Linux some definitions

absolute pathname -A pathname that starts with the root directory (represented by /). An absolute
pathname locates a file without regard to the working directory.

access – In computer jargon, a verb meaning to use, read from, or write to. To access a file
means to read from or write to the file.

Access Control List  -See ACL.

access permissions-Permission to read from, write to, or execute a file. If you have write access permission to a file (usually just called write permission), you can write to the file. Also access
privilege.

ACL -Access Control List. A system that performs a function similar to file permissions
but with much finer-grain control.

active window On a desktop, the window that receives the characters you type on the keyboard.
Same as focus, desktop

address mask See network mask on page

alias A mechanism of a shell that enables you to define new commands.

alphanumeric character– One of the characters, either uppercase or lowercase, from A to Z and 0 to 9,
inclusive.
ambiguous file reference-A reference to a file that does not necessarily specify any one file but can be used to specify a group of files. The shell expands an ambiguous file reference into a list of  filenames. Special characters represent single characters (?), strings of zero or more characters (*), and character classes ([]) within ambiguous file references. An ambiguous file reference is a type of regular expression (page 1185).
angle bracket – A left angle bracket (<) and a right angle bracket (>). The shell uses < to redirect a
command’s standard input to come from a file and > to redirect the standard output.
The shell uses the characters << to signify the start of a Here document and >> to append output to a file.

animate– When referring to a window action, means that the action is slowed down so the
user can view it. For example, when you minimize a window, it can disappear all at
once (not animated) or it can slowly telescope into the panel so you can get a visual
feel for what is happening (animated).

anti-aliasing Adding gray pixels at the edge of a diagonal line to get rid of the jagged appearance
and thereby make the line look smoother. Anti-aliasing sometimes makes type on a
screen look better and sometimes worse; it works best on small and large fonts and
is less effective on fonts from 8 to 15 points. See also subpixel hinting

API Application program interface. The interface (calling conventions) by which an
application program accesses an operating system and other services. An API is
defined at the source code level and provides a level of abstraction between the
application and the kernel (or other privileged utilities) to ensure the portability of
the code.FOLDOC

append To add something to the end of something else. To append text to a file means to
add the text to the end of the file. The shell uses >> to append a command’s output
to a file.

applet A small program that runs within a larger program. Examples are Java applets that
run in a browser and panel applets that run from a desktop panel.

archive A file that contains a group of smaller, typically related, files. Also, to create such a
file. The tar and cpio utilities can create and read archives.

argument A number, letter, filename, or another string that gives some information to a command
and is passed to the command when it is called. A command-line argument is
anything on a command line following the command name that is passed to the
command. An option is a kind of argument.

arithmetic expression-A group of numbers, operators, and parentheses that can be evaluated. When you
evaluate an arithmetic expression, you end up with a number. The Bourne Again Shell uses the expr command to evaluate arithmetic expressions; the TC Shell uses @, and the Z Shell uses let.

ARP- Address Resolution Protocol. A method for finding a host’s MAC address
(page 1174; also Ethernet address) from its IP address. ARP allows the IP address to
be independent of the MAC address. See page 377 for more information.FOLDOC
array- An arrangement of elements (numbers or strings of characters) in one or more
dimensions. The Bourne Again, TC, and Z Shells and awk/mawk/gawk can store and
process arrays.
ASCII- American Standard Code for Information Interchange. A code that uses seven bits
to represent both graphic (letters, numbers, and punctuation) and CONTROL characters.
You can represent textual information, including program source code and English
text, in ASCII code. Because ASCII is a standard, it is frequently used when
exchanging information between computers. See the file /usr/pub/ascii or give the
command man ascii to see a list of ASCII codes.
Extensions of the ASCII character set use eight bits. The seven-bit set is common; the
eight-bit extensions are still coming into popular use. The eighth bit is sometimes
referred to as the metabit.
ASCII terminal- A textual terminal. Contrast with graphical display

ASP– Application service provider. A company that provides applications over the Internet.

asynchronous event– An event that does not occur regularly or synchronously with another event.
Linux system signals are asynchronous; they can occur at any time because they
can be initiated by any number of nonregular events.

attachment A file that is attached to, but is not part of, a piece of email. Attachments are frequently
opened by programs (including your Internet browser) that are called by your mail program
so you might not be aware that they are not an integral part of an email message.

authentication The verification of the identity of a person or process. In a communication system,
authentication verifies that a message really comes from its stated source, like the
signature on a (paper) letter. The most common form of authentication is typing a
user name (which might be widely known or easily guessable) and a corresponding
password that is presumed to be known only to the individual being authenticated.
Other methods of authentication on a Linux system include the /etc/passwd and
/etc/shadow files, LDAP, biometrics, Kerberos 5, and SMB.FOLDOC
automatic mounting-A way of demand mounting directories from remote hosts without having them
hard configured into /etc/fstab. Also called automounting.

avoided An object, such as a panel, that should not normally be covered by another object,
such as a window.

back door A security hole deliberately left in place by the designers or maintainers of a system.
The motivation for creating such holes is not always sinister; some operating systems,
for example, come out of the box with privileged accounts intended for use by
field service technicians or the vendor’s maintenance programmers.
Ken Thompson’s 1983 Turing Award lecture to the ACM revealed the existence, in
early UNIX versions, of a back door that might be the most fiendishly clever security
hack of all time. The C compiler contained code that would recognize when the
login command was being recompiled and would insert some code recognizing a
password chosen by Thompson, giving him entry to the system whether or not an
account had been created for him.
Normally such a back door could be removed by removing it from the source code
for the compiler and recompiling the compiler. But to recompile the compiler, you
have to use the compiler, so Thompson arranged that the compiler would recognize
when it was compiling a version of itself. It would insert into the recompiled compiler
the code to insert into the recompiled login the code to allow Thompson entry,
and, of course, the code to recognize itself and do the whole thing again the next
time around. Having done this once, he was then able to recompile the compiler
from the original sources; the hack perpetuated itself invisibly, leaving the back
door in place and active but with no trace in the sources.
Sometimes called a wormhole. Also trap door.

background process-

A process that is not run in the foreground. Also called a detached process, a background process is initiated by a command line that ends with an ampersand (&). You do not have to wait for a background process to run to completion before giving the shell additional commands. If you have job control, you can
move background processes to the foreground, and vice versa.

base name– The name of a file that, in contrast with a pathname, does not mention any of the
directories containing the file (and therefore does not contain any slashes [/]). For
example, hosts is the basename of /etc/hosts.FOLDOC
baud The maximum information-carrying capacity of a communication channel in symbols
(state transitions or level transitions) per second. It coincides with bits per second
only for two-level modulation with no framing or stop bits. A symbol is a
unique state of the communication channel, distinguishable by the receiver from all
other possible states. For example, it might be one of two voltage levels on a wire
for a direct digital connection, or it might be the phase or frequency of a carrier.FOLDOC

Baud – is often mistakenly used as a synonym for bits per second. baud rate Transmission speed. Usually used to measure terminal or modem speed. Common baud rates range from 110 to 38,400 baud. See baud.

Berkeley UNIX- One of the two major versions of the UNIX operating system. Berkeley UNIX was
developed at the University of California at Berkeley by the Computer Systems Research Group and is often referred to as BSD (Berkeley Software Distribution).

BIND- Berkeley Internet Name Domain. An implementation of a DNS  server developed and distributed by the University of California at Berkeley.

BIOS– Basic Input/Output System. On PCs, EEPROM-based system software that provides the lowest-level interface to peripheral devices and controls the first stage of the bootstrap  process, which loads the operating system. The BIOS can be stored in different types of memory. The memory must be nonvolatile so that it remembers the system settings even when the system is turned off. AlsoBIOS ROM.

bit– The smallest piece of information a computer can handle. A bit is a binary digit:
either 1 or 0 (on or off ).

bit depth- Same as color depth.bit-mapped

display-A graphical display device in which each pixel on the screen is controlled by an
underlying representation of zeros and ones.

blank character- Either a SPACE or a TAB character, also called whitespace . In some contexts, NEWLINEs are considered blank characters.

block -A section of a disk or tape (usually 1,024 bytes long but shorter or longer on some
systems) that is written at one time.

block device A disk or tape drive. A block device stores information in blocks of characters and
is represented by a block device (block special) file. Contrast with character device.

block number- Disk and tape blocks are numbered so that Linux can keep track of the data on the
device.
blocking factor– The number of logical blocks that make up a physical block on a tape or disk.
When you write 1K logical blocks to a tape with a physical block size of 30K, the blocking factor is 30.

Boolean The type of an expression with two possible values: true and false. Also, a variable
of Boolean type or a function with Boolean arguments or result. The most common Boolean functions are AND, OR, and NOT.FOLDOC

boot See bootstrap.

boot loader A very small program that takes its place in the bootstrap process that brings a
computer from off or reset to a fully functional state. See “GRUB: The Linux Boot
Loader”.

bootstrap Derived from “Pull oneself up by one’s own bootstraps,” the incremental process of
loading an operating system kernel into memory and starting it running without
any outside assistance. Frequently shortened to boot.

Bourne Again Shell-bash. GNU’s command interpreter for UNIX, bash is a POSIX-compliant shell with
full Bourne Shell syntax and some C Shell commands built in. The Bourne Again
Shell supports emacs-style command-line editing, job control, functions, and online
help, Bourne Shell sh. This UNIX command processor was developed by Steve Bourne at AT&T Bell
Laboratories.

brace- A left brace ({) and a right brace (}). Braces have special meanings to the shell.
bracket A square bracket  or an angle bracket .

branch- In a tree structure, a branch connects nodes, leaves, and the root. The Linux
filesystem hierarchy is often conceptualized as an upside-down tree. The
branches connect files and directories. In a source code control system, such as
SCCS or RCS, a branch occurs when a revision is made to a file and is not
included in subsequent revisions to the file.

bridge– Typically a two-port device originally used for extending networks at layer 2 (data
link) of the Internet Protocol model.

broadcast- A transmission to multiple, unspecified recipients. On Ethernet, a broadcast packet
is a special type of multicast  packet; it has a special address indicating
that all devices that receive it should process it. Broadcast traffic exists at several
layers of the network stack, including Ethernet and IP. Broadcast traffic has one
source but indeterminate destinations (all hosts on the local network).

Broadcast address- The last address on a subnet (usually 255), reserved as shorthand to mean all hosts.
 
Broadcast network-A type of network, such as Ethernet, in which any system can transmit information
At any time, and all systems receive every message.
 
Buffer- An area of memory that stores data until it can be used. When you write information
To a file on a disk, Linux stores the information in a disk buffer until there is
Enough to write to the disk or until the disk is ready to receive the information.
 
Bug- An unwanted and unintended program property, especially one that causes the
Program to malfunction.
 
Built-in (command)-A command that is built into a shell. Each of the three major shells—the Bourne
Again, TC, and Z Shells—have its own set of built-ins.
 
Byte- A component in the machine data hierarchy, usually larger than a bit and smaller
Than a word; now most often eight bits and the smallest addressable unit of storage.
A byte typically holds one character.
 
C programming language-A modern systems language that has high-level features for efficient, modular programming as well as lower-level features that make it suitable for use as a systems programming
Language. It is machine independent so that carefully written C programs can be easily transported to run on different machines. Most of the Linux operating system is written in C, and Linux provides an ideal environment for programming in C.
 
C Shell- csh. The C Shell command processor was developed by Bill Joy for BSD UNIX. It
was named for the C programming language because its programming constructs
are similar to those of C.
cable modem- A type of modem that allows you to access the Internet by using your cable television
connection.
 
Cache-Holding recently accessed data, a small, fast memory designed to speed up subsequent
access to the same data. Most often applied to processor-memory access but also used
for a local copy of data accessible over a network, from a hard disk, and so on.FOLDOC
 
calling environment-A list of variables and their values that is made available to a called program. Refer
to “Executing a Command” 
 
Cascading windows- An arrangement of windows such that they overlap, generally with at least part of
the title bar visible.
 
 case sensitive- Able to distinguish between uppercase and lowercase characters. Unless you set the
ignorance parameter, vim performs case-sensitive searches. The grep utility performs
case-sensitive searches unless you use the –i option.

catenate– To join sequentially, or end to end. The Linux cat utility catenates files: It displays

them one after the other. Also concatenate.

chain loading The technique used by a boot loader to load unsupported operating systems. Used
for loading such operating systems as DOS or Windows, it works by loading
another boot loader.

characterbased-A program, utility, or interface that works only with ASCII (page 1151) characters.
This set of characters includes some simple graphics, such as lines and corners, and
can display colored characters. It cannot display true graphics. Contrast with GUI
 
characterbased terminal– A terminal that displays only characters and very limited graphics. See characterbased.

character class- In a regular expression, a group of characters that defines which characters can occupy
a single character position. A character-class definition is usually surrounded by square brackets. The character class defined by [abcr] represents a character position that can be occupied by a, b, c, or r. Also list operator.
In GNU documentation and POSIX, used to refer to sets of characters with a common characteristic, denoted by the notation [:class:]; for example, [:upper:] denotes the set of uppercase letters. This book uses the term character class as explained under “Brackets” on

character device-A terminal, printer, or modem. A character device stores or displays characters one
at a time. A character device is represented by a character device (character special) file. Contrast with block device.

check box A GUI widget, usually the outline of a square box with an adjacent caption, that a user can click to display or remove a tick . When the box holds a tick, the option described by the caption is on or true. Also tick box.

checksum A computed value that depends on the contents of a block of data and is transmitted or stored along with the data to detect corruption of the data. The receiving system recomputes the checksum based on the received data and compares this value with the one sent with the data. If the two values are the same, the receiver has some confidence
that the data was received correctly.The checksum might be 8, 16, or 32 bits, or some other size. It is computed by
summing the bytes or words of the data block, ignoring overflow. The checksum
might be negated so that the total of the data words plus the checksum is zero.
Internet packets use a 32-bit checksum.
child process- A process that is created by another process, the parent process. Every process is
a child process except for the first process, which is started when Linux begins
execution. When you run a command from the shell, the shell spawns a child
process to run the command.

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