Filesystem Hierarchy Standard

The Filesystem Hierachy Standard is the official convention that defines the Directory structure and contents in Linux distributions and other UNIX-like operating systems. It is maintained by the Linux Foundation.

UsrMerge

Historically the root level and usr level directories were separate, due to Storage-imposed space limitations meaning that root and usr were separate Hard Disks. However, we no longer physically separate them, because the space constraint no longer exists, and because the separation causes confusion. Merging the two makes snapshotting and atomic updates easier.

In practice, the merging just means that every root level instance of a directory like bin is Symlinked to its usr equivalent.

Root

• / is the root/trunk/base/head of the filesystem, within which everything else lives.

Home

• /home is where user owned directories live, e.g. /home/alice, /home/bob
• /root is the home directory equivalent for the Root User

Binaries

• bin contains general purpose binaries that everyone can use
• sbincontains binaries that only the Root User should be using, i.e. “System Binaries”

Directory	Binaries	System Binaries	Purpose
/	/bin	/sbin	Bare minimum
/usr	/usr/bin	/usr/sbin	Standard. Package Manager.
/usr/local	/usr/local/bin	/usr/local/sbin	Manually installed.
See [[#usrmerge	UsrMerge]], i.e. bin -> usr/bin, sbin -> usr/sbin

Libraries

• lib contains 32 bit libaries
• lib64 contains 64 bit libaries

Directory	32 bit	64 bit	Purpose
/	/lib	/lib64	Bare minimum
/usr	/usr/lib	/usr/lib64	Standard. Package Manager.
/usr/local	/usr/local/lib	/usr/local/lib64	Manually installed.
See [[#usrmerge	UsrMerge]], i.e. lib -> usr/lib, lib64 -> usr/lib64

Optional

• /opt is for manually installed optional software.
  • e.g. /usr/bin/google-chrome -> /opt/google/chrome/google-chrome

Hardware

• /dev contains handles to hardware via files
  • e.g. hard drive might be at /dev/sda
  • e.g. webcam might be at /dev/video0
• /media or /mnt are where the system automatically “mounts” removable media devices into the filesystem

Configuration

• /etc entirely contains files relevant to system configuration. “Et-cetera”.
  • /etc/passwd is a list of all User accounts; usernames, user ids, group ids, home directory paths, and the type of shell they use
  • /etc/shadow has encrypted passwords and account expiration information
  • /etc/sudoers defines which users and groups are allowed to runn commands as root using sudo
  • /etc/hostname oneliner file that contains the hostname of the machine
  • /etc/os-release is basically the ID for the machine
  • /etc/fstab is the “Filesystem Table” that tells the OS which hard drives, partitions, or networks shares to automatically mount upon boot
  • /etc/hosts is a local address book that maps IPs to hostnames. A DNS cache.
• $HOME/.config

Temporary

• /tmp is where “Temporary”/Ephemeral data gets stored. It is automatically wiped.
• /run is a temporary storage area for system data that has accumulated during the runtime of the OS since the last boot. It stores PIDs and Sockets. It also holds information about logged-in users and temporary network settings.

Magic

• /proc is a window into the Linux Kernel that essentially acts as a File-driven API for either querying information from the kernel, or requesting the kernel to do something. Instead of some special command to query some information from the kernel, you can simply just read a particular file in /proc to get that information. Because /proc is magic, the inodes located there are posers! They are ephermeral, exist only in RAM, and are generated on-demand.
  • e.g. /proc/1234/ contains information about Process number 1234
  • e.g. /proc/cpuinfo contains information about the CPU
  • e.g. /proc/meminfo contain information about RAM usage
  • e.g. /proc/sys can be written to tune kernel configuration
• /sys stores a structures view of every piece of hardware connected to the machine. Unlike, /proc, /sys alows you the change thing by writing the files. Unlike /proc/sys, /sys is for hardware interaction, whereas the former is for kernel interaction.

Misc

• /var is used for holding persistent data. “Variable”.
  • /var/log holds logs
  • /var/lib contains persistent state data, e.g. database tables or container volumes
  • /var/cache contains cached data, e.g. package manage might cache data here