Book: LPI Linux Certification in a Nutshell
## Glossary This Glossary contains a complete list of terms that you should be familiar with before taking the LPI exams. Knowledge of these terms will be important in preparing for LPI exams, but no exam question should depend on knowledge gleaned solely from this list. This Glossary is copyrighted by the Linux Professional Institute (http://www.lpi.org/) and is provided under the terms of the LPI Open Content License (http://www.lpi.org/license.html#OCL). This list was compiled by (in alphabetical order): Les Bell, David DeLano, Alan Mead, Tom Peters, Richard Rager, with additions by Jeff Dean and editing by O'Reilly & Associates, Inc. *nix A term for any operating system resembling Unix, including Linux and a large number of free and commercial systems; also Un*x. 100BaseT Ethernet over UTP cables, using hubs to produce a star topology; supports a maximum (theoretical) transmission of 100 MBps. 10Base2 So-called Thin Ethernet, using RG-58 coax cables and BNC connectors to construct a chain of cables; supports a maximum (theoretical) transmission of 10 Mbps. 10Base5 The older Thick Ethernet, which used vampire taps into a single cable; supports a maximum (theoretical) transmission of 10 Mbps. 10BaseT Ethernet over UTP cables, using hubs to produce a star topology; supports a maximum (theoretical) transmission of 10 Mbps. access To connect to and utilize a device (computer, printer) or file. account The symbol or number that refers to a user for accounting purposes. address 1. A location in memory; specifically, the I/O port used by a device to communicate with the processor. 2. A unique identifier assigned to an interface on a network-attached device such as a network interface card. Notice: a host can have multiple interfaces, hence multiple addresses. 3. The name or number given to a computer, device, or resource so it can be identified, found, and accessed on a network. administer To control the operation and use of a computer or other device; the task of a system administrator. algorithm A formal description of a procedure that, when suitable input is entered, will generate output as a result that satisfies specific requirements. alias 1. Within a shell, a substitute word for a command string (e.g., alias dir = "ls -- color"). 2. An additional IP address on an interface. 3. Refers to another name given to an email account, in order to accept mail for one email address and forward it to another. Alt The Alternative key on a keyboard. analog Refers to a physical measure that can take any value within a continuous range (e.g., the voltage used to encode loudness when transmitting a signal over a conventional copper telephone line). See also digital. ANSI Short for American National Standards Institute (http://www.ansi.org/). A standards body responsible for many protocols. API Short for Application Programming Interface. A specification that allows simple access to functionality of a library or other system resources when writing a program; operating system functionality is made available through an API. application A program that runs on top of an operating system. application layer The top layer of the four-layer TCP/IP protocol model. Includes protocols such as Telnet, FTP, HTTP, SMTP, and so on. archive 1. A backup of data to be preserved. 2. A file that contains one or more components and an index (e.g., in tar, cpio, rpm, or deb format). argument A piece of information passed to a command or function (usually typed in behind it) that modifies its behavior or that is operated upon by the command or function (i.e., in cat motd, motd is the argument). See also parameter. ARP Short for Address Resolution Protocol. A protocol that, given an IP address on the local network, returns the Ethernet address of the corresponding interface. ASCII Short for American Standard Code for Information Interchange. A specification of characters widely used in the Unix world and beyond. aspect ratio The ratio between the width and the height of a pixel on a computer display. assembler A program that compiles programs written in assembly language into object code. assembly language A low-level computer language that can be translated directly to the object code of the computer processor. background A state of process execution that does not produce output to the terminal (execution may stop if the process tries to write to the terminal); it is common to run system processes and long-running user applications in the background. See also foreground. backup 1. A copy of essential data stored on- or off-site as insurance against failures of system hardware, software, or user. 2. To make a backup. binary 1. Taking two discrete values (e.g., bits), as opposed to decimal (taking ten discrete values). 2. A file that is not intended to be read by applications or the operating system instead of by humans; especially in plural ("binaries") for compiled sources. See also text. BIOS Short for Basic Input/Output System. A simple, low-level operating system that supplies a uniform API to higher-level operating systems; BIOS is generally implemented in ROM of some sort. bit The smallest entity of information; can have one of two states (0/1, on/off, open/closed, etc.). bitplanes The number of bits available for each display pixel to code for visual appearance (color, proximity, etc.). block device A device that exchanges data with the operating system in sizable blocks (e.g., 512 bytes) at a time. boot To cause the operating system to begin to function. Takes its name from "pulling oneself up by the bootstraps," a whimsical analogy applied to the BIOS loading itself and then running the boot loader. "Reboot" is the term for a repeat of the process. boot loader Software, usually installed on the MBR of Intel machines, which exists to load the operating system kernel and begin its functioning. BOOTP Short for Boot Protocol. A protocol used to provide information to diskless workstations or devices over a network. See also DHCP. broadcast A frame or datagram addressed to all interfaces on a network. BSD Short for Berkeley Software Design. A variant of Unix originally developed at the University of California, Berkeley. The BSD TCP/IP stack is the model for most subsequent TCP/IP implementations. buffer Temporary storage. See also cache. BUGTRAQ A mailing list for discussions regarding network security (daemons, programs, operating systems, routers, etc.). build To run a sequence of compile and link steps to produce a new version of an executable program. bus A cable for transmitting signals between various components within one computer system. byte A datatype of 8 bits. C A compiled computer language closely associated with Unix. C++ An object-oriented computer language derived from C that needs a compiler. cache Any readily accessible storage area used to keep handy data that is (somehow) indicated to be needed again shortly; the purpose being to speed up the access of that data and improve system performance. Specifically, the fast computer memory that is used as a buffer for data and program instructions between the CPU and the slower main memory. See also RAM. caching-only DNS A domain name server that does not have any domains files. Caldera OpenLinux A commercial Linux distribution. call Execute a subprogram or library procedure in an executable program. card Any device that can be plugged into a computer expansion slot. CD-ROM A removable medium of considerable popularity that comes in several variations, the most popular being ISO9660. CERT/CC A team of people that studies Internet security and provides incident response services; see http://www.cert.org/. CGI Short for Common Gateway Interface. A standard for allowing server applications to be executed as part of an HTTP request. CHAP Challenge Handshake Protocol, an authentication scheme used in PPP. char C datatype (usually 1 byte) used to store letters. See also character. character A letter or sign usually represented by 1 byte in ASCII code. character device A device that exchanges data with the operating system in one character (or byte or even word) at a time. child process Any process created by another so-called parent process; usually used in reference to a particular parent process. CIDR See variable length subnet mask. CIFS Short for Common Internet File System. Microsoft's successor to SMB, a suite of protocols for sharing file and print services (among Windows machines or Un*x machines running CIFS servers like Samba). clean In reference to a drive being mounted, "clean" means that the drive was unmounted properly and thus (theoretically) does not need to be checked; otherwise, a drive is "dirty." client A computer or process that connects to and receives a service from a server computer or process. coax Short for "coaxial." A type of cable with inner and outer conductors used for TV cables and for Ethernet LANs, where the computers usually have T-joints to attach to a single chain of cables that needs to be terminated by resistors. colormap A table used to encode a palette of colors for images. command-line interface An interactive user interface that allows commands to be given to a computer program or shell through a text-based terminal (or terminal emulator in a window within a graphical user interface). compiler A program that examines program source code and translates it into an equivalent object code file. See also interpreter. compression Removal of redundant information from a file or data stream to reduce its size, the storage space it needs, or the time needed for transmission. Lossy compression actually discards information that is considered nonessential and is appropriate only for data such as images or sound. computer A digital, electronic, general-purpose, programmable, information-processing automate. console The primary, directly attached, user interface of a computer. Some system administration functions may be performed only at a console. control panel A collection of buttons, switches, lights, or displays used to configure and control a router, printer, computer, or other device. core dump The content of memory written to a file on disk when a program crashes (usually called a "core"). corrupted Damaged (said of a file or disk contents). CPU Short for Central Processing Unit. The main component that makes a computer work; these days, usually a "microprocessor" on a single silicon chip. See also processor. crack To gain access to a computer system without proper authorization (e.g., by guessing a legitimate user's password) and possibly interfere with its normal operation or integrity. cracker Someone who tries to crack into another computer system. See also hacker. A software program used to crack, for instance, by guessing passwords. crash A sudden stop of normal operation. Supposedly, the original hard drives would sometimes experience a catastrophic failure in which the read/write heads would crash into the media, possibly sending the media flying; hence, a crash is an unintentional termination of software or hardware due to some failure or error -- especially a termination in a final, catastrophic, or unpleasant way. CSLIP Short for Compressed SLIP. SLIP with added VJ compression of IP headers. Ctrl The Control key on a keyboard. current working directory The location within a filesystem where a program works. This is the default location for many commands unless another directory is explicitly defined. cylinder A number of tracks located at the same radius on the several surfaces of a hard disk. A hard disk with four platters has eight surfaces, so that at each position of the read/write heads, eight tracks can be read without head movement; these eight tracks form a cylinder. daemon A program that runs in the background to offer system services. data "That which is given," for instance, as input to a computer. See also information. data link layer Layer two of the ISO/OSI seven-layer model. Responsible for establishing an error-free communications path between network nodes over the physical link layer, frames messages for transmission, checks the integrity of received messages, manages access to and use of the media, and ensures proper sequencing of transmitted data. These functions are generally provided by a network card driver. database 1. A usually large collection of ordered and readily accessible data. 2. A program to manage a database and extract information from it. datagram packet Especially as used in UDP. Not IP-specific; other protocols use the term "datagram" in their documentation. Debian A GNU/Linux distribution built by a volunteer organization. default The value of a parameter that a program uses if it is not explicitly given a value. Del The Delete key on a keyboard. delete Remove or erase a file, character, directory, and so on. dependency A state in which other libraries, programs, or packages are required to make a program work. DES Short for Data Encryption Standard. A U.S. government-sanctioned standard for the encryption of data now considered insecure to high-end brute force attacks. desktop The screen from which all programs are started and run on X. device A "peripheral" piece of hardware that is an optional part of or can be attached to a computer (even one that is actually housed within the computer's casing): interface cards, drives, printers, and so on. The software interface used within Unix (Linux) to represent a computer peripheral: interface cards, drives, printers, and so on; see /dev directory. DHCP Short for Dynamic Host Configuration Protocol. Provides for automatic downloading of IP address and other configuration data from a server to a client. Allows for reuse of IP addresses so that the number of hosts can exceed the number of available IP addresses. DHCP is an upgrade to the BOOTP protocol. dial-in, dial-up, or dial-out Refers to a connection made over the Public Switched Telephone Network (PSTN), as opposed to a permanent, or leased-line, connection. die To cease execution, especially in a final or complete manner. digital Refers to an entity that can assume only a limited number of discrete states and not any value (e.g., binary). See also analog. directory A special type of file that contains information about other files, such as filename, location, permissions, size, and so on. dirty A filesystem that was not cleanly unmounted. See also clean. disk Rotating magnetic media that support direct or random access. See also floppy disk, hard disk. display A human-readable device to display text, graphics, or other data. distribution A (usually) complete collection of software needed to operate a computer including the Linux kernel and various utilities and applications. DMA Short for Direct Memory Access. A hardware protocol that allows a special controller circuit (DMA controller) to transfer a block of data from a peripheral device's buffer memory directly to main memory without CPU involvement. See also PIO. DNS Short for Domain Name System. A hierarchically structured distributed directory service that translates human-intelligible names like http://www.lpi.org/ into the corresponding IP addresses. domain 1. One or more computer networks that serve an organizational group. 2. The name assigned to a network domain. domain name server A system running BIND serving DNS-name-to-IP-address translations. drive Any device that can store and retrieve data in a relatively permanent fashion on media (which may be removable or built into the device). EIDE Short for Enhanced Integrated Device Electronics. An improved version of the IDE interface, used to connect hard drives and CD-ROMS to a PC. email Electronic mail. emulate To simulate the actions of a device or program so that the simulation can actually perform the same functions as the original. emulator A program that emulates the functions of some device or other program. environment A collection of variables associated with a process so that it knows about the user preferences and configuration of the system; they are inherited by a child process. environment variables The variables that define an environment. ergonomic Easy to use by humans. Esc The Escape key on a keyboard. Ethernet A type of LAN computer interface using coax (10Base2 or 10Base5) or UTP cables (10BaseT or 100BaseT). execute To set to work (a program). See also run. execute permission Permission set on a file on a Unix filesystem so that it may be run as a program by the "operating system." executable A file that is a binary or a script that can be run as a program (may assume execute permission). export To share, as in a filesystem or volume. FAT Short for File Allocation Table. A simple filesystem that uses a table to index files on a block device (floppy or hard disk). It comes in the varieties of FAT-12 (MS-DOS), FAT-16 (MS-DOS, Microsoft Windows 3.x) and FAT-32 (Microsoft Windows 9x). FHS Short for Filesystem Hierarchy Standard. A proposed standard for the location of files on a Unix system. See http://www.pathname.com/fhs/. file A named sequence or stream of bytes at a known location in storage. filesystem The data structures placed on a logical disk or partition (by mkfs) that allow the operating system to record information about files stored there. filter To remove unwanted data. floating-point Used with numbers that may represent a fraction. See also integer. floppy disk A magnetic storage medium with a flexible disk inside. See also hard disk. floppy drive A device that can read and write floppy disks. font The shape of each of the letters in a character set. foreground The context in which a process is having access to a terminal for output (i.e., is not running in the background). The color of text on a computer display (as opposed to the text's background). Refers to the window "in front of " all others and with which the user is interacting. fork When an executing process creates an exact executing duplicate (except for the different PID) of itself. See also child process, spawn. format Specification regarding how data is stored. To apply the requisite format to storage media in preparation to making a filesystem. forwarding The act of receiving an email and then resending it to another destination. frame A packet as assembled and transmitted over the physical layer of a network (e.g., Ethernet, Token Ring, etc.). free Not costing anything. Not inhibited. As applied to source code, it allows modification, study, and adaptation, not inhibited by excessively restrictive commercial license terms. See also FSF, GPL. FSF Short for Free Software Foundation. The FSF is a tax-exempt charity that raises funds for work on the GNU Project; see http://www.fsf.org/. FSSTND Short for Filesystem Standard. A standard for the location of files on a Linux system; replaced by the FHS. FTP Short for File Transfer Protocol. A protocol for transferring files over the Internet and the software to accomplish the transfer. gateway A device that routes IP datagrams between networks in an Internet; a router. (In common usage today, a gateway is a general-purpose computer with a general-purpose operating system -- e.g., Linux -- that may be performing other functions, although security considerations may render this inadvisable. A router is a special-purpose computer with a special-purpose operating system -- e.g., IOS -- generally from a specialist supplier -- e.g., Cisco.) Gateways do not perform protocol translation. GB An abbreviation for gigabyte, or 1000 MB. GID Group ID. See also UID. global A variable, configuration section, procedure, and so forth having a scope that is unlimited (i.e., applies everywhere unless contradicted locally). GNU Short for "GNU's Not Unix." An FSF Project to build Unix-compatible utilities and programs exclusively based on free program source code. GPL Short for GNU Public License. The GPL is a license that permits the copying, modification, and redistribution of free software. It was created by the FSF for software developed by the GNU Project and has been applied to Linux as well. See http://www.gnu.org/copyleft/gpl.html. graphical user interface (GUI) An interactive interface using a graphics display. Refer to a "graphical user interface" only if there actually is a graphical interface (like X); do not use the term for interactive programs on text terminals (based on ncurses or slang). Use "interactive interface" as a catchall. See also command-line interface. graphics Images, pictures; in contrast to text. group Refers to a list of one or more users having the same access rights. See also /etc/groups. hack To accomplish a result in an unorthodox way. hacker Someone who hacks; a title assigned to people with remarkable computing skills. See also cracker. hang See crash. hard disk A computer device that uses solid disks as magnetic media to store data. See also floppy disk. hard link In Unix filesystems, an entry in a directory that points to a file in another directory on the same disk or partition and shares the inode of that file. See also symlink. hardware All physical parts making up the computer. high-level Refers to a computer language with a higher level of abstraction from the computer architecture than a low-level language. host Any computer attached to an IP-based Internet, especially computers that can act as a server to a client program or computer. HOWTO A series of documents, each on a particular topic, that form a significant portion of the documentation for Linux. HOWTOs originated with, and are generally published by, the Linux Documentation Project (LDP). HTML An abbreviation for HyperText Markup Language, a term coined by Tim Berners Lee. A standard for specifying the structure of a document indicated by tags in the document text; used on the World Wide Web with HTTP. HTML has been replaced with XHTML, an XML-like version of HTML. HTTP Short for HyperText Transfer Protocol. The succession of application layer protocols used for communication between a WWW browser and a WWW server. I/O Input/output. ICMP Short for Internet Control Message Protocol. A required protocol for the notification of errors between gateways and hosts on IP-based Internets. ID Short for "identifier" or "identity." IDE Short for Integrated Device Electronics. A popular interface on PCs to attach hard drives, constructed in such a way that much of the interface circuitry is integrated into the disk drive. See also EIDE. idle Inactive; waiting for a task or a wake-up call. IMAP Short for Internet Message Access Protocol. Protocol permitting access to electronic mail or bulletin board messages that are kept on a mail server. implement To create an actual object (program, device) that conforms to abstract specifications. include file A file that contains constants and parameters, possibly shared between two or more programs and included into the source code when these programs are compiled. information Something worth knowing, in contrast to just plain data. inode In Unix filesystems, a single block of administrative data defining a file on a disk partition. input Any data entered into a running program or into a file. install Transferring a new program to a computer's permanent storage (e.g., hard disk) and performing any necessary configuration or administration. integer A data type used to represent a whole (integer, nonfraction) number within a limited range. integrity In filesystems, a stable and noncorrupt state. interactive Adjective meaning having the property to be able to interact (i.e., respond to stimulation from the outside). Used in the context of programs or interfaces. interactive interface An interface between a computer and a user that allows them to interact and exchange input and output (commands and data). interface A connection (through a hardware device or through a software program) between different components of a computer system (usually performing some kind of translation between protocols internal to the components); used especially in the contexts of network communication or communication between computer systems and their users. Internet The worldwide distributed network of computers linked by the Internet Protocol. interpreter A program that examines a script or program source code and executes it, line by line. See also compiler. interrupt An electronic or software signal sent to a CPU to initiate a high-priority service, interrupting that which was in process. intranet A network (usually a LAN) based on IP, but unlike the Internet, allowing only restricted access. invoke Induce execution of. See also call. IO port The memory address peripheral devices use to communicate with the CPU. IP Short for Internet Protocol. The network layer protocol used on IP-based networks, including the Internet. IRQ Interrupt request. See also interrupt. ISA Short for Industry Standard Architecture. An increasingly obsolete PC bus standard for peripheral devices. ISDN Short for Integrated Services Digital Network. A technology for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second). Most ISDN lines offered by telephone companies give you two lines, called B channels, at once. You can use one line for voice and the other for data, or you can use both lines for data to give you data rates of 128 Kbps. ISO Short for International Standards Organization. One of several bodies that exist to promote standards, including computer standards. ISP Short for Internet Service Provider. A company that provides connections to the Internet. job A task that has been sent to the background or has been submitted for later execution. k or K A factor of 1000, but with computers usually 1024 (210 ). KB Kilobyte; 1024 bytes. Kbps Data transfer rate in units of 1000 bits per second. KBps Data transfer rate in units of 1024 bytes per second. kernel The core of an operating system, which provides multitasking (process creation, interprocess protection, interprocess communication), memory management, and basic I/O management. key 1. A token that is used to encrypt plain text or decrypt cipher text in an encryption system. 2. A database field that may be used as the basis of a query. 3. A marked switch on a keyboard that used to be a common computer input device. keyboard An input device having many keys marked with letters and other symbols. LAN Short for Local Area Network. A small network, usually with one or a few segments, which supports broadcasting and direct connections between hosts (e.g., Ethernet, Token Ring, AppleTalk, and ARCNet). See also WAN. LDP Short for Linux Documentation Project. A project intended to create free, high-quality documentation for GNU/Linux. See http://www.linuxdoc.org/. library A collection of (often-related) subroutines to be linked to a program. LILO The Linux boot loader; a program that loads the kernel so Linux can boot. LILO can also be used to boot other operating systems. link To bind a program to the subroutines it references (calls). These are typically located in object modules or libraries. Linux A Unix-like operating system first developed, still maintained by, and named after Linus Torvalds. It is freely available under the General Public License. load To transfer from disk into memory. local Within easy reach, on the local area network, not remote. log file Record of activities. low-level Refers to a computer language in which statements are similar to instructions for the processor (or in which statements are more like object code than in a high-level language). Mail User Agent An end-user program used to access, process, read, archive, compose, and send email messages. Such email programs often include some MTA functionality, in particular the ability to use SMTP to send email to an outgoing mail server, and POP3 or IMAP to download mail from an inbound mail server. See also MTA. manpage Standard Unix manual page (usually available on the computer system in nroff format, called with the man command). manual A document, often of book length, discussing the design or operation of a software package or device. By hand (as opposed to some more automated means). masquerade To pretend to be another host for the purposes of sharing one IP address among several local hosts hidden to the outside world for reasons of resource shortages or security. See also NAT. MB An abbreviation for Megabyte; 1000 KB (or sometimes 1024). Mbps Data transfer rate in units of 1 million bits per second. MBR Short for Master Boot Record. An area of the outermost cylinder of a hard disk that contains the partition table. The MBR contains four entries identifying the types, starting cylinder, and sizes of up to four partitions on the hard disk. One of the entries is flagged as active; this entry marks the partition from which the machine will boot. Floppy disks don't have an MBR, since they don't have a partition table. Instead, they just have a boot sector (same as a logical disk), which contains a Media Descriptor Table (MDT) and bootstrap loader. The MDT describes the format of a floppy disk or logical disk. media The physical device by which data is transmitted or (more commonly) stored. memory The place where a computer stores data and/or programs for direct access by the CPU. RAM or ROM (and also cache memory), not disks. mini-HOWTO A slimmer, more focused document; otherwise like a HOWTO. modem A device that converts between digital signals from the computer and analog signals for communication over a telephone line. mouse An input device that allows pointing to, selecting, and activating objects displayed in a graphical user interface. MTA Short for Message Transfer Agent. A program that routes email based on the header and invokes the correct delivery agent, especially SMTP, in order to route the mail toward its ultimate destination. For example: exim, qmail, sendmail, and smail are all MTAs. The term "Mail Transport Agent" is used in the online "Network Administrator's Guide" to refer to rmail, which is used to process incoming mail from UUCP before passing it on to sendmail. This usage is at least confusing, if not incorrect. MTU Short for Maximum Transfer Unit. Maximum size of an IP packet that will be accepted for transmission without fragmenting it into smaller datagrams. An optimal size is usually determined automatically; typical sizes are 296 bytes (40 header + 256 data for phone lines) and 1500 bytes (the maximum for an Ethernet connection). NAT Short for Network Address Translation. A generic description of the process whereby the IP address of a host on a private Internet is translated into an IANA-assigned unique address on the wider public Internet. This can be accomplished by several techniques: masquerading, circuit-level gateways such as SOCKS, transparent proxying, or application-level gateways. NetBEUI The current implementation of the NetBIOS protocol used in MS-DOS, MS-Windows, and OS/2. NetBIOS A lightweight transport protocol developed by Sytek, IBM, and Microsoft for use on personal computers. NetBIOS defines the protocol on the wire (datagram formats), the code that implements the protocol, and the API used to employ the protocol. An example of an application that uses the NetBIOS API is Microsoft Networks, the workstation and server code implemented in MS-DOS 3.0 and later, OS/2, and various Windows incarnations, though other applications do exist. NetBIOS employs name registration and broadcast discovery rather than addressing and is consequently a nonroutable protocol. See also NetBIOS over TCP/IP, SMB. NetBIOS over TCP/IP A layer of code that implements the NetBIOS API, but utilizing TCP and UDP datagrams, which are encapsulated in IP datagrams. Since IP is routable, this overcomes the most significant limitation of NetBIOS. netmask Short for network mask; the network part of an IP address. See also variable length subnet mask. network An interconnected set of hosts and other network devices that share a common physical layer such as Ethernet, X.25 and so on. See also LAN, WAN. network interface card An expansion board allowing a computer to access a network. network layer The layer of a network protocol stack that is concerned with addressing and delivery of datagrams across a network or Internet. In the TCP/IP protocol stack, the main network layer protocol is IP. NFS Short for Network File System. A protocol (developed by Sun Microsystems) enabling a *nix machine to mount a remote disk area as part of its local filesystem; widely considered of questionable security. NIS Short for Network Information Service (formerly "yellow pages"). Protocols to provide network services (such as authentication) for NFS. object code Instructions that can be executed by the computer processor. offline Not connected to a computer system or network. See also online. online 1. Connected to a computer system or network. See also offline. 2. Stored on and accessible through a computer system or network. operating system Central set of programs that manages the various components and devices of the computer and its interaction with application programs and users (e.g., MS-DOS, Windows 95/98/NT/2000, Mac OS, Unix, Linux, etc.). OSI Short for Open System Interconnection. A layered suite of protocols for network communications. The concept of a "stack" of protocols (hence "TCP/IP stack") is due to the OSI seven-layer model, even though TCP/IP has only four distinct layers (certain OSI layer concepts are combined in TCP/IP). output Any data generated by a process. owner The account that has its user ID (UID) number associated with a file. package A set of related files and programs, especially a single-archive file (tar, rpm) that contains them. packet A quantum of data transmitted over a network. Specifically, a unit of TCP traffic carrying the information necessary to deliver itself, especially using the UDP protocol (datagram). parallel Several bits at the same time, over time (over multiple wires). parameter A variable with a specific value that has a meaning or function, which belongs to a program function or command. See also argument. parent process A process that started one or more other, so-called child processes. partition An arbitrary region of a storage device (almost always a hard drive) created by partitioning software before data was stored. Specifically on IBM PC compatibles: one of up to four distinct areas on a hard drive that can be dedicated to different operating systems. One of the partition types, extended, supports further partitioning into a maximum of four logical disks. password A token that authenticates a user at login time. PC Short for Personal Computer. A computer designed to be used by one individual at a time. Often associated with computers compatible with the architecture of the original IBM microcomputer. PCI Short for Peripheral Component Interface. A PC bus to connect peripheral devices to the processor, PCI is thought of as a replacement for the original ISA bus. peripheral A device that is an optional attachment to the core components of a computer (CPU and memory). physical layer The lowest layer of both the ISO/OSI and TCP/IP protocol stacks. Consists of the cables, connectors, and associated hardware such as driver chips to implement a network such as Ethernet or Token Ring. PID Short for Process ID. A numerical identifier used to track processes by the kernel. PIO Short for Programmed Input/Output. A technique whereby the CPU executes a tightly coded loop in which it copies data from a peripheral device's buffer memory and writes it back out to main memory. See also DMA. pipe A data structure that connects a file handle in one process to a file handle in another; by convention, stdout of one process to stdin of the next. Established on the shell command line with the `|` symbol. pixel A "picture element"; the smallest addressable portion of a computer display. Screen resolution is usually cited in pixels. PLIP Short for Parallel Line Internet Protocol. IP protocol over a parallel cable (between two machines physically connected and not too distant). PnP Short for Plug 'n' Play. A specification intended to automate configuration of ISA peripherals in PCs. POP Short for Post Office Protocol. Protocol to retrieve mail from a mail server. Various software servers typically have names derived from "pop" such as ipop3d, ipop2d, and popper. port The name given to an individual, numbered "slot" that is available to Internetworking software. For example, HTTP servers generally listen to port 80. See also /etc/services, IO port. To adapt a computer program to operate in a new computing environment and/or in a new programming language. PostScript A page description language developed and marketed by Adobe Systems, Inc., widely implemented in laser printers, especially where high-quality output is required (e.g., phototypesetters), and under Linux, widely emulated in software for non-PostScript printers. PPID Short for Parent PID. The PID of a process' parent process. See also child process, parent process, PID. PPP Short for Point-to-Point Protocol. A physical layer protocol that can be used to encapsulate IP and other network protocols, making it an excellent way of extending LAN protocols to dialup users. PPP comprises an HDLC-like framing protocol, a link control protocol, and a family of network control protocols, each of which corresponds to a network protocol that PPP can encapsulate. PPP can also use PAP or CHAP for authentication. presentation layer The top layer of the ISO/OSI seven-layer model, which specifies character representation (e.g., ASCII) and graphics formats, such as NAPLPS (North American Presentation Layer Protocols). In TCP/IP, the presentation layer is subsumed into the application layer, but perhaps the closest equivalent standards are ASN.1, ANSI, and HTML/XML. process A running program; an instance of program execution. processor The main component that makes a computer work; these days, usually a "microprocessor" on a single silicon chip. See also CPU. program A sequence of instructions for the computer that implements an algorithm, especially when stored in a file in the form of either directly executable object code or source code for an interpreter or compiler. When loaded into memory and executed, the object-code program typically becomes a process. prompt An indication produced by a shell or application program that it is ready for further user commands or input. protocol A definition of data structures and formats to be exchanged by two programs over a network. queue A data structure that implements a first-in, first-out list (e.g., a print queue, which contains a list of jobs to be printed in order). RAM Short for Random Access Memory. Volatile, writable memory that a computer uses as its main memory. Comes in flavors such as EDO, ECC, SDRAM, and so on, which are not equivalent but from the perspective of a system administrator are very similar under normal use. See also ROM. read permission Authorization within a filesystem to display the contents of files and directories. README An important document that usually comes with a software package to call attention to important issues. README files usually have their name in uppercase letters, so that it appears at the top of a directory listing. reboot See boot. Red Hat A commercial Linux distribution offered by Red Hat Software, Inc. redundant Superfluous; said of information in the contexts of compression or the preservation of data integrity. regular expression A formal expression of a string pattern that can be searched for and processed by a pattern-matching program such as vi, grep, awk, or Perl. RFC Short for Request For Comments. Despite the name, a de facto specification of Internet protocols and standards. See http://www.cis.ohio-state.edu/hypertext/information/rfc.html. ROM Short for Read-Only Memory. Computer memory, usually involving some enduring medium, such as a silicon chip or a burnt laser disc, which can be read but not altered. This is inconvenient when the data can change, and just to be confusing, some special ROMs can be modified under certain circumstances. See also RAM. root The administrative account (`UID 0`) on a *nix system that has all privileges. See also superuser. The topmost, first, or originating node or object (e.g., root directory, / ). route The path across one or more networks from one host to another. To examine the destination network IP address in a datagram, and by consulting a table, direct the datagram to the next router along the path to the destination or to the destination itself. router A device that routes IP datagrams between networks in an Internet; a gateway. RPM Short for Red Hat Package Manager. A system that eases installation, verification, upgrading, and uninstalling of Linux packages. See the RPM HOWTO for more information. run To let it work (a program). See also execute. runlevel Mode of operation of a Unix system, offering different services on each level. Samba An open source project to implement the SMB protocol and its network functions, including file and printer sharing. script A computer program that is written in an interpreted programming language and therefore stays in human-readable text format. See also binary, executable. SCSI Short for Small Computer System Interface. A multidrop bus cable architecture particularly suitable for both internal and external attachment of mass storage devices such as hard drives, tape drives, and CD-ROMs. segment A (limited) length of cable. Segments can be joined by repeaters (rare), bridges (common), routers, or switches (which are hardware logic bridges and routers). serial One bit after another, over time (over a single wire). server A process, or a host computer, that provides a particular service to client processes (e.g., web server, print server). service A process that accepts requests and returns responses in an almost endless loop; a daemon. session layer The ISO/OSI session control layer establishes and controls system-dependent aspects of communications sessions between specific nodes in the network. It bridges the gap between the services provided by the transport layer and the logical functions running on the operating system in a participating node. In the TCP/IP network stack, there is no session control layer; its functions are implemented partially in the transport layer and partially in the application layer. shell A program that mediates between the user and the operating system, typically accepting commands and invoking the corresponding programs. signal A logical interrupt to a process, which the process must generally deal with synchronously. A form of interprocess communications. single mode Single-user mode, runlevel 1. SLIP Short for Serial Line Internet Protocol. A way of encapsulating IP datagrams for transmission over asynchronous modem connections. See also PPP. SMB Short for Server Message Block. A Microsoft protocol developed originally to transport MS-DOS, and later OS/2 and Microsoft Windows, API calls, and their arguments across a NetBIOS LAN. Primarily used under Linux as a protocol for file and print sharing with Windows machines. See also Samba. SMTP Short for Simple Mail Transport Protocol. A conversational protocol used by mail servers for delivery of email via the Internet. SNR Short for Signal-to-Noise Ratio. The relative amount of useful information in a signal, as compared to the noise it carries. socket A TCP application layer connection. software A computer program. source code The plain text code written in a programming language that specifies the detailed operation of a program. Source code needs to be processed by a compiler to produce a program that can be run (i.e., be executed) by the computer. sources The files containing the source code for a program or program system, from which the executable program or library can be built or ported to another computer platform. spawn To create a child process by means of a fork( ) and an exec( ). stderr The standard Unix error output device (by default to the terminal display). stdin The standard Unix input device (by default, the terminal keyboard). stdout The standard Unix output device (by default, the terminal display). sticky bit A permission bit on an executable file that causes the kernel to keep the memory image of the process after it has terminated, in order to avoid the overhead of reloading it when it is re-invoked. stream A sequence of data bytes with sequencing and flow control, such as that implemented by the TCP protocol within TCP/IP. subnet mask A value used in configuring the TCP/IP stack that specifies which part of a 32-bit IP address is the network address and which part is the host address. SUID Short for Set User ID. A permission bit for files in Unix-compatible filesystems based on the UID of the user who owns the file, rather than the user who created the process. superuser The user of the root account. SuSE A commercial Linux distribution. swap space Virtual memory; called swap space because processes swap location between fast RAM and slow virtual memory if their priority changes. symlink Symbolic link. In Unix filesystems, a symlink is an entry in a directory that points to another filename in the filesystem. See also hard link. synchronize To make the events in two separate sequences happen at the same time (used in communications). To make the content and state of data stored in two separate locations identical (e.g., cache, FTP sites). syntax The formal rules that determine how keywords or commands and their components need to be combined when writing the source code of a computer program or forming shell commands. sysadmin System administrator. A person who administers a computer system and keeps it working. system A computer system; a term loosely used to refer to hardware and/or software. See also operating system. TCP Short for Transmission Control Protocol. A session-oriented streaming transport protocol that provides sequencing, error detection and correction, flow control, congestion control, and multiplexing. See also UDP. TCP/IP A suite of protocols basic to Internet transmissions, which includes TCP and IP. terminal The outlet of a computer, usually consisting of a display for output of text (or possibly graphics) and a keyboard (and possibly a mouse) for input, used as a device for interaction between the computer and a user. See also workstation. terminate To disconnect, end, finish, quit, stop, and so on. terminator A resistive load to indicate the end a chain of devices, usually a SCSI chain or a coax network chain. text A series of characters that can be displayed on a terminal display or printed on paper for human reading. TFTP Short for Tiny FTP. A protocol similar to FTP, but much simpler and even less secure. Used mainly for cracking computers and booting diskless network clients. third-party A company or organization not directly involved in developing an original product. For example, a software company (such as Red Hat Software) that offers new products to an existing piece of software (such as Linux). tools Programs or utilities that provide administrative functionality, such as a compiler or language interpreter. transport layer The ISO/OSI seven-layer model transport layer provides end-to-end control of a communication session once the path has been established, allowing processes to exchange data reliably and sequentially, independent of which systems are communicating and their locations in the network. The transport layer in TCP/IP is not defined in the same way, although TCP provides sequencing and error correction. UDP, which is also a transport layer protocol, does not have a session concept and is unreliable. The TCP/IP transport layer primarily provides multiplexing through the use of ports. troubleshoot The process of finding the reason(s) or problem(s) with networking, programming or hardware. tune To make small changes to configuration in order to produce more efficient operation. TurboLinux A commercial Linux distribution. UDP Short for User Datagram Protocol. A connectionless, unreliable transport protocol that provides multiplexing and error detection for applications that require a low-cost protocol for one-shot transactions. See also datagram packet, TCP. UID Short for User ID, the numeric identifier used on Linux and Unix systems to specify users. uninstall Remove hardware or software from a computer system. unload Remove services or software from a server so that more resources (CPU time, disk space, etc.) become available. unreliable In the TCP/IP sense of the term, a protocol that does not perform error correction (relying on "upper" layers to detect and correct errors, usually through retransmission). upgrade To update hardware or software to a better state. URL Short for Uniform Resource Locator. An identifier for an address on the Internet, preceded by the name of the protocol that must be used to reach that address (e.g., ftp://ftp.kernel.org/ or http://www.oreilly.com/). USB Short for Universal Serial Bus. A recently developed bus standard for connecting peripheral devices in a chain. user 1. The person that is using the resources of a computer. 2. A person's account or process; identification listed in /etc/passwd. user interface See interactive interface. UTC Short for Coordinated Universal Time. Official world time, formerly Greenwich Mean Time. utility A program to help you do a task easier. UTP Short for Untwisted Pair. Network cables with several parallel wires used for Ethernet. The network usually has a star topology with hubs and does not need terminators. variable length subnet mask See CIDR. vendor A company that provides a service or a product. virtual Functionality provided without additional hardware or software, often without the user needing to realize this economy (e.g., virtual memory, virtual console, or virtual web server). virtual memory Extra memory made available on a system by using space on a hard disk. Usually referred to as swap space. WAN Short for Wide Area Network. A network that links geographically widespread facilities (and often LANs at those locations) using point-to-point (leased-line, SLIP, or PPP) or packet-switched network (X.25, frame relay) links that does not support the broadcast and direct connection capabilities of LANs. wildcard A placeholder used to represent any character or group of characters. window A region on a graphical desktop; the user interface for I/O with a child process of the desktop. Windows NT A 32-bit operating system available from Microsoft. WinModem A modem that has only a Digital Signal Processor and uses MS Windows-specific software to encode and decode data. WINS Short for Windows Internet Naming Service. An automatic NetBIOS name database to resolve NetBIOS names to IP addresses. word A datatype consisting of 2 or 4 bytes; on i386 architectures, a word is 4 bytes (32 bits) in size. workspace Computer resources that are assigned to a computer user. workstation A computer, usually with a graphical display, for interactive use by an individual. See also server. write permission Authorization within a filesystem to modify the contents of files and directories. WWW Short for World Wide Web. Global distributed archive of HTML documents linked via HTTP. X Window System Also known as "X," the X Window System is a graphical windowing environment, originally created at MIT. yellow pages See NIS.