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a2 --- Glossary for beginners

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Chapter 745.   Glossary for beginners

(1)

In this chapter we briefly describe some terms commonly used in the Unix environments. The list is not complete and it is just meant to help the beginner.

745.1   Account

  • Account

    The term account literally represents an account, such as those that one may have in a bank. One has an account within a Unix system when one has been registered (and therefore has obtained a UID) and can access the system by using the access procedure.

  • GID

    Group identifier, Group ID or number that identifies a group of users.

  • Login, logout, access procedure

    The term access procedure refers to the process used by a user to access and interact with the syste, after an identification phase, that usually consists in the entry of a user-name and a password. In detail login identifies the user entry, whereas the logout identifies the conclusion of his/her activities.

  • UID

    User identifier, User ID or User Identification Number.

  • Password, passphrase, parola d'ordine

    It refers to a word or phrase used to verify the user identity in order to access some service.

745.2   File and file system

  • Second-extended, Ext2, Ext3

    The native file system of the GNU/Linux system is the type Second-extended, that has two variants: Ext2 and Ext3. The Ext3 variant contains some extensions that allow to reduce the possibility of data corruption, without loosing the compatibility with the Ext2 system.

  • FAT

    File allocation table. The FAT is one component of the file system in the Dos systems. It is so special that this type of file system is called with the same name: FAT.

  • Glob, globbing, wildcard characters, metacharacters

    The glob mechanism is used to identify many files (or directories) together and corresponds to the use of wildcard characters in the Dos environment. The more common ones are the asterisk, the question mark and the square parentheses.

  • Mount, unmount, mounting, unmounting

    In the Unix operating systems, when one wants to access data stored on a disk, one cannot refer to a device as it happens with Dos and similar systems. One must always refer to the global file system. To do this, all disks that must be accessed are connected together like branches on a tree. the term mounting, or mount, identifies a connection or link of the disk content to the global file system; the term unmounting, or unmount, identifies the disconnection or unlinking of a disk from the global structure.

  • Newline, change of line

    This term refers to the code required to identify the end of a line and the start of the next one. By using the name one should avoid to specify directly the actual code to allow different implementations on different systems.

    The more common Unix systems use the code <LF>. In the Dos and similar systems the couple <CR><LF> is instead used. For this reason, if one tries to print a text entered on a Unix system by using a printer configured for the Dos operating system, the result is a number of shifted lines.

    To solve the problem, since rarely such a printer can be configured to change line by checking only the <LF> code, it is possible to use a filter to transform the <LF> character into the <CR><LF>. This filter program is well known and is called unix2dos.

    Often in the Unix environments the term newline is confused with the <LF> code. This is a problem, because there are situations in which it is important to clarify that it is the <LF> code independently of the platform to which the concept is applied. Because of this, when this term is used, it is important to pay attention to the meaning of the text, by using some common sense.

  • Record

    The record is generally a record of amy time. In computing, the record usually corresponds to a line of a data file. A record is usually subdivided into fields, and can be considered analogue to a card in a card file: the fields of each card (i.e. record) are the different items written on the card.

  • Regular file

    In the Unix family operating systems, when the term file is mentioned, this can refer also to a directory and other objects with specific functions. The term regular file or normal file is used to specify that it is a simple file, by including in this category also the executables.

745.3   Network

  • Host

    The Host is the one who hosts. The term host is used in the network connections environment to define the nodes, considered as computers, which perform and host some type of service.

  • Node

    The network node is a computer or another specialized component which is connected to a network and which has a network valid address (address referred to the level 3, according to the ISO-OSI; chapter ##en-capitolo-rete-intro##).

  • Server, Server

    A server is generally a program which provides a network service. However this term is often used informally also to identify a network node which hosts inportant services.

  • Client, Client

    A client is generally a program which uses a service offered by a server. However this term is often used informally also to identify a network node which, in a given context, uses external services.

  • Protocol

    A protocol is a conventional language used by programs to communicate with other programs (for example, a client program communicated with a server by using a certain protocol).

  • MTU

    The name is an acronym of Max transfer unit and defines the maximum dimension of a (frame) which can be sent to the network through a given interface.

  • NFS

    A very important service in the local area networks consists in the possibility to share portions of a file system from and to other connected computers. The service is obtained by suing the NFS protocol and therefore allows sharing data in a network.

    NFS is often used as an acronym of Network file system and sometimes of Network file sharing. The two possible interpretations represent features of the same thing: the use of a file system that extends through the network and the consequent data sharing.

  • NNTP

    Network news transfer protocol. This is the protocol used to transfernews messages. A server NNTP is a computer that has the task to collect a copy of news messages from the discussion groups and to allow users to read and send messages to the same groups.

  • Proxy

    The term proxy is used in many cases to identify a service which has the task to provide or supply something. The typical example is the proxy and the servente which connects a local network to an external network, with the purpose to access the external network for the nodes of the local network, without allowing them a direct contact with the external nework. Usually, this type of proxy includes a cache memory to reduce repeated accesses to the external resources. However one should keep in mind that this definition applies also to other types of less known services.

  • SMTP

    Simple mail transfer protocol. It is a protocol used to transmit electronic mail. A server SMTP is a computer which has the task to manage the electronic mail of a group of users which use the computer as their dispatching central point.

  • URL, URI

    Uniform resource locator, Uniform resource identifier. These terms define an address which identifies exactly a network resource, such as a page HTML, a file of an FTP service and more. The two definitions have different extension; partcularly, URI includes URL and extends beyond.

745.4   Programs, execution and computing processes

  • Init, system initialization procedure

    Init is the program which is started by the kernel in order to start the system. Init uses a number of scripts to start the programs which work in the background to perform the processing needed to make the system available. Together they are known as system initialisation procedures, which includes both the start up and the termination, by identifying also different run levels.

  • Job

    The term job is often used in the Unix documentation with reference to various tasks, according to the context.

    • shell job

      The POSIX shell and especially the Bash, can manage the shell jobs which represent a set of processed generated by a single command.

    • Print Job

      It refers to printouts sent to the output queue (spool).

    • Scheduled Job

      It refers to commands whose execution is deferred to a specified time or queued until resources are available.

    There are other situations in which the term job is used, but the examples above should be sufficient to understand the variety of the meaning.

  • Log, log

    In computing, the log is equivalent to the history log of ships. The log is an automatic system that records significant events. The files that contain these records atr called log files and could also be identified as log files. Generally, the log is a file and what is written on them is a record.

  • PID

    Process identifier, Process ID or process identification number.

  • Pipe, pipeline

    The word refers to an imaginary pipe used to direct the output of a program into the input of another. The connection of programs in this way is managed by the shell and usually the symbol | is used to identify this operation. For the same reason, compatibly with the context, the symbol | is also called pipe.

  • Run level, run level

    When the system initialization procedure is used in mode System V, which is actually the normal mode, the run level can take different values, in order to define which system functions must be activated, according to the requirements.

    The run level is a positive number starting from zero, whose meaning depends on the configuration of the system. Usually the level zero is reserved for the shutdown phase, the level one is reserved to the single user mode and the level six is reserved to the system restart phase.

  • Script

    A script is a file of commands which is used in practice as an interpreted program. Normally the interpreter of a script is also a shell.

  • Shell

    The shell of an operating system is the program that has the task to interpret and execute commands entered by a user to the command line. The use of the term shell in this way has actually started with the Unix operating systems.

  • Standard error

    The file or device which is predefined as the destination of the error messages is the standard error. Usually it is the screen of the console or workstation used by the user. The standard error, usually, can be redirected by using the symbol 2> followed by the name of the file or device that one wants to use.

  • Standard input

    The file or device which is predefined as the data entry source, is the standard input. Usually it is the keyboard of the console or workstation used by the user. The data entry is terminated by entering an end of file character that is normally obtained by the combination [Ctrl d].

    The standard input, usually, can be redirected by using the symbol less (<) followed by the name of the file or device to be used, or the symbol pipe (|) when one wants to use the output of a command for input to the following command.

  • Standard output

    The file or device which is predefined as the destination of the output is the standard output. Usually it is the screen of the console or workstation used by the user. The standard output, usually can be redirected by using the symbol greater (>) followed by the name of a file or device to be used, or it can be redirected to another command by using the symbol pipe (|).

  • Unix domain socket, Unix domain socket

    It is a communication system between applications besed on a special file type: the socket. Some daemons offer services with this type of communication by listening to a connection request from client applications.

  • Utility, utility, utility program, service program

    A utility, or a utility program, or better a service program, is a useful and practical program, which performs its task simply and terse way. Usually, this type of programs are an integral part of the operating system.

745.5   Miscellany

  • Case sensitive, case insensitive

    The two definitions refer to the ``sensibility'' or not regarding the difference between upper case and lower case characters. Generally, the Unix systems are case sensitive, but there are situations when this sensitivity os missing or ignored.

  • Core

    In the Unix environments, core is a synonim of central memory, or RAM. The word is due to the fact the the original central memory forms were implemented as a network of magnetic iron nucleous: the nucleous memory, or core. For this reason, often, when a process teminates abnormally, the operating system dumps the image of its core memory. This file has the name core dump (obviously) and can be later on analysed by using diagnostic tools.

  • Daemon, demone

    The daemon, is a program that works in the background (background) and performs repetitive services. The term is typical of the Unix environments, whereas with other operating systems different definitions are used, such as server. Traditionally, most of the daemon programs have a name which terminate with the letter ``d''.

  • Domain, domain name

    Normally the term ``domain'' is used to refer to a given network node in an Internetnetwork. The name is composed of various elements, which are used to represent a hierarchy of domains, in a way comparable with what is done in the file system with the directory structure. A ``domain name'' may represent a partial position in the hierarchy, or the complete name of a node.

  • Regular expression, regular expression, regexp

    A regular expression is a way to define the search of strings by using a pattern. It is used by various service programs.

  • Implementation

    The verb to implement means to develop a practical solution to a project characteristic. In other word the implementation is the practical solution chosen to perform a given function. In summary, the implementation is the applied development of something in a given context.

  • Internationalization, i18n

    The internationalization is the mode in which a program is developed or modified in order to be compatible with the ``localization''. The code is made up by the fact that between the letter ``i'' and the letter ``n'' of internationalization there are 18 letters.

  • Localization, l10n

    The localization is the configuration that allows a program to be adaptable to local national-language features. The code is made up by the fact that between the letter ``l'' and the letter ``n'' of localization there are 10 letters.

  • Terminal, TTY

    Originally, the normal mode to interact with a computer was the use of a typewriter: teletype. From this name the code TTY is derived and is normally used to identify a generic terminal. The console is the main terminal directly linked to the computer. The term terminal is normally used to mean the unit made by the keyboard and the display.

    The data stream received from a terminal is normally referred to as standard input, i.e one refer to what has been entered through the keyboard. The data stream directed to the display is normally referred to as standard output, i.e. one refers to what is presented on the screen.

745.6   References

Appunti di informatica libera 2006.01.01 --- Copyright © 2000-2006 Daniele Giacomini -- <daniele (ad) swlibero·org>, <daniele·giacomini (ad) poste·it>


1) Translation last update on 2003.06.19 from Mario Pesce <mario (ad) datamission·co·uk>.


It should be possible to link to this page also with the name glossary_for_beginners.htm

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