The configuration of the system and most software is kept in various files under
/etc or in subdirectories. These files are supposed to be static; runtime configuration is kept under
/var/lib. An exception to this is the file
/etc/mtab. Moreover, the configuration under
/etc is often system specific and cannot be shared.
/etc once used to contain some system binaries, but they were moved long ago to
/etc/rmt of the GNU
tar package is a rare remnant. In some sense the same holds for the
init scripts under
/etc/init.d, although several packages install or expect scripts under specific subdirectories of
Software packages installed under
/opt sometimes use
/opt/etc to store their configuration.