Introduction to Programming the WWW I

1
Introduction to Programming the WWW I

• CMSC 10100-1
• Winter 2003
• Lecture 14

2
Introduction to Perl

• What is Perl?
• How do you run it?
• From command line
• From Web
• How do you write Web pages with it?

3
Accessing HTML Files Over the Internet
4
Web Application Program

• Carries out many dynamic tasks, such as the
  following
• Input a search term, search the WWW, and return
  the results
• Calculate and display the number of times that a
  page has been viewed
• Verify the input fields on a Web form
• Save a Web form into a database
• Display a special graph, or return the results of
  a calculation based on data input from a form

5
The Common Gateway Interface

• a standard that enables Web browsers to exchange
  data with computer programs located on a Web
  server
• first appeared in the NCSA HTTPD Web server
  software built by the National Center for
  Super-computing Applications (NCSA).
• one of the first widely used Web servers
• was simple and the program source code was made
  available for free
• It is simple to use and available on a variety of
  Web servers.

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How Browsers and Web Applications Work with CGI
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An Interface with Different Programming Languages

• Web application programs that are developed
  specifically to work with the CGI standard are
  known as CGI programs.
• Lots of different programming languages can be
  used. For example,
• Perl,
• Visual Basic,
• Java,
• C,
• C,
• and UNIX shell scripts.

8
The Perl Programming Language

• Practical Extension and Reporting Language
• invented in 1987 by Larry Wall at NASAs Jet
  Propulsion Laboratory
• developed as a utility programming language for
  the UNIX operating system
• gained popularity because of its ease of use,
  free availability via the Internet, and its
  powerful combination of features

9
Why Perl is Popular

• Perl is a free language with lots of free
  applications
• Perl is easier to work with than many other
  languages
• Perl provides a CGI interface module
• Perl applications are portable.

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Internet Service Provider Issues

• Some things to determine with your ISP
• Allow CGI programs to execute on its Web-server?
  Does it have Perl? What Version?
• Login and initial password on the Web server?
• Where do you put your Perl programs on the Web
  server and what permission settings?
• How much disk space for publishing?
• Support FTP and/or Telnet access? or SSH/SCP?

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Summary

• Web pages written in HTML are static and cannot
  interact with users.
• CGI is an interface standard that allows
  computer programs to communicate with Web
  servers. Several programming languages can be
  used with CGI.

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What You Need to Get Started

• SCP/SSH
• Connecting to the Web server
• Setting up your directories.
• Getting the location of the Perl interpreter.

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Navigating UNIX Directories
14
Navigating UNIX Directories - 2
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Finding The Location Of Perl

• Perl interpreter
• A program that translates Perl program commands
  into commands that are understandable to a
  computer.
• Runs your Perl programs and generates any output.
• Its command name is simply perl.
• It can be installed in any of several places on a
  Web server.

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Finding Location Of Perl

• SSH onto Web Server enter
• which perl
• where is perl

Perl Interpreter Location
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Starting Your Program Development Process

• Each time you develop and run a program
• Create a program file and copy (or save) it into
  the correct directory.
• Change your programs access permissions.
• Check your programs syntax.
• Run your program.

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Create Your Program File

• Editors are computer applications that enable you
  to create, change, and save files
• Microsoft Windows, Notepad is a simple editor
  that works well for Perl development.
• On UNIX systems, the Pico, Vi, and Emacs editors
  are popular choices.

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Starting Your First Program

• Start Editor and enter the following
• !/usr/local/bin/perl
• This program prints out a simple message
• print Steady Plodding Brings Prosperity\n

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Program Entered in Pico
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Run The Program

• Save the program file on the web server.
• Enter the full path to the program file to run.
• For example
• /home/kirby/html/cgi-bin/simple1.cgi

Home Directory
Directories On Web Server
Program File
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Change the Programs Permissions On A Unix Web
Server

• UNIX access permissions are used to define the
  access rights of your files
• read permissions define if the file can be read
• write permissions define if the file can be
  changed,
• execute permissions define if the file can be
  executed as a program
• You set access permissions for your user ID, your
  user IDs group, and everyone else

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To Check Your Program Syntax(on a UNIX Web
Server)

• establish an SSH session,
• navigate to the directory that contains the file,
• enter perl c filename, where filename is the
  program file whose syntax you want to. For
  example,
• cd html/cgi-bin
• perl c simple1.cgi
• If no syntax errors then you will seed
• simple1.cgi syntax OK

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Program with Syntax errors
Missing quote mark
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Running Your Program

• At least two different ways to run your Perl
  programs
• Directly on a Web server or PC without a browser
• Using your browser over the Internet.

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Getting Ready to Run Your Program Over the
Internet

• 1. To use a browser over the Internet, add the
  following MIME content-type line
• print Content-type text/html\n\n.
• !/usr/local/bin/perl
• print Content-type text/html\n\n
• This program prints out a simple message
• print Steady Plodding Brings Prosperity\n

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Change Program Process

• 1. Edit the program
• 2. Change the program.
• 3. Save the file.
• 4. Check the programs syntax.
• 5. Run the program.

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Running Your Program Over the Internet

• 1. Connect to the Internet.
• 2. Start your browser.
• 3. Enter the URL or Web address to your file
• 4. Check the programs syntax.
• 5. Run the program.
• For example, assume saved the in a file called
  simple2.cgi in my cgi-bin directory on the Web
  server. Can execute by the following
• http//people.cs.uchicago.edu/kirby/cgi-bin/simp
  le2.cgi

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Output Of Program Executed Over the Internet
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Dealing with Problems

• Many Web servers redirect the errors from CGI
  programs into a separate error log located on the
  server.
• You may receive a generic, cryptic message when
  running programs with errors.
• Two common messages are Internal Server Error
  (Figure 2.17) and 500 Server Error.

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Some Things to Check

• Verify the program syntax.
• Verify the access permission.
• Verify the file has the proper extension (.pl or
  .cgi).
• Verify the program is stored in the correct
  directory.
• Verify the correct Web address to your program.
• Verify the first line has the correct of the Perl
  interpreter.
• Confirm the accuracy of your MIME Content-type
  line.

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An Internet Server Error
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Generating HTML Statement from Perl Programs

• !/usr/local/bin/perl
• print "Content-type text/html\n\n"
• print "ltHTMLgt ltHEADgt ltTITLEgt Example
  lt/TITLEgtlt/HEADgt"
• print "ltBODYgt"
• print "ltBgtltFont Size5gtThis is a Test
  lt/FONTgtlt/Bgt"
• print "A very Interesting test"
• print "lt/BODYgtlt/HTMLgt"

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Program Output
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Another approach

• !/usr/local/bin/perl
• print "Content-type text/html\n\n"
• printltltEND
• ltHTMLgt
• ltHEADgt
• ltTITLEgtExamplelt/TITLEgt
• lt/HEADgt
• ltBODYgt
• ltBgtltFont Size5gtThis is a Test lt/FONTgtlt/Bgt
• A very Interesting test
• lt/BODYgtlt/HTMLgt
• END

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What is this?

• Everything between ENDs gets treated as if it
  were inside a double quote
• Use this with require to print out a page with
  a fixed header, footer, body
• Use require foo.pl to execute the code in that
  file in that spot

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Summary

• There are several different configurations you
  can use to develop CGI/Perl programs.
• Using FTP and Telent are common
• Steps to create a program create with editor,
  enter program, set permissions, check syntax, and
  run the program.
• Two statements are required
• First line identifies Perl interpreter location.
• Second line specifies the MIME Content-type.