CGI and Perl

Introduction to CGI

CGI is short for "Common Gateway Interface", a protocol which allows programs which are run on a web server to communicate with visitors' browsers. CGI programs allow you to make your site more dynamic and interactive by including such features as counters, guest books and discussion boards. 

Most CGI programs are written in a language called Perl, but CGI programs can in theory be written in any programming language. The Efficient Hosting servers support CGI scripts written in Perl, Python, C, PHP and the common Linux shells. 

For more information about CGI and Perl, or to view archives of third party CGI scripts, you may follow the links to suggested CGI resources below. 

Uploading CGI Scripts

After uploading, the file permissions for any CGI scripts should be set to 755, in order to make them executable. 

For many CGI scripts it is also important that the permissions are set correctly for any files or folders which they need to access. For example, if a CGI script needs to write to a log file, then this log file will need to have write access enabled and the folder where it is located will need to give "execute" permission. If a CGI script needs to create, delete or rename files, then it will need "write" permission for the folder in which this is to be done. 

For third party scripts, the documentation provided by the author usually gives specific details of the permissions required. For instance, the instructions may direct you to "chmod 755" a file. 

Each number represents the sum of the values assigned for Read, Write, and Execute permission: 

Read - 4
Write - 2
Execute - 1 

Adding the numbers for each of the three permission groups gives you the setting.
In our example, the owner is granted read, write and execute permission (7=4+2+1), whilst both the group and others are granted read and execute permission (5=4+1).

The read, write and execute permission settings have different meanings for folders than they do for files.
For a folder: Read permission is required in order to list the files it contains.
Write permission is required in order to create, delete or rename files within it.
Execute permission is required in order to open files within it for reading and/or writing. 

File permissions can be modified by most popular FTP programs, including WS_FTP and CuteFTP. 

You can also check and update file permissions using the File Manager within your Control Panel. The permissions are displayed alongside all files and folders and can be changed by first clicking on the appropriate file name and then clicking "Change Permissions".

Useful Paths

Path to your "CGI-Bin" folder   /home/username/public_html/cgi-bin

Web Address of your "CGI-Bin" folder  http://www.yourdomain.com/cgi-bin

Web address of global "CGI-Bin" folder  http://yourdomain.com/cgi-sys

Path to Sendmail:  /usr/sbin/sendmail

Path to Perl:  /usr/local/bin/perl   (For convenience, /usr/bin/perl may also be used, so all Perl scripts should start either with the line #!/usr/local/bin/perl or with the line #!/usr/bin/perl

Troubleshooting CGI Scripts

If your script doesn't work, please make sure that the Permissions are set right for both the scripts themselves, and any files or folders they need to access. 

It is also worth double checking that all Paths are set correctly, and that all CGI scripts were uploaded as the correct File Type (usually "ASCII Text"). 

Another common problem is an output format error. Any output from a CGI program intended for a visitor's browser must be preceded by a valid "HTTP header". For example, if you are generating an HTML page, the first print statement that your program executes must be: print "Content-type: text/html\n\n"; 

Also, it is best to ensure that the very last line of a CGI script has a newline at the end. If not, it may not get processed when you run the script. 

It can be a good idea to check your CGI scripts for syntax errors before uploading them to your site. To do this, you will need to download Perl from www.perl.com and install it on your local machine. You can then type perl -c your_program.cgi in a DOS box to check your program for syntax errors. 

You may also be able to learn more about why a CGI script is failing to work correctly by checking the Error Log in the Web Statistics section of your Control Panel.

Further Reading and Related Links

CGI RESOURCES
CGI DIRECTORY
The Perl Homepage