Debugging Text-based CGIs
Debugging text-based CGIs can be a challenge because very little error information is available. All you see in the browser
is a generic "Server Error" message when something goes wrong. Here are some tips for more easily debugging text-based
CGIs.
Quick Checklist
Some errors are so common that it is helpful to run through this checklist first before trying other debugging techniques. Check
for these common errors:
- Make sure the first line of the CGI script has the correct path to the engine, and make sure there are no blank
lines or other characters before that line.
- Make sure that the CGI text file, any stacks in use, and the Revolution engine all have their permissions set to
executable. In most cases, this is chmod 755.
- Make sure that line endings in all text files are correct for the server the file is running on.
- If you are using OS X, make sure Personal Web Sharing is turned on in System Preferences.
- All data returned by the CGI must be preceded by the correct headers. At a minimum, this must be a
"content-type:"
declaration. You can add other headers if your script requires it.
- Make sure that all the correct Apache libraries are installed (see list below.)
Other Debugging Techniques
Write and debug the script in Revolution as much as possible
Debugging in the Revolution script editor makes it easier to catch syntax and logic errors. If possible, write as much of the
script as you can within the Revolution IDE and then, after testing, move it to a text file.
Check the server log
The server log is one of your most valuable resources. If any error information is available, it will be listed in the server
log. These errors can tell you about missing libraries, the line number where an error occurred, permissions errors, and "file not
found" errors, among others. On OS X and the various UNIX platforms, the server log is in
/private/var/log/httpd/, and you can
open
it in any text editor. If you are using a remote ISP, ask them how to access your server logs.
In addition to server errors, the server log will also list errors from the Revolution engine. If the first word of the error
message is "revolution" then you know the problem is related to your script, rather than to the server or its setup.
Write scripts in short sections
To debug while interacting with the server, write the CGI script one line or one section at a time, and add temporary lines of
code that return data to the browser at various points so you can see what the data is. This also one way to isolate the location
in a script where a failure is occuring (although the server log can often tell you the same thing.)
If the script runs without errors but the end results are unexpected, try returning a list of variables and their values at end
of the content output so you can see them for reference.
Put the result
Include put the result after lines of script that aren't working, or use try/catch structures to
identify problems. Any data
that is "put" will be displayed in the browser.
Server 500 Errors are usually script errors...
Server error 500, "Internal Server Error", or "Server error: premature end of header" are almost always script
errors. If
possible, move the script into a stack and test it in the Revolution IDE for syntax errors. Otherwise use one of the above
techniques to locate the problem.
...unless they are library errors
There is one occasion where a Server 500 Error may not be a script error, and that can occur if your server does not have the
correct library files installed. In this case, the error in the server log will reference the libraries that are missing, and
you'll see something similar to this:
Premature end of script headers: /usr/local/apache/cgi-bin/echo.cgi
revolution: error in loading shared libraries: libXext.so.6:
cannot open shared object file: No such file or directory
This tells you which library you (or your ISP) will need to install. Revolution requires these libraries, some of which are
often omitted by ISPs:
libXext.so.6
libX11.so.6
libm.so.6
libc.so.6
ld-linux.so.2
Use shell
Run a shell session in a terminal program. First cd to the cgi-bin folder and then type ./engineName
scriptName. For example,
type ./revolution hello.cgi to run the "hello.cgi" script.
Determining Environment Variables
Servers are all configured differently, and not all of them support every possible environment variable. The best way to
determine which ones your server can work with is to run a CGI script that reports them.
You can download the echo.mt script for this purpose. Place the script into your cgi folder, set its permissions to 755, and
call it from your browser like this:
http://localhost/cgi-bin/echo.mt
If you are running the script from a remote server, substitute the IP or domain address of the server for localhost.
Other online references
Here are some other sites that discuss using Revolution as a CGI:
Rodney Tamblyn's libCGI weblog
Nelson Zink discusses setting up a website to do e-commerce using MetaCard or
Revolution as a CGI
A brief overview from J. W. Mitchell high school about installing Revolution CGIs on Apache
Courtesy of Jacqueline Landman Gay
Hyperactive Software
info@hyperactivesw.com
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