Add the "Gentoo: Filesystem" page.

Author: paveloom

content/notes/Gentoo_Filesystem.md

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+title = "Gentoo: Filesystem"
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+
+Gentoo adheres to the following filesystem layout:
+
+- `/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
+
+On most Gentoo systems, though, some of those paths will be symlinks. For example,
+
+- `/bin -> usr/bin`
+- `/lib -> usr/lib`
+- `/sbin -> usr/bin`
+- `/usr/sbin -> bin`
+
+## References
+
+- [Gentoo Development Guide: Filesystem](https://devmanual.gentoo.org/general-concepts/filesystem/index.html)