The apachectl or command with the -e option can be used to specify the log level that should be used. For example, let's say you are attempting to debug a web server instance that is failing to start. The -e option can be used to log events at log level debug. This will also append the events to the error_log.

<web_server_root>/bin/apachectl -k start -e debug

Likewise, you can use the -X flag to debug a web server instance.

]# <web_server_root> -k start -X
/opt/IBMIHS90/bin/apachectl: line 96: 34966 Segmentation fault      (core dumped) $HTTPD "$@"