The Code Page 5

Let’s look at our program after we have collected our
variables:

The first thing I did was to make a call to the
ParseForm subroutine
and the GetDate subroutine. A subroutine is a block of code that has a
specific purpose or
does something specific within your program. By executing the code within a
subroutine you
can call it at any point during the program by executing a call like
below:

&parse_form;
&GetDate;

This comes in very handy if there is code you will
use repeatedly
during the program. The next thing I did was to define the first subroutine
in the
program, the PrintHeader subroutine. It is necessary to send the
appropriate HTTP header
to the browser so that it will know how to translate the information it is
being passed.
This is the most straightforward part of the CGI protocol. All subroutines
start with the
word sub and then the name we wish to give it.

sub PrintHeader {
print “Content-type: text/htmlnn”;
}

Whenever text or HTML is output to the
browser, we want
to define our HTTP header as text or html exactly as above. This subroutine
will be used
several times throughout the program.

This next section examines the query string to see
what information
we are passing it after the question mark (?). In other words, when you
first called the
program, you called it with the syntax “scipt.cgi?exec” with exec
being the query
string. The program evaluates query strings with a standard if statement in
this format
” if (this expression is true) { Do This! } “

if ($ENV{‘QUERY_STRING’} =~ /exec/i) {

In the above line, I took it one step further by
allowing the client
or user to pass it any string which contains the word exec in a non-case
sensitive format.
This is a wonderful example of Perl’s powerful regular expression and
pattern
matching. A regular expression is a string or series of characters that can
be used to
define a search pattern, such as a keyword. “ExECdog” will
evaluate as true and
thus execute the code block because we are searching for the regular expression
“exec.” If we wanted to require the user to type in a specific
query string we
would change the statement to if($ENV{‘QUERY_STRING’} eq “exec”).
You will
notice that we no longer have the “=~” which searches for an
instance of a
string or pattern, and we have omitted the /…/i which tells Perl to
not worry about
the case of the word, i.e. exec is the same as EXEC. When we use eq
“string” we
are searching for the literal equal of the string, and thus only
“exec” will
evaluate as true. Note that we do not need the semicolons to terminate the
end of our if
expressions.

And then, if the expression evaluates as true, we
execute our code
block.

&PrintHeader;
print “Hey man it worked!n
“;
print “I’m executed!n
“;
print “$daten
“;
}

After we make a call to the PrintHeader subroutine
(which prints the
appropriate HTTP header), the program executes several simple print
statements. You will
note that, at the end of each of these lines, I put a Perl new line
character (n). These
are not necessary for the execution of the program, but they do make it
simple for more
complex programs that will print a lot of information to your screen. The
new line
character is the equivalent of hitting the return key in your text editor.
You should also
notice that on the final line we print a scalar variable $date. You can
include variables
within your print statements exactly as if they were their normal string or
numerical
counterparts. Then we terminate our If statement and finish executing our
block of code
with the closing curly bracket. You should be made aware that certain
characters are
reserved by Perl and cannot be placed within a print statement, such as
quotation marks.
In order to print a line such as “Hey Bob” we would need to place
a before the
quotation marks. The line would look like this:

Print ” “Hey Bob” “;

Now, if you didn’t pass the program a query
string– if you
just called it as script.cgi– it will execute the main portion of the
program. We
accomplish this by using an else statement, which says: “If you
don’t pass me
any query strings, I am going to do this!” The format is very similar
to the if
statement, only it does not need an expression to evaluate as it always
will appear after
an if type of statement, and thus, will execute only if the earlier
statement evaluated as
false. Below, we tell the program to execute the LogMe subroutine, or to
record the
information about the user:

else { # beginning of else statement
&LogMe # Call the LogMe subroutine
} # terminate else statement

The LogMe subroutine is the part of our program that
does most of
the work. When the subroutine is invoked, it opens a text file (which we
named in our
variable) and appends a line of text to the bottom of it containing some
environmental variables. Environmental variables are bits of information set by the
server that your
program can access. These variables can contain information about your
server, your
program, and, in this case, the visitor.

sub LogMe {

The first thing we do in the LogMe subroutine is to
open our text
file. Whenever we work with a file in Perl, we assign it what is called a
file handle.
This file handle can be anything. Below, for clarity’s sake, I used
the name
FILE—but I could have used any word for the name. The next thing I do is tell the
program how to
access the file. By using >>, I am telling the program to append to
the end of the file. There are three main ways you can open and access a file.

You can specify nothing and open the file for reading only

open(FILE,”$ServerPath$FileName”)

You can open the file for appending

open(FILE,”>>$ServerPath$FileName”)

You can open the file for writing, and erase any previous information

open(FILE,”>$ServerPath$FileName”).

The pipe operators (||) or the logical or
statement
tells the program if you can’t open the file then write an error to
the access log.

open(FILE,”>>$ServerPath$FileName”)
|| die (“no
luck with opening file $dir$file”);

We then print all of our variables to the file,
using our file
handle to specify which file we are writing to. You will notice that we are
writing a mix
of variables, our delimiter(|), and the environmental variables, which are
part of the
special %ENV associative array (don’t worry about it for now).

# Print the Environmental variables about the user to
the text file
separated by a | character
print FILE
“$shortdate|$time|$ENV{‘HTTP_REFERER’}|$ENV{‘REMOTE_ADDR’}|
$ENV{HTTP_USER_AGENT}|$ENV{‘DOCUMENT_NAME’}n”;

Then we have to make sure that we close our file
handle.

close(FILE);

Just to be sure that it worked, we again call our
PrintHeader
subroutine and print a simple line letting us know that we have been
logged.