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Introduction to
CGI and Perl
CGI/Perl
Programming
By Diane Zak
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Objectives
• In this chapter, you will:
• Review basic Internet terminology
• Learn about the CGI protocol
• Create a CGI script using the Perl
language
• Run a CGI script from the command
line and Web browser
• Debug a CGI script
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Internet Terminology
• Internet
– Global network that connects millions of
computers and many networks together
• IP Addresses
– IP = Internet Protocol
– Used to uniquely identify machines on
the Internet
– Made up of 4 sets of numbers separated
by periods
– Example: 216.148.218.195
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Internet Terminology
• Domain Name
– Friendly name that is easier to
remember than an IP address
– Association of a name to an
address
– Example: www.redhat.com is
associated with 216.148.218.195
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World Wide Web (WWW)
• Web browsers are used to view
Web pages written in HTML that
have been served up by Web
servers
– HTML = Hypertext Markup
Language
– Web pages have .html or .htm
extensions and are stored on web
servers
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World Wide Web (WWW)
• URLs are used to tell the
browser exactly which web
page to request
– URL = Uniform Resource Locator
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World Wide Web (WWW)
• URLs are made up of 4 parts:
– Communication Protocol
• Typically http://
– HTTP is the default communication protocol, so
URL www.redhat.com = http://www.redhat.com
– Web server domain name
• IP address or domain name of the server
– Path or directory (optional)
• A specific directory on the web server
– Web page filename (optional)
• A specific file on the web server
• If a filename is not typed, a default document
(if it is known to the web server) will be sent
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Static Web Pages
• A static web page will only change if
the page is manually edited or
updated
• Usually used for information that
doesn’t change often
• Not interactive
• Most web pages are static
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Dynamic Web Pages
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• Interacts with user
• Uses programs or scripts to create a
dynamic HTML page using CGI, ASP,
Java applets, etc.
– Programs:
• Compiled beforehand, so the computer can
read the code
• Faster than scripts
– Scripts:
• Each line is converted into code as the script
is running
• Easier to create and change
• Works on most platforms or operating
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systems without much updating
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Introduction to CGI
• CGI = Common Gateway Interface
– Protocol that allows communication between
web server and CGI scripts
– CGI scripts usually stored in cgi-bin directory
– Scripts usually have .cgi or .pl extensions
– Can use scripting languages or compiled
languages to create CGI scripts
• Scripting languages: Perl, AppleScript, UNIX shell
• Compiled languages: C, C++, Visual Basic
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How CGI Works
• Web browser requests
CGI script from web
server using HTTP
protocol
• Web server sees that it
is a CGI script and runs
the script
• The CGI script sends
output back to the web
server which sends the
output to the browser
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First CGI Script and Perl
• Finding Perl interpreter
– Need to know where the Perl
interpreter is, to include in your
program
– whereis perl (UNIX)
– Start --> Search --> For Files or
Folders --> perl.exe (Windows)
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First CGI Script and Perl
– shebang line - tells the operating system
where the Perl interpreter is located necessary on UNIX
– comment line – ignored by the Perl
interpreter
– End statements with semicolon
– newline character – to end a line and
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create a new line
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First CGI Script and Perl
• Content-type header
– tells the script what type of output
there will be
– print “Content-type: html/text\n\n”;
• print is used to output information within
the quotation marks to the browser
• HTML tags are created dynamically
using print statements
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Testing a Script
1. Open shell or DOS prompt
2. cd path
path=directory where the Perl script is located
Example: cd cgi-bin/chap01
3. perl -c filename OR perl -w filename
filename = the name of the Perl script
Example: perl -c first.cgi or perl -w first.cgi
-c option or switch will check for syntax errors
but will not execute the script
-w option or switch will check for execution15
errors and will execute the script
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Testing a script
Results of
perl -c and
perl -w on
UNIX
Results of
perl -c and
perl -w on
Windows
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Testing a Script
• Additionally, can test a Perl/CGI script
with a Web browser
– On a UNIX machine, change the file
permissions of the script so that it can be
executed
• chmod 755 filename
– In the browser, type in the URL of the script:
• http://yourservername/cgi-bin/chap01/first.cgi
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Testing a Script
Results of testing the first.cgi
script on a Web browser
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Debugging a Perl Script
• When you use the -c or -w
switches with Perl, the line
number of the error will be shown
– Edit the Perl script, and go to that
line number
– Examine that line, and the line
directly above it
– Make the corrections, resave the
file, and try testing the script again
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Debugging a Perl Script
• Look for simple errors, such as:
– Missing semicolons at the end of
lines
– Uppercase commands or functions
(remember that Perl is case-sensitive)
– Missing quotation marks at the
beginning or end of print statements
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Summary
• Web pages are either static or dynamic.
• The contents of a static web page are
established when the page is created. A
web server will transfer the contents of a
static page to a browser, and the browser
will interpret and display the HTML
instructions or code.
• Dynamic web pages interact with the
user, and will use an HTML document and
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a program or script.
Summary
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• The CGI protocol defines how a web server
communicates with CGI scripts or programs.
• UNIX and Perl are both case-sensitive.
• Perl CGI scripts on UNIX must start with the
shebang line, which tells the location of the
Perl interpreter
• The Content-type header is necessary to
send HTML output to a browser:
– print “Content-type: text/html\n\n”;
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Summary
• The newline character is \n.
• perl -c scriptname will check for syntax
errors in the script, but will not execute
the script.
• perl -w scriptname will check for
execution errors in the script, and will
also execute the script.
• Perl statements must end with a
semicolon (;).
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