Perl Highlights

Perl is a very commonly used language for CGI scripts. It's popular because it's a very powerful language and because it is interpreted rather than compiled. Compiled languages, like C, require that the program be run through a compiler, which converts the English-like program statements into an executable file that can't be read via a text editor. Interpreted languages, on the other hand, are "compiled" on the fly so there's no reason to generate a separate executable file. That makes it easier for the programmer to make changes. It also makes the programs very portable -- compiled programs can only be run on the operating system that they were compiled for. You can't compile a C program on a PC and then run it on a UNIX server. With Perl, you just upload the text file, make a few tweaks, and it will run on almost any operating system.

In order to start installing pre-written CGI scripts on your web site, you just need to have a high-level familiarity with Perl. You need to know what it looks like, what lines are safe to modify, and what lines to leave alone. You need to know how to make changes in order to install the script in your environment. You don't need to understand Perl to the extent that every line of the script makes sense to you.

Editing perl files

Probably the first question most beginners ask is this -- how do I edit a perl script?

Since perl is just plain text, you should edit the files with a text editor. A text editor is a simple program used for typing and editing. The most basic text editors are Notepad (PC) and Simple Text (Mac). Microsoft Word and similar programs are not text editors. Notepad and/or Simple Text will serve your purposes just fine, but there are other, fancier text editors available as well. These programs have features such as automatically formatting your perl files and showing line numbers to aid in debugging. Here are a few of my favorites:

BBEdit (Mac)
EditPlus (PC)

The first line

The first line of every UNIX Perl CGI script* contains the path of the perl executable on the server. That is, where is the perl program installed on your ISP? Open any perl script in your favorite text editor and you will see something like this:

#!/bin/perl

Where /bin/perl can be replaced with a different path.

You must know where perl is installed on your ISP in order to configure this line properly. You can look in your ISP's FAQS page for this information or email tech support. If you have telnet access, you can find out the path yourself.

The most common paths to the perl program are:

/bin/perl
/usr/bin/perl
/usr/local/bin/perl
/usr/sbin/perl

If you don't have this line correct, your script won't work at all. Most likely you will get a 500 server error when you try and run the script from the web.

* Note: This line is ignored on NT systems

Comment lines

Any line that begins with pound sign (#) symbol in perl is a "comment". That means the text following the pound sign is not perl code, but is just extra text inserted for clarity. Usually comments are used to help explain the code to the reader.

Non-comment lines

All other lines in the file are considered to be perl code and must be properly formatted in order for the script to function.

Every non-comment line must end with a semicolon (;) in perl. However, not all perl "lines" are on a literal line in the text file. For example:

 $mystring = "
 hello
 world
 ";

is only a single "line" in the strict sense, although it physically spans several text lines. So just make sure you don't delete any semicolons's that are in the file.

Strings

Strings are defined as a group of characters enclosed in either quotation marks ("string") or single quotes ('string'). The quotation mark or single quote is called the "delimiter" of the string: it defines the beginning and the end.

Many times you will need to modify strings in order to install a CGI script. For example, in the setup file for Links 2.0 from Gossamer Threads:

$db_script_path   = "/home/ashley/www/links/cgi-bin/admin";

You would need to edit the string in order to put the correct path for your admin script.

It is generally safe to edit strings as long as you don't accidentally erase the delimiters. However, if you wanted to use the delimiter symbol inside the string, you must precede it with a back slash. For example:

$string = "my dog says \"hello\"";

would set the variable $string to [my dog says "hello"].

All of the backslashes can get pretty confusing, so there is another way around this. You can replace the delimiter marks to any symbol you like by using the notation

qq!string!

It is very common to see this syntax with either an exclamation mark (!) or a tilde (~) as the delimiter symbol.

So, if you see a structure like this:


qq!
Some stuff here
!;

you can edit anything between the exclamation marks, but make sure you don't accidentally erase the exclamation marks themselves! You also cannot have any exclamation marks inside the text itself unless they are preceded by a backslash.


qq!
I want and exclamation mark \! here
!;

Similarly, if you see


qq~ text here ~;

you must put a backslash before any tilde (~) characters within the text.

Variables

Anything that starts with a dollar sign ($) is what's known as a variable. You can either assign a variable to a number, a string, or another variable. If you are assigning it to a string (ie a group of letters and/or numbers), you must enclose the string in single or double quotes (or another delimiter as described above). You must put a semicolon at the end of the line.

Arrays

Anything that starts with an ampersand (@) is what's known as an array. That is, a *list* of numbers, variables, or strings. Your list must begin with an open parenthesis [(] and end with a close parenthesis [)]. Again, there must be a semicolon at the end of the line. Within the parentheses, you can have a series of numbers, strings, or other variables, each separated by a comma. Again, strings must be enclosed in delimiters.

From the Web Store ES setup file:

 @sc_db_index_for_detail  = (
                            $db{"item"},
                            $db{"name"},
                            $db{"description2"},
                            $db{"image2"},
                            $db{"price"},
                            $db{"weight"},
                            $db{"shipexempt"},
                            $db{"taxexempt"},
                            $db{"size"},
                            $db{"color"},
                            $db{"options"}
                            );

Due to the special meaning of the ampersand symbol in perl to designate arrays, if you write an email address anywhere in a perl file, you must write it like this: ashley\@enscript.com with a backslash before the ampersand. This is a very common mistake!