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Unix filesystem hierarchy

Traditional Unix systems have a common filesystem layout with hierarchical tree-based structure. This isn’t standardized and as a consequence has varied over time and among various systems. Nevertheless and before proceeding any further, POSIX compliance requires the following directories

/ Root directory
/dev Contains null, tty, and console
/tmp Writeable directory

Also the binaries /bin/sh and /usr/bin/env are required.

In this post I won’t go into historical significance of its directories and the reason they were created in initial times of Unix. Rather, this will be a presentation alongside some commentary for modern systems.

FHS

An attempt to standardization comes from Linux that has introduced Filesystem Hierarchy Standard which defines the directory structure and theirs contents for Unix systems and an annex for Linux-based operating systems. It is based on the following distinctions among files.

  • Shareable vs. unshareable where shareable are files stored on one host and (can be) used on others.

  • Static vs. variable where static are those that do not change without administrator intervention.

The following top-level (root) hierarchy structure are the non-optional (sans /home and /root) and the Linux specific directories specified in the latest version (FHS v3.0, released 2015).

/ Root filesystem
/bin Essential user command binaries
/boot Static files for the boot loader
/dev Device files (virtfs)
/etc Host-specific system configuration
/home User home directories (optional)
/lib Essential shared libraries
/media Mount point for removable media
/mnt Temporary mount point
/opt Application software packages
/proc Kernel and process information (virtfs)
/root Home directory for root user (optional)
/run Run-time variable data
/sbin Essential system binaries
/srv Data for system provided services
/sys Kernel and system information (virtfs)
/tmp Run-time temporary files
/usr Secondary hierarchy
/var Variable data

The /bin must contain no subdirectories. In contrast files in /etc are recommended be stored in subdirectories. The /usr and /var directories are a bit complex. The /usr is host-independent whence shareable, read-only data and its hierarchy has three similar directories to root with the following structure. (Paths relative to /usr.)

./bin User command binaries
./include Headerfiles (optional)
./lib Libraries
./local Tertiary hierarchy
./sbin Non-essential system binaries
./share Architecture-independent data
./src Source code (optional)

Optionally exists ./libexec that includes binaries run by other programs, which is common practice in some environments. The ./lib can also be used for this purpose and as older FHS versions did not support ./libexec using ./lib became the common practice.

The ./local subdirectory mirrors /usr itself and is for use by the system administrator for local software. The ./local hierarchy is supposed to be managed manually, /usr is managed through a package manager, and /opt by both with subdirectories managed either through package manager or manually. Packages installed in /opt contain all their static files in their directory rather being put in pre-specified directories like Unix software traditionally is. That said, some files are eventually copied into /etc/opt and /var/opt.

The ./share is used by programs to contain static, read-only data. A subdirectory is recommended, except if a single file is used in which case ./share/misc may used instead. The following mandatory and some optional directories can be inside it. (Paths relative to /usr/share.)

./man Primary manual pages directory
./misc Miscellaneous data
./dict Word lists (optional)
./doc Miscellaneous documentation (optional)
./info GNU Info system (optional)

The /var has a structure showing its purpose for holding variable files for system operation. (Paths relative to /var.)

./cache Application cache data
./lib Variable state information
./lock Lock files
./log Log files and directories
./opt Variable data for /opt
./run Run-time variable data
./spool Application spool data
./tmp Reboot-preserved temporary files

The /var/lib also contains data not exposed to consumers. In contrast, data for use by services is put in /srv. This top-level directory provides space for services requiring a single tree for read-only data, writable data and scripts. The /srv has no specific subdirectory structure and is a free namespace, something that is often abused.

Virtual file systems

The userspace or virtual file systems are filesystems not presented on a storage medium and show the strength of the everything is a file abstraction found in Unix systems. I’ve noted virtual file systems in root hierarchy with virtfs. The /dev, using devfs, is the the location of special or device files. The /proc, using procfs, offers a snapshot of kernel and userspace processes instantaneous state. It can be used for retrieval of kernel, process, and memory information. Finally, /sys, using sysfs, exposes kernel’s volatile objects to userspace. Those are information about devices, drivers, and kernel features. It Should be noted that /proc and /sys are the only Linux specific directories of the root hierarchy. Also technically on many systems /tmp is a virtual file system as well. Specifically it is called a tmpfs, storage space that resides in memory rather a permanent storage device.

/usr merge

There is some redundancy which though it has historical justification, this isn’t the case anymore. Most obvious and seen in detail above, /usr being mostly a mirror of /. The recent decade steps were taken by major distros to amend this by making a merged /usr directory scheme with /{bin,sbin,lib} becoming symlinks to /usr/{bin,sbin,lib}. Though simplification wasn’t the purpose, rather it was improved compatibility with other Unixes and GNU buld systems, it also succeeds in reducing the hierarchy complexity. The reasoning for moving directories to /usr rather back to the root is that /usr could be mounted read-only by default and root could remain r/w and contain only empty mount points, symlinks, and host-specific data. This means that /usr can be shared read-only across several systems and it would contain almost the entire system.

After the /usr merge, the base of the system is composed of the following directories.

/usr Installed system; shareable
/etc Configuration data; non-shareable
/var Persistent data; non-shareable
/run Volatile data; non-shareable

Systems that clean /var on reboot are volatile and is a step before stateless which means a system bootable without any configuration data (empty /etc). The idea behind it is that software should have its own defaults making /etc closer to its goal as a space for host-specific configuration.

Continuing on historical artifact directories, there’re some others that are mostly unusable in modern systems. An example being /media where removable media is mounted. Media traditionally referred to diskettes and optical discs. Those are mounted in /media/floppy and /media/cdrom respectively. On systems using udisks, a D-Bus daemon that offers storage related services, removable storage devices such as USB flash drives are mounted under /run/media/$USER instead. On the topic of storage, interestingly there isn’t a directory where internal drives are supposed to be permanently mounted. It is common for /mnt subdirectories be used for this but that conflicts with tradition for /mnt itself being a mount point. Stealing from OS X which contains a subdirectory for each mounted disk in /Volumes, keeping with Unix tradition of lower-case abbreviated names, and refraining from making new directories in root, I mount local disks in /srv/vol.

The stali distro, once a suckless.org project, follows its own simplified filesystem. Rather moving directories to /usr it loops /usr back to root, that is /usr on stali is a symlink pointing to /. Also /sbin is symlink to pointing to /bin. Finally /share and /include are introduced in root, result of their peculiar merge. The entire hierarchy is then the following sans the three virtual file systems (dev, proc, sys).

/ Root directory; also root home
/bin Every executable
/etc System configuration
/home User directories
/include Header files
/lib Static libraries
/mnt Mount points
/sbin -> /bin  
/share Architecture-independent data
/sucks Anything else
/usr -> /  
/var Variable data

Obviously /media is removed and instead everything is mounted as subdirectory in /mnt. Something worth noticing is the absense of /boot directory. The kernel as an executable resides in /bin. Also, as stali follows a static link philosophy /lib can be ommited and /usr is only for compatibility. It can be ommited if software is compiled with prefix the root.

/home

A standard directory on desktop systems but may not exist in servers. Because its importance for desktops, efforts for standardizing its layout have also taken place. The convention which is adhered nowadays is the XDG Base Directories specification.

The directories are configurable using the $XDG_*_HOME and $XDG_*_DIR variables. The former have to be configured during the session initialization whereas the later (sans the runtime one which is with the previous) are configured in $XDG_CONFIG_HOME/user-dirs.dirs. The file specifies the current set of user directories, and it is in shell format.

The former are alongside their defaults

$XDG_CONFIG_HOME $HOME/.config User configuration files
$XDG_CACHE_HOME $HOME/.cache User non-essential data files
$XDG_DATA_HOME $HOME/.local/share User data files
$XDG_RUNTIME_DIR /run/user/$UID User non-essential runtime data files

The three dot directories were created to fix the dotfiles mess. Specifically, traditionally user applications stored their configuration, cache, and data on a dot directory (name starts with .–dot–which means they’re hidden by default) in $HOME. Now the files are split in those three directories making it simple to backup configuration and user introduced data files alone without the data files automatically generated by the program and aren’t of importance.

Though XDG directories are supported by many programs there’re also many still using the traditional model. Some may optionally use XDG directories by using one or more environmental variables. See the Arch wiki for a list of programs supporting, not supporting, and supporting XDG by the use of environmental variables alongside those variables.

The later are alongside their defaults (relative to $HOME)

$XDG_DESKTOP_DIR ./Desktop
$XDG_DOCUMENT_DIR ./Documents
$XDG_DOWNLOAD_DIR ./Downloads
$XDG_MUSIC_DIR ./Music
$XDG_PICTURES_DIR ./Pictures
$XDG_TEMPLATES_DIR ./Templates
$XDG_VIDEOS_DIR ./Videos

Those folders are the usual default on desktop systems. Personally since I’m doing mainly programming-related work that isn’t very useful to me. My home hierarchy, taking inspiration from root and /usr, is the following.

~/bin Executable files
~/doc Documents
~/etc User configuration files
~/opt Software packages
~/src Sources
~/srv Mostly static files
~/tmp User temporary workspace
~/var Mostly variable files

The ~/var has (paths relative to it)

./backups Old files; basically hoarding directory
./tmp User persistent temporary files
./trash Symlink to freedesktop Trash/files directory
./urbit Urbit piers ie ship (instances) states

And ~/srv has (paths relative to it)

./music XDG directory
./pictures XDG directory
./public XDG directory
./share Network shareable files
./seed Seeded torrents
./templates XDG directory
./videos XDG directory
./www Local sites

The XDG directories can be set in this format using the following dirs file. To disable a directory, it can be pointed to $HOME. In my configuration the desktop directory is disabled.

XDG_DESKTOP_DIR="$HOME"
XDG_DOCUMENTS_DIR="$HOME/doc"
XDG_DOWNLOAD_DIR="$HOME/var/tmp"
XDG_MUSIC_DIR="$HOME/srv/music"
XDG_PICTURES_DIR="$HOME/srv/pictures"
XDG_PUBLICSHARE_DIR="$HOME/srv/public"
XDG_TEMPLATES_DIR="$HOME/srv/templates"
XDG_VIDEOS_DIR="$HOME/srv/videos"

The ~/etc holds my “curated” dotfiles, the ones I’ve version controlled, which I then symlink to their location in $HOME and ~/.config. Though few tools exist for this, configurable for every conceivable purpose which makes it easier using the same dotfiles across hosts, I just have ~/etc mirror the actual hierarchy without dot prefix and then use a simple script to make the links. Meaning that it takes the following stucture and how files out of it are symlinked to the files in it. (Relative to ~/etc.)

./config/<file> ~/.config/<file> -> ~/etc/config/<file>
./config/<dir> similar
./<file> ~/.<file> -> ~/etc/<file>
./<dir> similar

Nix can also be used for the purpose of dotfiles management and deployment. That said I’ve used that system for time before I moved to NixOS and never went around switching my home configuration to Nix. For anyone interested, it is simple using home-manager, as it is possible to do

home.file.".<file>".source = ~/etc/<file>;
home.file."./config/<file>".source = ~/etc/config/<file>;

TODO: Make post on dotfiles maintenance

The ~/tmp is actually a symlink to ~/var/tmp. Ideally it should mounted in tmpfs, and be volatile similarly to root. Unfortunately I tend to hoard stuff. Also ~/var/trash isn’t really good idea as it can mess up tools working with the Trash directory.

TODO: Export a virtual file system for Trash

The root home directory though recommended to be left as /root, it may be determined by preference. In order to simplify top hierarchy a bit, I’m moving it to /home/root. This can be done modifying the /etc/passwd file. NixOS users may use the following configuration.

...
  users = {
    users.root = {
      home = pkgs.lib.mkForce "/home/root";
    };
  };
...

Concluding, though standardization attempts have happened, filesystem hierarchy remains a mess. There’re some proposed alternatives that perform more drastic changes. NixOS and its inspired-from Guix System are of them. GoboLinux actually was made for this very purpose; of redefining the filesystem hierarchy. Home directory is still all over the place since many programs do not adhere to XDG.