The basic filesystem layout and purpose is as follows:

• /bin: Boot-critical applications
• /etc: System administrator controlled configuration files
• /lib: Boot-critical libraries
• /opt: Non-standard layout applications
• /sbin: System administrator boot-critical applications
• /tmp: Temporary data
• /usr: General applications
  • /usr/bin: Applications
  • /usr/lib: Libraries
  • /usr/local: Non-Portage applications. Ebuilds must not install here.
  • /usr/sbin: Non-system-critical system administrator applications
  • /usr/share: Architecture independent application data and documentation
• /var: Program generated data
  • /var/cache: Long term data which can be regenerated
  • /var/lib: General application generated data
  • /var/log: Log files

Where possible, we prefer to put non-boot-critical applications in /usr rather than /. If a program is not needed in the boot process until after filesystems are mounted then it generally does not belong on /.

Any binary which links against a library under /usr must itself go into /usr (or possibly /opt).

The /opt top-level should only be used for applications that do not conform to the standard filesystem layout. This particularly includes prebuilt software packages that expect being installed in a single directory.

The /usr/local hierarchy is for non-Portage software. Ebuilds must not attempt to put anything in here.

The /usr/share directory is for architecture independent application data which is not modified at runtime.

Try to avoid installing unnecessary things into /etc every file in there is additional work for the system administrator. In particular, non-text files and files that are not intended for system administrator usage should be moved to /usr/share.