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The
Common Gateway Interface (CGI) is a standard for
interfacing external applications with information
servers, such as HTTP or Web servers. A plain HTML
document that the Web daemon retrieves is static, which
means it exists in a constant state: a text file that
doesn't change. A CGI program, on the other hand, is
executed in real-time, so that it can output dynamic
information.
Since a CGI program is executable, it is basically the
equivalent of letting the world run a program on your
system, which isn't the safest thing to do. Therefore,
there are some security precautions that need to be
implemented when it comes to using CGI programs. Probably
the one that will affect the typical Web user the most is
the fact that CGI programs need to reside in a special
directory, so that the Web server knows to execute the
program rather than just display it to the browser. This
directory is usually under direct control of the
webmaster, prohibiting the average user from creating CGI
programs. There are other ways to allow access to CGI
scripts, but it is up to your webmaster to set these up
for you. At this point, you may want to contact them about
the feasibility of allowing CGI access.
If
you have a version of the NCSA HTTPd server distribution,
you will see a directory called /cgi-bin. This is the
special directory mentioned above where all of your CGI
programs currently reside. A CGI program can be written in
any language that allows it to be executed on the system,
such as:
-
C/C++
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Fortran
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PERL
-
TCL
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Any
Unix shell
-
Visual
Basic
-
AppleScript
It
just depends what you have available on your system. If
you use a programming language like C or Fortran, you know
that you must compile the program before it will run. If
you look in the /cgi-src directory that came with the
server distribution, you will find the source code for
some of the CGI programs in the /cgi-bin directory. If,
however, you use one of the scripting languages instead,
such as PERL, TCL, or a Unix shell, the script itself only
needs to reside in the /cgi-bin directory, since there is
no associated source code. Many people prefer to write CGI
scripts instead of programs, since they are easier to
debug, modify, and maintain than a typical compiled
program.
What is Perl?
Perl is a high-level programming language with an eclectic
heritage written by Larry Wall and a cast of thousands. It
derives from the ubiquitous C programming language and to
a lesser extent from sed, awk, the Unix shell, and at
least a dozen other tools and languages. Perl's process,
file, and text manipulation facilities make it
particularly well-suited for tasks involving quick
prototyping, system utilities, software tools, system
management tasks, database access, graphical programming,
networking, and world wide web programming. These
strengths make it especially popular with system
administrators and CGI script authors, but mathematicians,
geneticists, journalists, and even managers also use Perl.
Maybe you should, too.
Configure the script
In order to run your script, you'll have to modify the
first line with the correct path to the Perl interpreter.
"/usr/bin/perl" is the path to your perl
interpreter, and it should appear after '#!' (also called
'shebang') on the first line of the script.
#!/usr/bin/perl
Normally,
anything following '#' is considered to be a comment, but
the first line in a script is the exception.
Upload
it to the server
The most important thing to remember is to use ASCII mode
when you upload the script to the server. If you think you
have everything configured properly but the script won't
run, reload it using ASCII mode, just to be sure.
Create a directory called "cgi-bin" under your
/web directory and put your .pl files under it.
File Permissions
Every file has both an owner and a group. By default, the
owner and the group will be the same as the user (or
process) that created the file. There are three levels of
permission for every file, and they are the owner's
permissions, the group's permissions, and everyone else's
permissions. Each level may have read, write, or execute
access for a file.
The permissions on a file are easy to understand as a
matrix, in the form shown below.
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Three
columns designate three groups, Owner, Group, and
Everyone. Owner is the person who owns the file (you).
Group is designated for any groups that are set up by your
network administrator, and may or may not apply to your
script. Check with your network administrator if you're
not sure. Everyone refers to the world-at-large, like
anyone who will use your script on the Internet.
The three rows of the matrix are the three actions that
may be performed on your file: Read, Write, Execute. Read
means that the file can be read, but not changed in any
way. Write means that data can be written to the file, and
Execute means that the file can be run as a program. Each
is valued in the following way: Read=4, Write=2,
Execute=1. The values for each column are added up, then
the three digit number is used to change the permissions
on your server.
Here's an example for the permissions needed by CGI
scripts:
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In
other words, anyone can read or execute the script, but
only the owner can write to the file. Please note that you
do NOT want to assign write privileges to Group or
Everyone! The only updates to your files should be done by
your script, and not by any hacker who happens upon your
site.
The sum of the columns is the number you'll use with chmod,
like this chmod 755 myscript.pl
If you're using a graphical FTP program, you should be
able to make the appropriate choice with radio buttons.
See your program's documentation for more information.
One final point about moving your files correctly is
regarding the file extension. Servers may be configured to
accept only certain extensions, such as .cgi, or .pl.
Check with your hosting service to see if you need a
specific extension on your scripts.
Test the script
Now that you know how to install a script, you'll need a
script you can use for testing. The steps are the same no
matter what the scripts do.
test.pl
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