Before starting to explore troubleshooting commands, I first want to cover locations of useful information. Useful information is a bit of a vague term, pretty much every file, directory, or command can provide useful information. What I really plan to cover are places where it is possible to find information for almost any issue.
Log files are often the first place to start looking for troubleshooting information. Whenever a service or server is experiencing an issue, checking the log files for errors can often answer many questions quickly.
By default, RHEL and most Linux distributions keep their log files in /var/log/, which is actually part of the Filesystem Hierarchy Standard (FHS) maintained by the Linux Foundation. However, while /var/log/ might be the default location not all log files are located there (http://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard).
While /var/log/httpd/ is the default location for Apache logs, this...