The OS promises not to touch /usr/local other than to set it and its subdirectories up. This allows two things: 1. /usr/local is usually set up to take priorit

The OS promises not to touch /usr/local other than to set it and its subdirectories up. This allows two things: 1. /usr/local is usually set up to take priority over /usr, so if you want a newer C compiler or whatever you put it in /usr/local but keep the system compiler in /usr because you might need it for an OS patch or something. 2. If you see an error from something in /usr/local you know not to bother your OS vendor about it because it came from an outside source. It's largely the same theory as /opt, except stuff in /opt shouldn't link to any libraries in /usr while stuff in /usr/local can (stuff in /opt should be self-contained).