/etc = system & application settings
/etc/skel = “skeleton” files used to create user accounts
/lib = security files, system modules, & libraries containing software libraries & information databases
/opt = pre-loaded software packages
/usr/lib = program libraries (shareable system program modules)
/usr/man = manual pages
/usr/doc = program documentation
/usr/info = program documentation
/var = spooling, logs, & all messaging files
