What is CGI? The Common Gateway Interface (CGI) is a mechanism that allows Web clients to execute programs on a Web server and to receive their output. CGI applications are often used to produce HTML pages on the fly; they are also used to process the input from an HTML form. A gateway is a program that converts information to a format that a client can use; on the Web, a gateway is a program that takes non- HTML input or data and produces as output an HTML page that can be displayed by a Web browser.

How the CGI program being executed? You can execute a CGI application with your Web browser by requesting the URL that corresponds to the application. Most Web servers store CGI applications in a directory named cgi-bin. But Webmasters can define different directories for CGI applications if they so choose. One example of the URL for CGI application could be bin/MyCGIApp.cgi. bin/MyCGIApp.cgi For the information how to publish your website in LSBU server, you can visit to find more information.

How can you write a CGI application? You can write a CGI application in any language that can be executed on your Web server, compiled or interpreted. This may include the following, C, C++, FORTRAN, Perl, etc. The Perl is most popular language to write the CGI program.

CGI methods To execute a gateway program using CGI, you make a request to the server to run an application via a method. A method is a way of invoking a CGI program. In fact, methods are one of the underlying structures of HTTP; CGI methods rely on HTTP methods. The method you use defines how the program receives its data. There are three methods used for HTTP. They are: The GET method –When you use the GET method, your script receives its data in the QUERY_STRING environment variable. Your script will have to parse the QUERY_STRING environment variable to interpret the data. –The Get method should be used when the object of the request is to obtain information from the server, as opposed to changing or adding to the information on the server. The Post method –Your script receives its data from stdin (standard input) with the POST method. The server will not mark the end of the input for your script; there will be no EOF. Your script must use the CONTENT_LENGTH environment variable to determine how much data to read from standard input.

CGI Environment variables Environment variables are a series of hidden values that the web server sends to every CGI you run. Your CGI can parse them, and use the data they send. Variable Name Value DOCUMENT_ROOT The root directory of your server HTTP_COOKIE The visitor’s cookie, if one is set HTTP_HOST The hostname of your server HTTP_REFERER The URL of the page that called your script HTTP_USER_AGENT The browser type of the visitor HTTPS "on" if the script is being called through a secure server PATH The system path your server is running under QUERY_STRINGThe query string (see GET, below) REMOTE_ADDRThe IP address of the visitor

CGI Environment variables REMOTE_HOST The hostname of the visitor (if your server has reversename-lookups on; otherwise this is the IP address again) REMOTE_PORT The port the visitor is connected to on the web server REMOTE_USERThe visitor’s username (for.htaccess-protected pages) REQUEST_METHOD GET or POST REQUEST_URI The interpreted pathname of the requested document or CGI (relative to the document root) SCRIPT_FILENAME The full pathname of the current CGI SCRIPT_NAME The interpreted pathname of the current CGI (relativeto the document root) SERVER_ADMIN The address for your server’s webmaster SERVER_NAME Your server’s fully qualified domain name SERVER_PORT The port number your server is listening on SERVER_SOFTWARE The server software you’re using (such as Apache 1.3)

Basics of a Perl Script Perl language is something like HTML. It has a clearly defined syntax, and if you follow those syntax rules, you can write Perl as easily as you do HTML. Rules for the Perl programming: –Always put this line at the top of your Perl script, no excuses: #! /usr/bin/perl This line tells the server that this is a Perl script, and where to find the Perl interpreter. Make sure it is the place where Perl interpreter is, and you can always use which perl or whereis perl to find the correct place. –Always put a semicolon at the end of every line. If you don’t, you’ll never, ever, ever get a script to work

First Perl Program First perl script example: first.pl –#!/usr/bin/perl –print “Hello, world!\n”; In the Unix shell, you will need type: chmod 755 first.pl to change the file permission to allow you to run the program. And then when you type./first.pl You will get the output: Hello, world! On your screen.

Basics of a CGI Script A CGI program here is still a Perl script. But one important difference is that a CGI usually generates a web page such as form-processing CGI etc. To write a CGI that’s going to generate a HTML page, you must include this statement somewhere in the script, before you print out anything else: Print “Content-type:text/html\n\n”; This is a content header that tells the receiving web browser what sort of data it is about to receive – in this case, an HTML document.

First CGI example First CGI example first.cgi: –#!/usr/bin/perl –print "Content-type:text/html\n\n"; –print " Test Page \n"; –print " \n"; –print " Hello, world! \n"; –print " \n"; Here if you change the file permissions and run./first.cgi, you will find the script will just print out a bunch of HTML. If you use your browser call It will return a web page with the “Hello, world!” phrase on it.

First CGI example Another way to write the above CGI, without multiple print statements, is as follows: –#!/usr/bin/perl –print "Content-type:text/html\n\n"; –print <<EndOfHTML; – Test Page – – Hello, world! – –EndOfHTML This is the “here-doc” syntax, and it is very useful for you to get rid of the typing work.

Perl Variables A Scalar Variable –It stores a single (scalar) value. Perl scalar names are prefixed with a dollar sign($). So if I want to assign a value to the variable $stuff, here is how I’d do it: –A word: $stuff=”rules”; –A phrase: $stuff=”I like SBU!” –Numbers: $stuff=2 –You do not need to declare a variable before using it. A scalar can hold data of any type. You can even add and subtract like this –$stuff = 2+2 –$stuff = 4-2

An example for perl scalar variables: #!/usr/bin/perl $classname = "Apllied software for IE"; print "Hello there. What is your name?\n"; $you = ; chomp($you); print "Hello, $you. Welcome to $classname.\n"; Here the STDIN is standard input. And the chomp($you) is the function to remove the carriage return from the end of the variable $you

Perl Variables Arrays –An array variable is a variable that holds many scalar variables in numbered slots. These slots are added as needed so the variable can grow dynamically. Array variables usually have (at symbol) in front of them. Here is an example to declare an array: = (“red”,”green”,”blue”) –In Perl, just C, array indices start with 0, so $color[0] is equal to “red”. Here $ is again to refer to the single value of an array.

Perl Variables Here is another example to show how arrays work: –#!/usr/bin/perl –# this is a comment –# any line that starts with a "#" is a comment. = ("red","green","blue"); –print "$colors[0]\n"; –print "$colors[1]\n"; –print "$colors[2]\n";

Perl Variables Array Functions = ("red","green","blue","cyan","magenta", "black","yellow"); –$elt = # returns "yellow", the last value of the array. –$elt = # returns "red", the first value of the array. –You can also add data to an array: = ("green", "blue", "cyan", "magenta", "black"); # adds "orange" to the end of array now becomes ("green", "blue", "cyan", "magenta", "black", "orange"). = ("purple","teal","azure"); # appends the values to the end now becomes ("green", "blue", "cyan", "magenta", "black", "orange", "purple", "teal","azure"). RedGreem Blue Cyan magentablack yellow

Here are a few other useful functions for array manipulation: = ("green", "blue", "cyan", "magenta", "black"); # sorts the values # alphabetically now becomes ("black", "blue", "cyan", "green", "magenta" ). You can also use reverse to change the sorting order

Perl Variables Hashes –A hash is a special kind of array- an associative array, or paired group of elements. Perl hash name is prefixed with a percent sign (%), and consist of pairs of elements – a key and a data value. –Hash Name key value –%pages = ("fred", " "beth", " "john", " ); –Another way to define a hash would be as follows: –%pages = ( fred => " beth => " john => " );

Perl Variables An example for using hash: –#!/usr/bin/perl –# –# the # sign is a comment in Perl –%pages = ( "fred" => " –"beth" => " –"john" => " ); –print "Content-type:text/html\n\n"; –print <<EndHdr; – URL List – – URL List – –EndHdr

Perl Variables –foreach $key (keys %pages) { –print " $key \n";} –print <<EndFooter; – –EndFooter Foreach is a special for loop iteration

Perl Variables Output: URL List URL List john fred beth

Perl Variables Hash Functions –delete $hash{$key} # deletes the specified #key/value pair, and returns the #deleted value –exists $hash{$key} # returns true if the specified #key exists in the hash. –keys %hash # returns a list of keys for that hash –values %hash # returns a list of values for that #hash –scalar %hash # returns true if the hash has # elements defined (e.g. it’s not an #empty hash)

Perl Operators Assignment Operators 1.=Makes the value of the variable on the left side equal to whatever is on the right. 2.+=Adds the value of the right side to the left side and makes the variable on the left equal to it. 3.-=Same as above, only subtracts instead of adds.

Perl Operators Comparison Operators 1.< Returns a true value if the value on the left is less than that on the right. Otherwise false. 2.> Same as above, but the other way around. 3.>= Returns a true value if the value on the left is greater than or equal to that on the right. False if not. 4.<= Are we seeing a pattern here? 5.== Returns a true value if the values on both sides are equal; otherwise returns a false. 6.eq The same as above, but rather than comparing values, it compares strings of text. 7.!= Returns a true value if the value on the right is not equal to that on the left. False if they are equal. 8.ne Again, same as above, but for strings of text.

Perl Operators Mathematical Operators 1.* Multiplies the two values together. 2./ Divides two values. 3.+ Adds two values. 4.- Subtracts two values Adds 1 to the value on the left (so if $i were equal to 1, $i++ would equal 2) Subtracts 1 from the value on the left.

Flow Controls The For Loop –for ($i = 0; $i <= $#stuff; $i++) { print "$stuff[$i]";} The While Loop –$stuff = ; –while ($stuff ne "bob") {print "$stuff"; $stuff = ;}

Flow Controls Foreach Loop –foreach $slotnum {print "$slotnum";} –The foreach loop is really useful for running through associative arrays since their slots aren't numbered. Check this out: –foreach $slotname (keys (%stuff)) {print "$stuff{$slotname}";}