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Perl CGl Programming: No experience required.
(Publisher: Sybex, Inc.)
Author(s): Erik Strom
ISBN: 0782121578
Publication Date: 11/01/97

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Table of Contents


Glossary

a

Apache
A UNIX Web server that has recently become popular. Apache supports all of the server-side includes.
applet
A small program that is run from an <APPLET> tag in an HTML document. At this writing, only Java programs can be run as applets from a Web page.
array
A number of values stored sequentially under a single variable name. A list in Perl. The first element has an index of 0 and can be referenced as a scalar variable: $array[0]. See list, scalar.
associative array
In Perl, a special kind of array in which each element consists of two parts: a key and a value. Variables that refer to associative arrays are prefixed with the percent sign (%).

b

binary numbering system
A base-2 numbering system (as opposed to decimal numbering, which has a base of 10). The binary system has only two digits, 0 and 1, which makes it convenient for representing transistor switching states in a computer (0 is “off” and 1 is “on”). All computer instructions eventually boil down to binary numbers.
browser
A program that is capable of translating and displaying documents formatted in the Hypertext Markup Language. Web browser software is the medium through which most communication happens on the World Wide Web.

c

C
The programming language on which Perl is based. Also, the programming language in which most of the UNIX operating system was written.
C++
An extension of the C programming language that is object oriented. In C++, data structures are tightly bound as objects that can be added to and passed around as a whole. The intent is to allow the programmer to deal with a data structure as a piece, rather than a loose collection of pieces.
CERN
The acronym for Centre Européen pour la Recherche Nucléaire, the European Laboratory for Particle Physics, where the World Wide Web was born. Also the name of one of the two most popular UNIX Web servers.
CGI
An acronym for Common Gateway Interface, the standard for communication between a Web server and a program running on the server machine. CGI is the pipeline for using an independent program to process information entered on a Web page.
Common Log Format
A structure for recording Web server transactions in a log file that is used on the NCSA and CERN servers for UNIX. A similar logging format is used by the Sambar server, and Microsoft’s Internet Information Server can be configured to conform to the Common Log Format.
concatenate
Literally, to link or combine. In programming languages, the term usually refers to strings: “Now is the” concatenated with “time for all” would result in “Now is the time for all”. In Perl, the dot (.) operator is used to concatenate strings.
conditional expression
In programming, a construct that allows the program to make decisions on how it will proceed based on the truth or falseness of a set of conditions. Conditional expressions say something along the lines of “if this is true, then do this; otherwise, do that.” In Perl, the if, else, and elsif statements are used to build conditional expressions.
counter
In programming, a variable used to count, usually through iterations in a loop.

d

die
A commonly used Perl function that allows the programmer to print a message and exit the script semi-gracefully when a fatal error condition is encountered.
directory functions
A set of Perl functions that allow directories to be treated generally like files. See opendir, readdir, seekdir, telldir, and closedir in Appendix A.
DTD
The acronym for Document Type Definition, the specification for a descriptive markup language such as HTML. Usually used in reference to the Standard Generalized Markup Language, from which HTML is derived. See HTML, SGML.

e

environment
An area of the computer’s memory where information is stored in variables that are accessible to more than one program. The environment is the mechanism through which information is passed to a Perl program from the Web server, through CGI.
environment variable
A variable accessible to more than one program in which environment information is stored. Environment variables are important in CGI programming, because they are used to pass data from the Web server to the CGI application. In Perl, the environment is available through the %ENV associative array. See Table 3.1 in Skill 3.

f

file handle
The variable created when a file is opened in Perl (and many other programming languages). All operations on the file are performed through the handle variable.
firewall
A program or an entire computer that stands between the Internet and a local area network (LAN). Firewalls are used to control access—both in and out—to the LAN and the Internet. Usually set up to make the LAN more secure from intruders.
flag
In programming, a variable that is either true or false. Also known as Boolean values, after the 19th-century British mathematician George Boole, who invented an algebra based on symbolic logic.
#flastmod
An SSI directive that returns the last modification date and time of a file. See server-side includes.
#fsize
One of the server-side includes directives. Returns the size of a file.
Free Software Foundation
Most noted for the GNU project, it was started by Richard Stallman at the Massachusetts Institute of Technology in 1983 as a way to avoid proprietary software. FSF’s philosophy is that software should be free and freely passed around. Fascinating background is available at www.gnu.org/gnu/gnu-history.html.

g

GET
The default HTML method for sending information from a form to a CGI application. The GET method sends the information in a URL, which is passed to the CGI program in the QUERY_STRING environment variable.
Ghostscript
A freeware interpreter for the PostScript page-formatting language. Ghostscript can be used from a Perl script to create dynamic graphical images on a Web page.
global variable
A variable that is visible to the entire program. In Perl, this would be any variable that is not declared with the local() function.
GNU
A project of the Free Software Foundation to provide freeware versions of UNIX utilities—and UNIX itself. See Free Software Foundation.
guest book
On a Web site, a page where visitors can enter information about themselves and search for information about other visitors. See Skill 7.

h

hashing
In Perl, the process of translating the key-value pairs of an associative array into a numeric value that can be looked up very quickly.
hexadecimal numbering system
A number system with a base of 16, as opposed to decimal numbering, which has a base of 10. Hexadecimal, or “hex,” numbering is convenient for dealing with binary numbers because each hex digit represents exactly 4 binary digits, or bits. The hexadecimal system uses all of the numeric digits, plus the characters A through F. See binary numbering system.
“hosts” file
A file on the local system that associates computer names with IP addresses. The first place TCP/IP processes look when names have to be resolved into addresses.
HTML
The acronym for Hypertext Markup Language. HTML is used to format documents for display on the World Wide Web.
HTML input types
The various types of data-entry tools that can be used in an HTML form. See Table 5.1 in Skill 5.
HTTP
An acronym for Hypertext Transfer Protocol, which is the transmission format used for communicating between servers and clients on the World Wide Web.

i

Internet
The worldwide network of computers over which the World Wide Web and every other significant computer communication protocol operate. The Net is not so much a thing as a collection. It is, in the truest sense, a network.
IP address
The numeric address of a computer on a TCP/IP network. The Internet itself is a TCP/IP network. IP stands for Internet Protocol. In their familiar form, IP addresses are specified as numbers separated with periods, such as 127.0.0.1. TCP/IP software, including the Perl functions that deal with networking, packs the address into a single number, which is used to find the computer being referenced.
ISP
An acronym for Internet service provider. ISPs typically purchase a high-bandwidth connection to the Internet, then sell dial-up access to it to users for a monthly fee. ISPs generally will host users’ Web sites, too, for an additional fee.

j

Java
A proprietary programming language that has become popular on the World Wide Web because it can be used to run graphical programs on a client that has connected to a Web server.

l

list
In Perl, a list—or array—of elements designated by a variable with the prefix @.
local variable
A program variable that is visible only in the module (a subroutine in Perl) in which it is declared. In Perl, local variables are declared with the local() function. See global variable.
logical operator
The syntax used in a programming language for doing logical, or Boolean, operations. In Perl, the logical operators are && for AND, || for OR, and ! for NOT.

m

MIME
The acronym for Multipurpose Internet Mail Extensions, the standard that is used to define the types of data that can be sent and received over the Internet.
multitasking
In operating system parlance, an OS that appears to run more than one program simultaneously by either sharing one processor for the instructions from several programs or sharing the instructions among multiple processors in a computer with more than one. Windows NT, Windows 95, and UNIX are all multitasking operating systems.

n

NCSA
The acronym for National Center for Supercomputing Applications. Based at the University of Illinois Urbana-Champaign campus. Mosaic, the first graphical Web browser, was developed at NCSA by a team that included Marc Andreeson, who later went on to found Netscape. The NCSA Web server is one of the two most popular UNIX servers.
null
In many programming languages, including Perl, a value that has no value. Many Perl functions return null on an error condition or to indicate that an array or list is empty.

o

operating system
The software on a computer that allows it to run programs and interface with peripheral devices such as keyboards, mice, disk drives, and monitors.

p

Perl
The Practical Extraction and Report Language, though it’s hardly ever referred to in such lofty terms. Perl is a C-like, interpreted scripting language that is heavily used in CGI programming for Web sites because of its adeptness with text processing. Perl is relatively easy to learn and quite easy to use.
polling
An opinion poll. Webmasters can use HTML forms and CGI applications to gather and collate opinions on any subject from visitors to their Web sites.
POST
One of the HTML methods for sending information in a form to a CGI application. In the POST method, the information is received by the CGI program via the standard input (STDIN).
printable characters
Characters that can be displayed on the screen, generally. Technically, the ASCII characters with values from 32 to 127. In a URL, printable characters that have a special meaning in URLs are encoded as hexadecimal numbers preceded by a percent sign. See Table 5.2 in Skill 5.

r

regular expression
An expression that defines a search string through various weird and wonderful UNIX conventions. Regular expressions are perhaps the most powerful aspect of Perl as a programming language.

s

Sambar
A freeware Web server that runs on Windows 95 and NT. The Sambar server’s most current release supports all of the server-side includes. Look at www.sambar.com for more information.
scalar
In Perl, a single value prefixed with $, as opposed to an array or list.
script
In the Perl-CGI context, a Perl program, so called because the source code is run as a script. It is not compiled into an executable file.
server-side includes
An extension to standard Web server software that allows you to send commands to the server from within an HTML document. See Table 8.1 in Skill 8.
SGML
The acronym for Standard Generalized Markup Language. SMGL is used to create descriptive markup languages. HTML was derived from it.
SHTTP
Secure Hypertext Transfer Protocol, a proposed standard to allow encrypted data to be transmitted over the World Wide Web.
SSL
An acronym for Secure Sockets Layer, a Netscape invention for allowing encrypted data to be transmitted on the World Wide Web.
standard input, output
Often abbreviated as STDIN and STDOUT in Perl and C programming. These terms usually refer to the computer keyboard for input and the monitor for output. However, input and output can be redirected to files or other programs. CGI makes extensive use of output redirection to get information between the Web server and CGI applications.

t

TCP/IP
Short for Transmission Control Protocol/Internet Protocol. TCP/IP is the standard by which all communication takes place over the Internet. It defines the lower-level layers of communication and how data will be sent and received.

u

URL
The acronym for Uniform Resource Locator, which is how addresses—even of CGI programs—are specified on the World Wide Web.

v

variable
A symbolic storage place for a value in most programming languages. Variables allow the programmer to give descriptive names to the values that will be used in the program.

w

Winsock
Short for Windows Sockets, Microsoft’s implementation of the Berkeley sockets application programming interface for network communication. All TCP/IP communication in the Windows operating systems takes place through Winsock.
World Wide Web
A collection of computers running software that understands the Hypertext Transfer Protocol. Truly worldwide; anyone with Web browser software and an Internet connection can cruise through the Web. The WWW has become explosively popular since 1990. It may even become a new standard of commerce if more secure standards of data transmission are adopted.
World Wide Web Consortium
W3C for short, a group that was formed in 1994 to develop and monitor standards for the World Wide Web. W3C currently is led by Tim Berners-Lee, who was instrumental in starting the Web in 1990. The group maintains a Web site at www.w3c.org.


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