Table of Contents


Appendix A
TERMS AND DEFINITIONS


Term Definition

address A number specifying a location in the computer's memory. May also specify a line number or range of line numbers when used inside a line editor.
alias A single name used to reference one or more commands.
ANSI American National Standards Institute. An institution formed to set standards. One example is the ANSI standard definition for a terminal.
append mode A mode in a text editor which allows you to append text after the current buffer position.
ASCII American Standard Code for Information Interchange. Defines the bit sequence codes used to represent control characters, letters, numbers, and symbols on a computer system.
argument The part of a command line specifying the data to be used by the command. A command may or may not use arguments. An argument may consist of letters, numbers, or text strings. An example is cat -v myfile. The argument is myfile, the command is cat, and -v is an option.
array A consecutive set of data elements referenced by the same name. Each element is referenced using subscripts to index the location in the array.
background A state of execution referring to commands that have been started from your terminal but are not interactive. The commands execute and are associated with your terminal but do not require input from your terminal. This allows you to continue executing other commands from your terminal.
baud rate The transfer rate of data from a computer to a peripheral device or from one device to another. Modems and terminals are two common peripheral devices that use BAUD ratings for data transfer.
binary file A file composed of bits. Usually special application files or compiled machine executable programs are binary. This is more efficient for disk usage and for execution speed.
bit A binary digit. May be a 0 or 1. Binary means base two numbers.
block special file A device driver that preforms I/O (reads and writes) data blocks at a time. A disk drive driver is usually a block special file.
boot block A logical block of information stored on magnetic media (usually disk) containing machine instructions used to boot the system.
branching A decision point in a computer program that changes the logical flow of a process, program, or structure.
buffer A temporary storage area, usually in the computer's memory, used by programs to perform data manipulations. For example, the vi test editor copies the contents of your file to a buffer in the computer's memory. Any changes you make to the file affect the buffer image of the file, not the disk copy. When you save your changes the buffer is copied back to disk.
built-in command A command processed within another program. The shell has many built-in commands.
built-in editors Editors within the ksh and csh used to edit the command line. The ksh has two, vi and emacs. The csh has a psuedo version of ed.
byte A computer character composed of either seven or eight bits. The number of bits depends on the hardware architecture.
character special file A device driver that performs I/O (reads and writes) on a character-by- character basis. Terminals are usually a character special device.
child directory See subdirectory.
child process A process spawned (started) by another (parent) process.
command A request for the shell to perform. A command may be a built-in command, an alias, function, program, variable assignment, or programming construct.
command interpretor A program that reads commands, interprets the input, translates the input commands to instructions understood by the system. Thus the shell is a command interpretor that acts as an interface between you and the computer.
command mode A mode in a text editor where the characters you type are interpreted as editing commands. This mode allows you to perform certain actions within the editor.
command substitution The replacement of part of a command with the output of another command.
compiler A computer program that converts source code written in a high-level computer language to a machine-executable binary file. Once the source code is compiled you execute the new program by entering the filename as a command.
concatenate To combine separate elements into one. You concatenate files when you combine multiple files into one.
constant A value used in computer programming languages that remains constant throughout the entire program.
control character An ASCII character with a value from octal 000 to 037. Control characters usually have special meanings. For example, Ctrl-S is often used to stop text from scrolling off your screen and Ctrl-Q is used to restart the text scrolling.
CPU Central Processing Unit. The part of the computer that reads machine code instructions and performs the associated task. The Intel 80286 and 80386 are microchip CPUs.
current directory See present working directory.
cursor A rectangle or underscore displayed on your terminal indicating where you may enter or delete characters on your display.
daemon A program usually started when the system enters multiuser mode and remains running to service related user programs' requests. For example, the print spooler daemon processes all print requests queued by the lp (lpr on BSD) command.
default The value or condition automatically assigned (assumed). You must explicitly change the value or condition to keep the default from being used in most situations.
delimiter A character or characters that logically separate words or fields of data. For example, a space and tab are used to delimit options and arguments on a command line.
device A physical unit used by the computer for a specific task. Examples are printers, hard disks, floppy disks, modems, and tape drives.
device driver A special program written to control a specific device. Device drivers on UNIX are accessed as files. When the file is accessed the major and minor numbers of the device actually map to specific system calls used to execute the appropriate code for the device.
diagnostic program A program used to isolate problems on a computer.
diagnostic message A message displayed by a program when an error has occurred.
directory A file containing the names and inode numbers of other files. The other files may be any type of file (special, directory, or regular files).
disk A type of magnetic storage device, usually containing several hard disks resembling phonograph albums. This media provides large amounts of data storage with fast access capabilities.
EBCDIC The standard used by IBM to define what set of binary digits (bits) represent a single character. The code defines alphabetic characters, numerals, control characters, and special characters.
EOF End-of-file. The Ctrl-D character is used to indicate the EOF mark from your keyboard. But UNIX does not actually use any character to mark the end of a file on disk. Instead the byte count of the file is used.
EOL End-of-line. The end of a line marker. UNIX usually uses a new-line.
electronic mail The exchange of information via a computer and other electronic devices. UNIX provides mail/mailx for reading and writing mail messages.
environment The conditions which control your computer processes while you are logged in to the system. Your environment on UNIX allows you to customize your login shell.
environment file The file executed each time the shell begins execution. Only used by ksh and csh.
erase character The character used to delete the previous character you just typed on your terminal. UNIX allows you to reset this to any character using the stty command.
escape The ability to leave a program and then return at a later time at the same point from which you exited, such as in the vi editor when you type :sh<CR>.
execute The act of processing machine instructions by the computer. A program is loaded into memory, then the computer's CPU begins operations based on instructions within the program.
executable file A file containing instructions that are processed by the system. The program is stored in a file. UNIX allows shell scripts to be executable files. When you enter the name of an executable file UNIX checks the contents then executes the file accordingly.
export To pass variables to a child process.
external command A standard UNIX command requiring a process ID for execution, as opposed to an internal command which executes within the current process.
FIFO See queue.
file A collection of data. A sequence of bytes. Files may contain data, programs, or various other text. Data is read from and/or written to the file. The file may be an executable script or program.
file descriptor A number associated with an open file.
file system A large logical structure used to store boot blocks, inode blocks, and data blocks. All files are stored in data blocks within the file system.
filename A sequence of characters used to identify a file. The total number of characters must not excede 14 characters in System V. A slash (/) cannot be used in UNIX filenames.
filter A program that reads from the standard input and writes to the standard output. The standard input of the filter may be your terminal keyboard or the standard output of another program. The standard output of the filter may be sent to your terminal screen or the standard input of another program.
firmware Executable machine code stored in EPROMS, PROMS, or ROMS. Usually used to perform initial boot procedures.
flag A command option. See option.
foreground The execution of one command from your terminal. UNIX executes the first command. After the first command completes, the computer prompts you for another command.
fork The act of creating a new process. The new process is a child process of the parent which forked it. There is a system call used to spawn (start) a child process.
full-duplex A type of data communications where the computer can transmit and receive data simultaneously. Your terminal usually communicates with the UNIX System in full-duplex mode.
full pathname A pathname that begins with the root directory (/) and continues to the specific directory or file.
function A list of commands referenced by a command name. A function is user definable. It is stored in a file and loaded into the shell's internal memory. When you reference the function's name the function is executed.
global Applies to the entire file or environment. For a file, global is used in text editor terminology, referring to lines 1 to EOF. In the environment, global refers to variables that are known throughout the environment.
group ID A number relating to a specific group of users. Used by the system and utilities to control who has access at the group level.
hardware The physical components combined to create a part of a computer or the complete system.
here-document Lines of input redirected by the shell to a program. The input immediately follows the program and is read by the shell and written to the standard input of the program.
hidden character A character that is not displayed on your terminal. This is usually a control or special character.
history file The file used by the shell to save each command you entered at the prompt.
input/output The input and output of data by the computer to and from its associated devices.
input/output device A physical electronic device that can receive input from another device and/or send output to another device. Your keyboard is an input device and your terminal is an output device.
inode Information node for a file. Contains all pertinent information concerning a file.
inode number The number of a file's inode contained in the second field of a directory entry.
insert mode A mode inside a text editor. The characters you type in this mode are entered into the editor's buffer as text.
interactive An operating system that you interact with moment by moment. You enter a command and wait for a response. When the program resonds back, you enter further information or another command.
interactive program A program that communicates with its user. The program requests information and the user responds appropriately. Usually the input device is a keyboard or mouse and the output is to a terminal screen or printer.
internal command Same as a built-in command.
interpreter A program that reads commands and executes them.
interprocess communication The ability for two or more separate programs to communicate with each other.
keyword parameter The assignment of a value to a variable on a command line. Thus the variable is only known by the command being executed on the command line.
keyword command A word in the shell that has special meaning. The keywords in the UNIX shell perform certain program control.
line editor A program that allows you to edit text line-by-line. Still useful in shell scripts.
library A collection of commonly used utility programs. Most computers have source code, object code, and compiled program libraries. This eliminates the need for programmers to write commonly used programs.
login The procedure you use to gain access to the computer system.
login name See user name.
metacharacter A character that has special meaning to a computer program. The shell uses metacharacters to perform filename matching and expansion.
modem An electronic communications device used to transmit data accross phone lines.
multitasking A computer operating system that allows multiple tasks (processes) to execute simultaneously. Actually, the execution of each task is serial. The operating system shares the CPU with multiple tasks by swapping the tasks in and out of the CPU.
multiuser A computer operating system that allows multiple users to use the system's resources simultaneously. A multiuser system implies that it is multitasking.
operating system The software on a computer system that is loaded at boot time. All other software is controlled by the operating system. All computer resources (devices, memory, etc.) are also controlled by the operating system.
option Special arguments on a command line that control how the program will function. On UNIX the convention for specifying an option is to precede it with a dash (-).
ordinary file A file containing text or data.
output The writing of information from the computer to an output device such as a printer, terminal, or even a file.
parameter A variable or constant placed on a command line. Options and arguments are command parameters.
parent directory The directory directly above a directory. Referred to relatively as "..".
parent process A process that spawned (started) a new process. The new process is referred to as a child process.
parity A checksum technique used to varify proper transmission of data. Even parity adds all the 1 bits in a byte and sets the parity bit to 1 or 0 to make the sum of all 1 bits even.
parse The process of scanning and deciphering data based on certain criteria. A program parses its command line deciding which are arguments and which are options.
path A list of directories searched for specific types of files.
pathname Directory names separated by slashes (/) preceding a file or directory name. See full path and relative path.
peripheral device Devices controlled by the main computer. Used to input, output, and store data. Terminals, printers, disk drives, and modems are all peripheral devices.
permissions Modes on directories and files that control which users can access certain files for reading, writing, and executing. The chmod command is used to change permissions.
pipe The process of sending the output of one command to the input of another. The shell creates a pipe when a "|" character is encountered.
pipeline Multiple filters separated by the pipe character "|." The output of the first filter becomes the input of the next.
ports The physical devices where external peripheral devices are connected to the computer. Terminals and printers are the most common type of peripherals connected via physical ports.
positional parameter The parameters (arguments) of a shell script command line that have been assigned to variable names beginning with 0 and continuing through the number of arguments on the command line.
present working directory The directory in which you are currently located. The dot (.) is used to represent the current directory.
primary prompt See prompt.
printer A physical device used to print data on paper or some other physical media.
process A program in execution. Includes the contents of memory, register values, files statuses, current directory, and login related information.
program A set of instructions that can be executed by a computer to perform specific tasks.
prompt A string displayed on your terminal to inform you the shell is ready to accept the next request. The value of the prompt is stored in the variable PS1 (ksh) and prompt (csh).
queue A structure to control the processing of data or programs. The first data in the queue is processed first. Often refered to as a FIFO, first-in-first- out.
RAM Random Access Memory. Used to store programs during execution. Allows direct access to any memory location.
read-ahead The ability to read input while processing output or previous input.
regular expression Any string of text used to search for matching text in a file. Special regular expression characters allow various matching patterns.
relative pathname A pathname to a directory or file starting from your present working directory.
remote system Any system other than the one on which you are currently working.
ROM Read only memory. Once the memory has been loaded with instructions it can only be read from by other programs.
root The top level directory of the UNIX file system hierarchy. Designated by a slash. Also the user name of the super-user.
screen editor An editor which edits text relative to the position of the cursor. The changes are visually updated as you make them.
script A program written in a shell language.
secondary prompt The string displayed by the shell if the command you typed at the primary prompt was incomplete.
shell A program that reads requests, interprets them, and requests tasks from the operating system. It is an interface between you and the computer.
shell script See script.
signal A message which is sent from one process to another.
source code The English version of a program. The source code must be compiled into object code and then linked into machine code.
special character See metacharacter.
special file A file used to interface with a device. Referred to as device drivers in the kernel. The /dev/ttyXX files are special files used to communicate with your terminal or printers.
spooler A subsystem of the computer system which stores print requests on disk until the printer is available for printing.
standard input The default file assigned to file descriptor 0. Usually this is set to /dev/ttyXX for your shell to read input, where XX is the number of the tty on which you are logged in to the system.
standard output The default file assigned to file descriptor 1. Usually this is set to /dev/ttyXX for your shell to write output.
standard error The default file assigned to file descriptor 2. All programs display their messages to this file descriptor.
stream A sequence of input data to or output data from a program.
string A set of characters. Any pattern of characters combined together, which may include metacharacters.
subdirectory A directory contained within another directory. If you list the current directory, the subdirectory is displayed as a file.
subscript A numerical expression contained within brackets ([ ]) after a variable which evaluates to a specific index within an array.
subshell A shell started (spawned) by a parent shell.
super block The second block of a file system which contains critical information about the file system.
super-user The "root" login. The super-user is a user who has operating system authority to perform any task on the system. Equivalent to other systems' "supervisor" level users.
system A complete and functional computer system. Usually refers to the hardware and the software as a whole.
system administrator The person in charge of systems analysis, operations, maintenance, and various other administrative tasks on a UNIX operating system.
task A unit of work to be performed by the computer.
terminal A device used to input data to and display output from a computer system. It allows you to interact with the computer.
timesharing A computer operating system which allows multiple users to share the resources of a computer. It appears to user that the resources of the computer are dedicated to him.
tty Teletype. A device resembling a typewriter that provided interactive access to the computer. The predecessor of the terminal.
UNIX System A time sharing interactive computer operating system and the associated application and language software.
user A person who uses a computer or computer operating system.
user ID A number assigned to each user for identification by the system and utilities.
user name A string of characters used to identify a user on the computer. Each user name on a system must be unique.
utility A software program used to perform routine tasks.
variable A symbolic name whose value may change throughout a program.
working directory See present working directory.


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