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. 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 earlier), 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 use relatively few of
them — usually only QUERY_STRING (or
PATH_INFO, less often). Of course, if you wrote
the script and all its libraries, you’ll know what
it used, but that isn’t always the case. So, to give
a concrete example, suppose we wanted to debug some script written in
C. We’d go into .../cgi-bin and
write a script called, say, debug.cgi, that
looked something like this:
#!/bin/sh QUERY_STRING='2315_order=20&2316_order=10&card_type=Amex' export QUERY_STRING gdb mycgi
We’d run it by typing:
chmod +x debug.cgi ./debug.cgi
Once gdb came up, we’d hit
r<CR>, and the script would run.[5]
A couple of things may trip you up here. The first is that if the
script expects the POST method — that is, if
REQUEST_METHOD is set to
POST — the script will (if it is working
correctly) expect the QUERY_STRING to be supplied
on its standard input rather than in the environment. Most scripts
use a library to process the query string, so the simple solution is
to not set REQUEST_METHOD for debugging, or to set
it to GET instead. If you really must use
POST, then the script would become:
#!/bin/sh REQUEST_METHOD=POST export REQUEST_METHOD mycgi << EOF 2315_order=20&2316_order=10&card_type=Amex EOF
Note that this time we didn’t run the debugger, for
the simple reason that the debugger also wants input from standard
input. To accommodate that, put the query string in some file, and
tell the debugger to use that file for standard input (in
gdb ’s case, that means type
r
<
yourfile).
The second tricky thing occurs if you are using Perl and the standard
Perl module CGI.pm. In this case, CGI helpfully
detects that you aren’t running under Apache and
prompts for the query string. It also wants the individual items
separated by newlines instead of ampersands. The simple solution is
to do something very similar to the solution to the
POST problem we just discussed, except with
newlines.