UNIX Hints & Hacks
Appendix E: Glossary
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UNIX Hints & Hacks |
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Appendix E: Glossary |
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access permission This allows you to read, write, or execute various files depending on the permissions that are set on the file that you are trying to access.
alias UNIX commands that you can define with new shorter names. This table is a mechanism that is associated with the C and Korn shells. You can take a long command that can be over 15 characters in length and type two keys to perform the same command after you have made an alias of it.
append When a command is executed, the output can be attached to the end of another file. This append process is more typically performed by having two greater than signs ( >>) at the end of a command that points to another file. The output from the commands is attached to the bottom of the other file.
argument This is a string type that can be a number, letter, or filename that is passed to a command at the time it is called to provide additional instructions to the command. A command-line argument can be anything on a command line following the command name.
background process A process that can be started with an ampersand ( &) at the end of a command. The process in the foreground can be placed in the background by pressing Ctrl-Z and typing bg. It requires no user interaction, but if there is a pause in the program that runs in the background, it might stop and not proceed.
baud rate The term baud is equal to the duration of the shortest signaling element. This is not the same as a unit of information (a bit), but actually the rate at which a communication channel is measured across a wire. In today's standards this usually relates to the speed of a modem. The current claim is that the common baud rates range from 110 to 19,200 baud. The most common use for terminal servers is 9600.
Berkeley UNIX This version of UNIX was developed at University of California at Berkeley by the Computer Science Department. Berkeley Software Distribution is sometimes known as BSD.
Bourne shell This shell was developed by Steve Bourne at AT&T Bell Laboratories for the standard UNIX System V command processor. See shell.
BSD See Berkeley UNIX .
buffer This is an area of memory that stores and maintains data until it can be used.
buffer cache Similar to a regular buffer, information gets written to a file on a disk, the UNIX system 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.
C shell Developed by Bill Joy for Berkeley's BSD UNIX, it is another command processor like the Bourne shell. Its programming structure when writing scripts is similar to those that use the C programming language.
call This is one way to call a program into action.
case-sensitive UNIX is extremely case sensitive; there are certain occasions where being able to distinguish lowercase characters is a must.
child process A process that gets created by a parent process. In reality, every process is a child process except for the very first process, and that one is started when the system boots and begins execution. As you execute your scripts, the shell spawns a child process and there is nothing you can do about it.
command When you sit at a shell prompt, this is what you type to have the computer perform some type of task. This task executes a utility, a program, or a script.
command line This is a term that refers to a line that you enter in response to a shell prompt.
command-line argument See argument.
console terminal Typically called the console. It is the main system terminal that is locally connected to the physical CPU. All system error messages are directed to display on the console. Consoles are usually connected to the serial port the computer, but with the advancements of graphic consoles, this doesn't have to be the case.
CPU Central Processing Unit. This is the heart of the computer where all the processing of calculations and manipulations of the system takes place.
crash This is an occurrence that takes place. One we do not like to happen. Suddenly the system just stops, halts, goes down, loses power, and ceases to function. In many cases, a total system failure occurs, brought on by some type of failure that is taking place within the system. If the system continues to crash without coming up--Ouch--you have really big problems.
.cshrc file This file is probed by the C shell when you log in to a system and sets the variables and aliases that you use while logged in.
daemon A process that runs in the background performing a function without being locked to a terminal. If an error were to occur, the output would be logged in the system messages or log file and would also be displayed on the console.
device A device is simply a type of peripheral. It can be anything from a disk drive or printer to a terminal, network card, or any other I/O unit that is attached internally or externally to the computer.
device driver These are usually built in to the UNIX Kernel that controls a device such as a disk drive, tablet, or a terminal.
device file All devices attached to the computer are accessed through an actual device file that is located off the /dev directory. A device file is also sometimes referred to as a special file.
directory In UNIX, directory is actually short for directory file . It is a file that contains a list of other accessible files and directory files within it.
disk partition When a disk is sliced up into portions, this is called partitioning. This is often done to balance the access time of I/O operations across the disk. It prevents total corruption of all the data. If one partition fails on a disk, the data can sometimes be salvaged on the rest of the disk.
dummy files These are files that really serve no purpose except for testing. Large dummy files are created and burn a system by testing I/O throughput on disk drives. Small dummy files are used for testing the network, sending mail, and printing.
editor A utility or tool that is used to create and modify text files. The vi and ed editors are part of the UNIX system. Many UNIX systems also come with others and there are lots of different ones available on the Internet.
environment Everyone that logs in to the system has an environment that is referenced and made available to the command processor. Within the environment, a list is made of all the variables you have access to and their values for each of those variables.
exit status The status returned by a process; either successful (usually 0) or unsuccessful (usually 1). These values can be passed to a variable and can be useful in certain shell scripts.
filename extensions This the part of a filename that follows the period at the end of the name. There are many types of extensions, and it seems like there are more types every week. Some include .uu, .tar, .gz, and .Z, to name a few.
filesystem A type of data structure that resides on part of the disk. All UNIX systems have a root filesystem and most have at least a few other filesystems depending on the configuration of the system and the amount of disk space it has. Each filesystem is made up of three things: a superblock that contains information about the filesystem, the inodes that contain control information about individual files, and data blocks that hold all the information in the files.
flag Similar to an argument, a flag indicates a particular option or modification to take place when the command is executed. In many cases, flags begins with a hyphen in front of them.
foreground Where the background doesn't have to attach to a terminal, foreground must run under direct control of the terminal. During this time, the terminal cannot be used for anything else until a foreground job finishes or is halted. A foreground job can be placed into background by using the command Ctrl-Z, followed by bg, for background, at the shell prompt.
fork A term that is used when one process creates another process. This is also known as spawning a process.
GECOS field The fifth field in the password file. This is often referred to as the comments field. The GECOS field holds all the necessary contact information on a user: name, phone number, location, and any other identifying information.
graphic consoles These are consoles that support a windowing environment and is still connected physically to the local CPU. A multiple console window can be displayed to more than one user to monitor possible error messages that are sent to the console. Other windows can be utilized on the graphic console to perform normal daily tasks.
group Refers to a collection of users. Groups are set up as a basis for determining file access permissions between groups of users. If you are not the owner of a file and you belong to the group the file is assigned to, you can then be granted access to the file. On BSD systems, a user can belong to several groups at the same time. On AT&T systems, each user can belong to only one group at a time, but a user may change his or her group with the command newgrp.
group ID This ID number exists in the fourth field of the /etc/passwd file and directly relates to the number that must be defined in the /etc/group file. If this number does not have a group name associated with it and is not defined, the user does not belong to any groups and can be considered a security risk.
history This is a command in the C shell and a function in the Korn shell that allows a user to execute previously executed commands.
home directory This is the working area that users first log in to. The path of this location is set from the sixth field of the password file and is stored in the shell variable HOME.
hotkeys These are keys that perform a function when that predefined single key is hit. One place these can be set up is in the vi editor.
HTTP Hypertext Transfer Protocol, the transport mechanism that Web pages use to move from a server to a Web browser client.
implementation The process of performing a task through to its completion. This generally refers to piecing together a new systems loading the OS bringing it online turning it into a production system for the users.
inode You hear the word a lot. Here, finally, is what it means. An inode is a data structure that contains information about a file. The inode for a file keeps track of all the following information: the file's length, when the file was last accessed and modified, access privileges, owner and group IDs, number of links, the time the inode was last modified, and pointers to the data blocks that contain the file itself. There is always one inode per file. When parts of the inode are lost, they pop up in the directory lost+found within the partition they once existed in.
installation There are a couple of meanings to this word. It is a term that refers to loading or building a system with the UNIX operating system. Another use of the word is going to a specific location to work on a computer.
interactive A program, utility, or tool that allows ongoing dialog with the user.
I/O device Input/output device. Also see device.
kernel It is the center of the UNIX operating system universe. They wouldn't call the actual kernel file vmunix or UNIX if it wasn't. The kernel allocates the necessary resources that are needed and controls the processes that are running.
kill This forces a process to die, terminate, exit, or stop processing. There are flags that can be passed to the kill commands that allows the process to exit gracefully or die hard without warning.
Korn shell The Korn shell was developed at AT&T Bell Laboratories by David Korn. This command processor is becoming a growing favorite over the C shell and Bourne shell users.
line editor This is an editor that can be run on a terminal without any emulation to support it. When the user wants to make a change to the file, they have to go to the line they want changed and, then, indicate the change desired. This is all done from a command-line interface.
link A pointer to a file. There are two kinds of links--hard links and symbolic (soft) links. AT&T systems have only hard links whereas BSD versions support hard links and symbolic links. A hard link associates a filename with a place on the disk where the contents of the file are located. A symbolic link associates a filename with the pathname of a hard link to a file.
.login file A file that is executed by the C shell when you log in. It provides the ability to set environment variables and run commands that you want executed at the beginning of each login session.
login name The name is the account name that you are assigned and log in to the system with. Every login name has a corresponding unique user ID, which is the numeric identifier for the user. The login name is the first field and the user ID is the third field in the /etc/passwd file.
login shell The shell that you are using when you first log in. The login shell can fork other processes that can run other shells as well as running utilities and other programs.
.logout file The C shell executes this file from your home directory when you log out. You can put commands in this file that you want run each time you log out.
macro A single piece of instruction that a program replaces with several more complex instructions. These can be set up within vi to save you time when trying to save a little time.
map This assigns a new interpretation of a terminal key. This is used in the vi editor.
multitasking UNIX has the ability to run multiple complex applications, programs, scripts, and other processes all at the same time.
multiuser One of the two great reasons for using UNIX is the number of users that UNIX can handle on each machine. UNIX servers can handle numbers into the thousands. Multitasking is the other great reason.
NFS A remote filesystem that can be seen on the network and appear on your local system as if it is a local mounted device.
network Interfacing, linking up, or connectivity of multiple systems over the same LAN or WAN.
null device This is a special file in /dev/null. So special that anything sent to it is discarded into thin air. You can copy files to it or redirect output; if you have huge files being created, you can point a symbolic link to it.
OSI Open Systems Interconnection. It is a model that shows data is moved across a network and is made up of seven layers that data traverses through: the application layer, presentation layer, session layer, transport layer, network layer, the link layer, and the physical layer.
PID Process identification. Usually followed by a number. When a new process starts, the operating system assigns a unique PID number to each process.
pipe A pipe is a connection between two programs in such a way that the STDOUT of the first program is sent through the STDIN of the next program.
power tools A generalization of powerful scripts, programs, and useful utilities that are written by UNIX administrators and for the most part are freely available on the Internet.
.profile file This file is used to run commands, set variables, and define functions for the Korn and Bourne shells.
recursive When relating to a directory structure, it traverses through the subdirectories until it cannot go any farther. Computer programmers, however, use this when writing source code to describe a program that continuously calls itself until some condition is met.
redirection The process of directing STDIN for a program to come from a file rather than from the terminal or console. It also goes the other way, by directing STDOUT or STERR to a file rather than to the terminal.
root directory The top level of all directories and the start of all absolute pathnames. The root directory is called or referenced by the slash symbol (/).
root filesystem This is the filesystem where all the system-related files live. It should never span across multiple disks. If you keep it on one disk you are able to bring the system up into single-user mode, as long as your disk is healthy.
root login In reference to the superuser account that is created with administrator privileges only. The bearer of this account has complete control over the entire system and all the files no matter who owns them. The login name of the superuser is typically root.
run This is not a physical command, but is only a reference to executing a script, program, or a command.
shell A UNIX system command processor for users and administrators alike to work in. There are three major shells: the Bourne shell, the C shell, and the Korn shell. There are several others available on the Internet: tcsh, zcshrc, and bash.
shell script In simple terms, this is really only an executable flat text file that consists of a series of shell commands.
single-user In single-user mode, only one account is allowed to log in at a time. Several services, such as networking, NIS, DNS, printing, and mail, are disabled. This is usually reserved for administrators to access.
smart terminal Although these terminals are supposed to possess some computer power, unfortunately, they are sometimes dumber than their counterparts, the dumb terminal. Evaluate these units before you buy them to make sure they are right for you.
special file See device file .
standard error (STDERR) Within a shell you can instruct a program to redirect all the standard errors out to a file. If you don't tell the program where to send them, it directs it out to the device that represents the current terminal.
standard input (STDIN) When a program receives input from a file or a terminal, the program is said to be receiving data from standard input.
standard output (STDOUT) When a program sends output to a file this is sending the data to standard output.
startup file A file the login shell runs when you log in. These are the files .profile, .login, and .cshrc.
string A sequence of characters.
subdirectory A directory that is located within another directory. Because the root directory is at the top, every directory except the root directory is a subdirectory.
superblock A block that contains control information for a filesystem. The superblock contains housekeeping information, such as the number of inodes in the filesystem and free list information.
swap The process that occurs when the operating system moves a process from memory to a disk or vice versa. Swapping a process to the disk allows another process to begin or continue execution.
symbolic Link A directory entry under BSD that points to the pathname of another file. In most cases, a symbolic link to a file can be used in the same ways a hard link can. A symbolic link can span filesystems and it can connect to a directory, where a hard cannot.
system administrator The person who is responsible for the day-to-day operations and maintenance of the system. The system administrator has the power to do anything, but don't abuse the privilege.
system console See console terminal .
System V One of the two major versions of the UNIX system. System V is a product of AT&T.
termcap Terminal capability. The termcap file contains a list of various types of terminals and their characteristics. AT&T-based systems replaced the function of this file with the terminfo directory.
terminfo Terminal information. The /usr/lib/terminfo directory contains many subdirectories, each containing the terminal emulation database information. Each one contains a summary of the functional characteristics of a particular terminal.
tty A terminal. tty is an abbreviation for teletypewriter.
UGU The UNIX Guru Universe Web Site for UNIX System Administrators. http://www.ugu.com
URL Uniform Resource Locator. This is the way a client-based Web browser knows how to address a web page, FTP site, or some other supported form of accessing servers through the Internet or intranets.
user A person using the computer system.
user ID The first field of the password number that the/etc/passwd file associates with a login name.
variable A name and an associated value. The shell allows you to create variables and use them in shell scripts. Also, the shell inherits several variables when it is invoked, and it maintains those and other variables while it is running. Some shell variables establish characteristics of the shell environment, while others have values that reflect different aspects of your ongoing interaction with the shell.
work buffer A location where ed and vi store text while it is being edited. The information in the work buffer is not written to the file on the disk until you command the editor to write it.
working directory The directory that you are associated with at any given time. The relative pathnames you use are relative to the working directory. Also known as the current directory.
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UNIX Hints & Hacks |
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Appendix E: Glossary |
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