CGI by rj

Basic terms that we know
 Web Application
• A Web application (Web app) is an application
program that is stored on a remote server and
delivered over the Internet through a browser
interface.
 Web Server
• A web server is a computer system that processes
requests via HTTP, the basic network protocol
used to distribute information on the World Wide
Web.

 Web Browser
• A web browser is a software application for
retrieving, presenting and traversing
information resources on the World Wide Web.
 HTML
• HyperText Markup Language (HTML) is the
standard markup language for creating web
pages and web applications.

 Static Pages
• static Web pages are those with content that
cannot change without a developer editing its
source code,
 Dynamic Pages
• while dynamic Web pages can display different
content from the same source code.

What is HTTP?
 HTTP (Hypertext Transfer Protocol) is the set of rules
for transferring files (text, graphic images, sound,
video, and other multimedia files) on the World Wide
Web.
 As soon as a Web user opens their Web browser, the user
is indirectly making use of HTTP.
 HTTP is an application protocol that runs on top of the
TCP/IP suite of protocols (the foundation protocols for the
Internet).
 TCP/IP (Transmission Control Protocol/Internet Protocol) is
the basic communication language or protocol of the
Internet. It can also be used as a communications protocol
in a private network (either an intranet or an extranet).

Get & Post Methods
 It specifies how to send the form data
to a web server.
 The data can be sent as URL
variables, by using the get (default)
method or as HTTP post, by using post
method.

Get method
 Get sends the data as part of the URL.
 Appends form-data into the URL in name/value pairs.
 The length of a URL is limited 2048 characters.
 Never use GET method if you have password or other sensitive
information to be sent to the server.
 Get is better for non secure data. Like query string in Google
(https://www.google.co.in/?
gfe_rd=cr&ei=KJWNWJ_lKKnT8ge0gYjQBw#safe=strict&q=what+i
s+php)
 Get can’t be used to send binary data. Ex. Images or word
documents.
 <form action=“myform.php” method=“GET”/>

Post Method
 http post requests supply additional data form the client (browser)
to the server in the message body.
 Appends form-data inside the body of the HTTP request (data is not
shown is in URL)
 Don’t have any restriction on data size to be sent.
 Post can not be bookmarked.
 Used to send ASCII as well as binary data. (img, word document)
 Is safe because the parameters are not stored in browser history or
in the web server logs.
 <from action=“some.php” method=“post”/>

What is CGI?
Web Browsing
 To understand the concept of CGI, let us see what happens
when we click a hyper link to browse a particular web page
or URL.
 Your browser contacts the HTTP web server and
demands for the URL, i.e., filename.
 Web Server parses the URL and looks for the filename.
 If it finds that file then sends it back to the browser,
otherwise sends an error message indicating that you
requested a wrong file.
 Web browser takes response from web server and displays
either the received file or error message.

What is CGI?
 However, it is possible to set up the HTTP server so that
whenever a file in a certain directory is requested that
file is not sent back;
 instead it is executed as a program, and whatever that
program outputs is sent back for your browser to display.
 This function is called the Common Gateway Interface or
CGI and the programs are called CGI scripts.
 CGI standard defines server-program interaction
• Developed at the National Center for Supercomputing Applications
(NCSA)
 􀂄 CGI was the first way of generating dynamic content.
 These CGI programs can be a Python Script, PERL
Script, Shell Script, C or C++ program, etc.

WWW
Client
CGI
program
WWW
Server
request
response
Invoke CGI
CGI output
internet server

How to Run Python on
XAMPP web server
 you have installed both xampp and
python on the C Drive:
 c:/xampp
 c:/python27 or c:/python35

Running Python Scripts via
CGI
 Modify http.conf
 Locate the configuration file http.conf in
c:xamppapacheconfhttpd.conf.
 In http.conf look for this line: AddHandler cgi-script
.cgi .pl .asp.
 Modify it so it looks like this: AddHandler cgi-script .cgi .pl
.asp .py
 You probably won't need to do this, but just to be sure, look
for this line: Options Indexes FollowSymLinks.
 Ensure that it looks like this: Options Indexes
FollowSymLinks Includes ExecCGI
 Restart Apache

 At the top of each Python script you create,
set the path to your version of Python.
 For instance, ours is in C:Python27 so we
write: #!/Python27/python
 Example script, create folder
at C:xampphtdocs and create script file
with .cgi extension containing the following
code within the folder:-

Simple CGI Application
#!/Python27/python.exe
print "Content-type:text/htmlrnrn"
print '<html>'
print '<head>'
print '<title>Hello Word - First CGI Program</title>'
print '</head>'
print '<body>'
print '<h2>Hello Word! This is my first CGI
program</h2>'
print '</body>'
print '</html>'

How to process html form data
with Python
 Create a file named "process_form" with the
code below and save it in a file with .py or
.cgi extension at this location
"C:xampphtdocscgirj" where the html form
file (myForm.html) above is located.
 Edit the html form "action" attribute to point to
the (cgi or py) python file above
(process_form.cgi).

CGI module
 There is one primary class in the cgi module that does all
the work: the FieldStorage class.
 This class should be instantiated when a Python CGI script
begins, as it will read in all the pertinent user information
from the Web client (via the Web server).
 Once this object has been instantiated, it will consist of a
dictionary-like object that has a set of key-value pairs.
 The keys are the names of the form items that were
passed in through the form while the values contain the
corresponding data.

 These values themselves can be one of three objects.
 They can be FieldStorage objects (instances) as well as
instances of a similar class called MiniFieldStorage, which
is used in cases where no file uploads or multiple-part form
data is involved.
 MiniFieldStorage instances contain only the key-value pair
of the name and the data. Lastly, they can be a list of such
objects.
 This occurs when a form contains more than one input item
with the same field name.

Cgitb module
 The cgitb module provides a special exception handler for Python
scripts. (Its name is a bit misleading. It was originally designed to display
extensive traceback information in HTML for CGI scripts. It was later
generalized to also display this information in plain text.)
 After this module is activated, if an uncaught exception occurs, a
detailed, formatted report will be displayed.
 The report includes a traceback showing excerpts of the source code for
each level, as well as the values of the arguments and local variables to
currently running functions, to help you debug the problem.
 Optionally, you can save this information to a file instead of sending it to
the browser.
 To enable this feature, simply add this to the top of your CGI script:
 import cgitb
 cgitb.enable()

Field storage
 To access the data submitted by the form in our script we
will need to make use of the FieldStorage() function
available in the cgi module of Python.
 For example, we would assign the data from the form to a
Python variable in the following way:
 formData = cgi.FieldStorage()
 The variable formData now contains every piece of
data submitted by the form.

 Other tool that provide Apache web
server on PC are WAMP or AMPPS.
 AMPPS is a stack of Apache, MySQL,
MongoDB, PHP, Perl & Python.

CGI: Pros and Cons
 Pros of CGI:
• Simple; suitable for small once-off tasks
• Supported by all web servers
 Cons of CGI:
• Slow; web server forks new process for every
request
• Parameter decoding tedious

Advanced CGI
Multi part form submission,
File upload,
Cookies

Cookie
 Cookies are basically key-value pairs
that a web server can set for a client's
web browser.
 This lets server programs track what
pages a user has visited or what actions
the user has performed. Cookies are
very useful for implementing things like
log-in sessions or shopping carts.

Using Cookies in CGI
 HTTP protocol is a stateless protocol. For a commercial
website, it is required to maintain session information among
different pages.
 For example, one user registration ends after completing
many pages. How to maintain user's session information
across all the web pages?
 In many situations, using cookies is the most efficient
method of remembering and tracking preferences,
purchases, commissions, and other information required for
better visitor experience or site statistics.

How It Works?
 Your server sends some data to the visitor's browser in the
form of a cookie.
 The browser may accept the cookie.
 If it does, it is stored as a plain text record on the visitor's
hard drive.
 Now, when the visitor arrives at another page on your site,
the cookie is available for retrieval. Once retrieved, your
server knows/remembers what was stored.
 The HTTP headers are a series of text parameters sparated
by carriage return and line feed characters.
 Cookies are set using the Set-Cookie parameter.

 Cookies are a plain text data record of 5 variable-length fields:
 Expires: The date the cookie will expire. If this is blank, the cookie will
expire when the visitor quits the browser.
 Domain: The domain name of your site.
 Path: The path to the directory or web page that sets the cookie. This
may be blank if you want to retrieve the cookie from any directory or
page.
 Secure: If this field contains the word "secure", then the cookie may
only be retrieved with a secure server. If this field is blank, no such
restriction exists.
 Name=Value: Cookies are set and retrieved in the form of key and
value pairs.

Setting up Cookies
 It is very easy to send cookies to browser.
 These cookies are sent along with HTTP
Header before to Content-type field.
 Assuming you want to set UserID and
Password as cookies.

File Upload Example
 To upload a file, the HTML form must
have the enctype attribute set to
multipart/form-data.
 The input tag with the file type creates a
"Browse" button.

CGI by rj

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