/opt
/opt is most often used for large third-party applications. It’s not uncommon to find /opt
empty on some systems and jam-packed on others. For example, by default, Oracle will
attempt to install itself in /opt. This directory doesn’t get all that much use on CentOS
systems, and on most of our servers it is still empty. Do not worry if you never have the
need to use this directory.
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/usr
This directory contains the vast majority of user software and is often the largest directory
on a machine. Although it’s good to know where your software actually resides, because
the default path includes /usr/bin (and /usr/sbin for root), most software “just works”
wherever you are located on the system.