| 1.) Background Information If you
run a website, you've probably heard of either Perl or CGI at some point. Whether you've
merely heard of it, installed a few scripts on your own, or taken one look at the code and
turned pale, this tutorial is for you.
Perl is a programming language. Many people think CGI is a language, as in, "I'm
going to learn CGI," but this is erroneous. Think of CGI as the method in which you
use Perl. You can write CGI scripts using other programming languages, but Perl is by far
the most common due to its relative simplicity and power.
Even as more efficient programming languages are starting to take away Perl's
popularity, Perl programming remains very popular and learning it is not a waste of time.
For example, PHP is similar to Perl in most respects. The syntax used is similar and a lot
of the basics are somewhat the same. If you know Perl, you've got a huge head start at
learning PHP, and a few other languages as well.
Although Perl may be less efficient than other programming languages like PHP or ASP,
there are still infinitely more freeware and shareware scripts out there written in Perl
than in any other language. In other words, Perl isn't going away anytime soon.
2.) A Few Small Rules
Now that you have a little background on Perl, let's get down to some of the basic
commands.
Like any programming language, Perl has rules. You will need to use certain symbols in
certain places to tell the script something. One of the most basic rules in Perl is that
every line must end with a semi-colon: ";" . As with every rule there are a few
exceptions which we'll go into later, but for now just realize that almost every line of
Perl code that you write will end with a semi colon. The semi-colon, in case you're
wondering, tells Perl that it has reached the end of a command.
Let's write a quick script to introduce you to some of the basics.
In any Perl-based script, a line like this goes at the very top:
#!/usr/local/bin/perl
This has been nicknamed the "shebang" line, and it is another requirement of
the Perl language. The shebang line points the browser to the path to the Perl interpreter
on your web host's server. Think of this line as a way of telling your browser,
"Here's where you can find a translator to turn my code into a functional
script."
Virtually every web host has Perl installed, although it will not always be in the
location specified above. If the above line doesn't work, contact your system
administrator and ask them for the path to Perl.
Note that you'll need to point to the Perl interpreter in each and every file that you
write. (Note: files that end in .cgi, .pl, and .perl can all be considered Perl files.)
This tutorial from now on assumes that you have a basic knowledge of HTML and form
fields. If you don't, head on over to http://www.HtmlGoodies.com
and read through some of the tutorials offered there.
Let's create our first script!
3.) Starting Off
The script we are about to create will take the information your user submits in a
standard feedback form and then load a page showing him what he just entered. Not terribly
useful, but a great example to get us started.
In this example, we'll have someone enter their name, city, and favorite food (after
all, that's the basic vital information for all of us, right?).
First, we create the HTML page with the form:
<html>
<head>
<title>Submit Your Information</title>
</head>
<body>
<h1>Submit Your Information</h1>
<p>
<form action="display.cgi" method="post" >
First Name: <input type="text" size="30"
name="first"><br>
City: <input type="text" size="30"
name="city"><br>
Favorite Food: <input type="text" size="30"
name="food"><br>
<P><input type="submit" name="submit"
value="Submit!">
<input type=reset name=reset value="Reset">
</form>
</body>
</html>
Do you see the names of those form fields? Those are the identifying values assigned to
each form field the user will be entering his information into. In this case they are
"first," "city," and "food." Think of the names given to
form fields as containers that will hold the data entered by a user.
The way this works is that when a user enters their name into the appropriate form field,
the name tag will "attach" itself to the data entered in order to identify it
for later use. If a user were to enter the word "Marvin" into the
"first" field, the word "Marvin" would be assigned the
"first" tag. As you'll soon see, we can use Perl to display whatever text has
been given the "first" tag. Starting to make some sense?
Now, as you'll notice, we have the form action set to "display.cgi." This
line in the form tells us that this information (with name tags attached, of course) will
be sent to a certain file. In this case, display.cgi. Since, display.cgi doesn't exist
yet, let's create it!
4.) Display.cgi
Now that we've got the form up (call the file whatever you like, just make sure that it
has a .html or .htm extension) we'll move onto creating the Perl script to work with the
data. The way this entire thing will work is:
1. A user will view a form and fill out the data
2. Each field on the form and the data entered will have a name
slapped to it
3. The data entered, together with the name of the form field, will
be sent to display.cgi for processing
Let's create display.cgi!
First, the shebang line:
#!/usr/local/bin/perl
Next, insert this:
require "subparseform.lib";
&Parse_Form;
Subparseform.lib is a widely used file that will aid us in the creation of this script.
Do a search for it on the web and you should be able to find it. If your search fails,
contact me and I'll give you some help: chris@mycoding.com.
Now, remember: we want this script to take the information from the form and display it
to the user in such a way as to say "This is the information you entered." To do
this we need to tell the script to "grab" the form information. But how? We
simply have to match up those tags!
Enter this next:
$first = $formdata{'first};
$city = $formdata{'city'};
$food = $formdata{'food'};
Look familiar?
Take notice of a few things: first, the ";" sign is at the end of every line,
as it should be. Second, the tag names: They match exactly with the tags on the form
information.
Take note of the equals sign ("="). That tells the script to assign the data
entered into the form to a variable or container which we can use inside the Perl script.
The first line is saying "make $first equal the information entered in the form field
with the name 'first'".
As you've probably noticed, first, city and food have a dollar sign in front of them.
Perl requires that all variables or containers used in a script begin with the $ sign.
That first line, as I said before, is telling the browser that whatever form field had
the name of "first" in the HTML file, now has the name of "$first".
What this allows us to do is use "$first" variable elsewhere in the Perl.
What will happen is that $first will be replaced by whatever the user entered into the
"first" field on the feedback form!
Starting to see where this is going?
Now, copy and paste this into the display.cgi file:
print "Content-type: text/html\n\n";
print "Thank you, here is the information you entered:<br>";
print
"<ul><li>$first</li><li>$city</li><li>$food</li></ul>";
That first line tells the script that you'll be using HTML in one or more of the
"print" statements. Now that the information from the form has been assigned new
variables beginning with the $ sign, all we have to do is type those variable names out
and the script will display what the user entered.
The print statement that you see above simply tells Perl to spit out the HTML code out
to the browser after parsing it. Here's what Perl "thinks" when reading the
above three lines:
Line One: Hmm... we'll be working with HTML statements in one or more of the following
print statements.
Line Two: Ok, this one is simple. I'll just spit out the code between the quotation
marks back out to the browser.
Line Three: This line contains three variables named $first, $city and $food that I
need to find and replace before sending out this line of code to the browser. I'll take
the value of $first from the form field named "first" in the form that was just
submitted to me, then I'll do the same for $city and $food. Now that I know what $first,
$city and $food stand for, I can send this line of code to the browser with the
replacements made.
Here's the entire display.cgi file:
#!/usr/local/bin/perl
require "subparseform.lib";
&Parse_Form;
$name = $formdata{'name'};
$city = $formdata{'city'};
$food = $formdata{'food'};
print "Content-type: text/html\n\n";
print "Thank you, here is the information you entered:<br>";
print " - First Name: $first<br> - City: $city<br>";
print " - Favorite Food: $food";
Let's assume the user entered the name "John," the city "New York,"
and the favorite food as "Pizza." Here's what John would see displayed in front
of him after filling out the form:
Thank you, here is the information you entered:
- First Name: John
- City: New York
- Favorite Food: Pizza
You should now have a basic grasp of some of the rules of Perl and how slapping
identifying "tags" on data can be useful. As you'll also learn later on, you can
tell Perl to send out emails using these same variables (tags). Imagine having someone
enter his email address and name, and getting an email personalized with his name in the
subject line!
In Part 2 we'll try to drill some syntax laws into your head, explore "if,"
"elsif," and "else" commands, and even explore using sendmail to send
out automatic confirmation/thank you emails!
[This second issue will be sent out next Friday. -Pete]
ABOUT THE AUTHOR
by Chris Bowyer
chris@mycoding.com
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