How do I determine when onpload has failed when it is run from the
command line is ksh? The exit code is inconsistent.

Most of the time it appears to write textual error messages to STDOUT.

However, I have seen it error out where there is no error message to
either STDOUT or STDERR but instead the error message appears in
/tmp/logname.log.

Once example is the following error message was written to the
/tmp/logname.log and not to STDOUT/STDERR:
Error accessing format, SQL error 100, ISAM error 0

I just need a set of rules for identifying errors so that they can be
processed.

The Informix Guide to HPL isn't very clear on this.

If anyone can help or point me to any documentation relating to this I
would be grateful.