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CGI Routines

CGI Routines

 
This is a brief introduction to using user CGI routines on the CSE systems. We regret that we cannot give you extensive help with writing or debugging your routines. Please consult one of the many HTML books for further details.
How CGI Routines Work
A CGI routine is invoked using an HTML statement such as
    <!--#exec cgi="/cgi-bin/counter-ord"-->
in an otherwise normal HTML file in your web directory. This statement looks just like a comment, and it will be treated as a comment unless you give the file containing this statement `execute' permission. (Which is how we have chosen to tell our web server to look more closely at a file it is serving, to see if there is something there for it to do.) If the file is named mystuff.html, do this:
    chmod 755 mystuff.html
The example used above, by the way, is how you embed an access counter in a web page at our site. A full example would be: 
    Wow, like farmed out, dood, yours is the
<!--#exec cgi="/cgi-bin/counter-ord"-->
access to my page since I installed this counter,
which makes you one groovy frood!
(Notice the way that is worded. This counter does not reliably count people, it counts the number of times the server is asked for the page. So one person may rack up several accesses as they bounce around your pages.)
The /user/cgi Directory
User CGI routines must reside in a directory /user/cgi/<username> where <username> is your login name. This directory is owned by you, so you can create, modify, and remove files there at will. The directory must remain world accessible (we create it that way, with permissions 755), and the routines you place in the directory must be world readable and executable (permissions 755).
If your username (login name) is knuth, then to invoke a routine in your /user/cgi directory, do this:
    <!--#exec cgi="/cgi-user/knuth/whatever"-->
where whatever is the shell script or binary executable you place in your cgi-user directory. To invoke a routine via a link, do this:
    <a href="/cgi-user/knuth/whatever">Some Text</a>
Note: Your cgi directory is now created when your account is created. If you have an old account and do not have a cgi-user directory, just send email to manager@cse.msu.edu and we'll be happy to create a directory for you -- but please check user/cgi BEFORE you request one be made for you.
Common CGI Problems
The first two lines of output generated by your CGI routine tell the web server how to interpret the rest of the output. The first line specifies the content type, and the second line is empty. For most CGI routines the first line of output should be: 
    Content-type: text/html
This tells the web server that the remaining output is HTML. Remember that the second line must be empty.
Any output generated by your CGI routine that is sent to stdout is interpreted by the web server and must be valid HTML. If the server gets confused, it complains about a ``server error'', but what it is trying to tell you is that it can't make sense out of what your CGI routine is generating.
Check that your web subdirectory, your HTML files, and your CGI routine have the correct access permissions!
As a last resort you can look through the mounds of information in /user/web/logs/error_log for clues. The following command is useful for finding errors. 
    tail -f /user/web/logs/error_log
Run this command, then activate your script, and watch what happens. Any output sent to stderr by your script will appear in the error log.