Introduction to CGI and Perl

1. Introduction to CGI and Perl CGI/Perl Programming By Diane Zak

2. 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

3. 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

4. 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

5. 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

6. World Wide Web (WWW) • URLs are used to tell the browser exactly which web page to request • URL = Uniform Resource Locator

7. 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

8. 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

9. Dynamic Web Pages • 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 systems without much updating

10. 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

11. 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

12. 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)

13. 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 create a new line

14. 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

15. 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 execution errors and will execute the script

16. Testing a script Results of perl -c and perl -w on UNIX Results of perl -c and perl -w on Windows

17. 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

18. Testing a Script Results of testing the first.cgi script on a Web browser

19. 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

20. 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

21. 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 a program or script.

22. Summary • 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”;

23. 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 (;).