ability to change its fundamental operation from underlying network components. Network processors are one means of implementing active networking concepts. Active networks have also been implemented as overlay networks . Active networking allows the possibility of highly tailored and rapid "real-time" changes to the underlying network operation. This enables such ideas as sending code along with packets of information allowing

378-454: The architecture. Active networking is an approach to network architecture with in-network programmability. The name derives from a comparison with network approaches advocating minimization of in-network processing, based on design advice such as the "end-to-end argument". Two major approaches were conceived: programmable network elements ("switches") and capsules, a programmability approach that places computation within packets traveling through

405-415: The compression of code as a measure of complexity is addressed via algorithmic information theory . One of the challenges of active networking has been the inability of information theory to mathematically model the active network paradigm and enable active network engineering. This is due to the active nature of the network in which communication packets contain code that dynamically change the operation of

432-446: The data to change its form (code) to match the channel characteristics. The smallest program that can generate a sequence of data can be found in the definition of Kolmogorov complexity . The use of real-time genetic algorithms within the network to compose network services is also enabled by active networking. Active networking relates to other networking paradigms primarily based upon how computing and communication are partitioned in

459-528: The dollar sign ( $ ). The C shell , csh , 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, Berkeley , and was widely distributed with BSD Unix . The C shell also introduced many features for interactive work, including the history and editing mechanisms, aliases , directory stacks , tilde notation , cdpath , job control and path hashing . On many systems, csh may be

486-549: The list. In shells that interpret ! as a history substitution, a leading caret ^ can be used instead. The operation of matching of wildcard patterns to multiple file or path names is referred to as globbing . In SQL , wildcard characters can be used in LIKE expressions; the percent sign % matches zero or more characters, and underscore _ a single character. Transact-SQL also supports square brackets ( [ and ] ) to list sets and ranges of characters to match,

513-400: The network. Fundamental advances in information theory are required in order to understand such networks. As the limit in reduction of transistor size is reached with current technology, active networking concepts are being explored as a more efficient means accomplishing computation and communication. More on this can be found in nanoscale networking . Unix shell A Unix shell is

540-747: The network. Treating packets as programs later became known as "active packets". Software-defined networking decouples the system that makes decisions about where traffic is sent (the control plane) from the underlying systems that forward traffic to the selected destination (the data plane). The concept of a programmable control plane originated at the University of Cambridge in the Systems Research Group, where (using virtual circuit identifiers available in Asynchronous Transfer Mode switches) multiple virtual control planes were made available on

567-408: The pattern 123? will match 123 and 1234 , but not 12345 . In Unix shells and Windows PowerShell , ranges of characters enclosed in square brackets ( [ and ] ) match a single character within the set; for example, [A-Za-z] matches any single uppercase or lowercase letter. In Unix shells, a leading exclamation mark ! negates the set and matches only a character not within

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594-468: The rest of the basic features considered common to all the later 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

621-441: The shell and are executed when loaded; they are usually used to set important variables used to find executables, like $ PATH , and others that control the behavior and appearance of the shell. The table in this section shows the configuration files for popular shells. Explanation: Variations on the Unix shell concept that don't derive from Bourne shell or C shell include the following: Wildcard character In software ,

648-401: The shell directly. On Unix systems, the shell has historically been the implementation language of system startup scripts, including the program that starts a windowing system, configures networking, and many other essential functions. However, some system vendors have replaced the traditional shell-based startup system ( init ) with different approaches, such as systemd . The first Unix shell

675-477: The shell itself for performance. But the most widely 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

702-627: 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 (for example, ".vimrc"), is a remnant of the RUNCOM ancestry of Unix shells. The PWB shell or Mashey shell, sh , was an upward-compatible version of

729-499: 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 later Unix shells, including piping, simple control structures using if and goto , and filename wildcarding. Though not in current use, it is still available as part of some Ancient UNIX systems. It

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