Apache: The Definitive Guide, Second Edition

4.5. Debugging Scripts

Because CGI scripts run underneath Apache, it can be awkward to debug them. When a script fails, you normally don't get much help on the browser screen, but the error log can be much more informative and is the first thing to check (by default, it is .../logs/error_log, but you can set it to what you like with the ErrorLog directive).

If you are programming your script in Perl, the CGI::Carp module can be helpful. However, most other languages^[‖] you might want to use for CGI do not have anything so useful. If you are programming in a high-level language and want to run a debugger, it is usually impossible to do so directly. However, it is possible to simulate the environment in which an Apache script runs. The first thing to do is to become the user that Apache runs as (often webserv). Then, remember that Apache always runs a script in the script's own directory, so go to that directory. Next, Apache passes most of the information a script needs in environment variables. Determine what those environment variables should be (either by thinking about it or, more reliably, by temporarily replacing your CGI with one that executes env, as illustrated above), and write a little script that sets them, then runs your CGI (possibly under a debugger). Since Apache sets a vast number of environment variables, it is worth knowing that most CGI scripts hardly use any—usually only QUERY_STRING (or PATH_INFO, less often). Of course, if you wrote the script and all ...