Filesystem Hierarchy Standard

UNIX-based operating systems uses one big file system tree, started with the root (/) directory. This filesystem can contain many distinct filesystems, mounted at various points, which appear as subdirectories.

• maintained by Linux Foundation
• specifies the main directories that need to be present and describe their purposes
• simplifies predictions of file locations
• additional directories not violates the standard
• components in directories other than the standard describe is violate FHS
• documentation

Data distinctions #

Shareable vs non-shareable #

• shareable: can be shared between different hosts, eg.: user home directory
• non-shareable: specific to a particular host, eg.: device lock files

variable vs static #

• variable: may change without sysadmins help, eg.: process files
• static: does not change without admin, eg.: binaries, libraries

/
├── bin
├── boot
├── dev
├── etc
├── home
├── lib
├── lib64
├── media
├── mnt
├── opt
├── proc
├── sys
├── root
├── sbin
├── srv
├── tmp
├── usr
├── var
└── run

/ #

• primary directory of the filesystem
• must contain all essential files required to boot and mount other filesystems
• according to FHS, no application should create a new subdirectory of the root
• only root user can write to this directory

bin #

• /binary
• contains all essential command binaries that needed for all user
• must contain all tools that needed in singel user mode or in recovery mode
• non-essential commands are placed in /usr/bin instead of /bin, like commands required only by non-root users
• recent distros do not separate /bin/ and /usr/bin, instead uses a symbolic link to /usr/bin

boot #

• contains static files needed to boot the system
• often a separate partition
• contains: kernel, initramfs images, boot configuration files and bootloader
• vmlinuz: compressed Linux kernel
• initramfs/initrd: initial RAM filesystem / initial RAM disk, mounted before the real root filesystem become available
• config: kernel compilation parameters
• System.map: kernel symbol table, used for debugging
• on EFI systems this is the EFI System Partition

dev #

• /devices
• contains special device files (device nodes)
• pseudo-filesystem
• old systems uses devfs, moderns uses udev as its filesystem
• files represents devices in your system
• contains character devices (byte-stream, eg.: /dev/hwrng) and block devices (eg.: /dev/sda)
• used to interact with hardwares and softwares
• on modern distros, udev is responsible for dynamically modifying files in /dev if devices are found (at boot time or on the fly) or removed
• network interfaces are too complex to be placed in /dev
• /dev is empty when the computer powered off

etc #

• /etcetera
• host specific system-wide configuration files
• applications may pre-populate this directory with its vendor-supplied configuartion files
• /etc/resolv.conf: the system’s DNS server
• /etc/passwd: user database
• /etc/shadow: users encrypted passwords
• /etc/group: list of groups and its users
• /etc/skel: template for new users
• /etc/init.d: contains daemon scripts for SysV

home #

• users home directories
• regular users probably use this for its home directory, eg.: /home/g0rbe for user g0rbe
• here is stored the user specific configurations, files and scripts
• shorthands to get logged in user’s home directory: ~ and $HOME

lib #

• /library
• contains essential shared libraries and kernel modules
• these libraries is needed to execute binaries in /bin and /sbin
• kernel modules and drivers stored in /lib/modules/
• PAM modules stored in /lib/security/
• recent distros uses symlink to /usr/lib

lib64 #

• essential libraries for 64bit system executables
• used when both 32 and 64 bit executables supported by the system

media #

• mount point for removable media
• when automatic mount is enabled, udev creates a folder and mount the filesystem there

mnt #

• /mount
• mount point for temporary filesystems

opt #

• /optional
• optional application packages that not part of the system distribution, but from an independent source
• used by packages to keep all of their files in one isolated folder instead of placing files to their proper place, eg.: configs in /etc, binaries in /bin, etc…

proc #

• /process
• virtual pseudo-filesystem called procfs
• used to give information about the system and the processes
• files anf folders in /proc are resides only in the memory
• every process have a subdirectory named as the process’s PID and contains information about the process, eg.: PID, name, parent process, resources used, etc…
• most of the file’s size is 0 while containing data
• like /dev, the /proc is empty on a non-running system
• /proc/sys is used to get and alter system configurations

sys #

• /system
• virtual pseudo-filesystem called sysfs
• used to give information about the system (devices, drivers, kernel modules, system configuration structures, etc…) and to alter system settings
• files and folders in /sys are resides only in the memory
• like /dev and /proc, the /sys is also empty when computer turned off
• strict standards about what can contain
• most files are contains only one line or value

root #

• home directory of the root user
• root specific configurations, files and executables
• located outside of /home in order to make sure the root user may log in even without /home being available

sbin #

• /system binary
• essential system binaries for booting, restoring, recovering and/or repairing the system and mounting other filesystems
• recent distros uses symlink to /usr/sbin

srv #

• /service
• data for services provided by this system
• rarely used

tmp #

• /temporary
• used to store temporary files (eg.: lock files)
• probably a ramdisk in memory called tmpfs
• probably flushed across a reboot
• can be accessed by anyone, the permissions are 0777
• should not used to store large files

usr #

• /user or today’s: /User System Resources
• multi-user (user-land) applications, utilities and datas stored here are not needed to boot the system
• contains a secondary hierarchy (/usr/bin, /usr/lib, etc…)
• typically a read-only directory
• /usr/share/man: man pages stored here
• /usr/lib: C and C++ API header file of system libraries

var #

• /variable
• variable datas that changes during system operations
• must be writeable
• stored here:
  • log files (/var/log)
  • spool file for further processing (/var/spool)
  • administrative data files
  • transient and temporary files, like cache (/var/cache)
  • user’s mailbox (/var/mail)
  • root for websites (/var/www)

run #

• /runtime
• used to store runtime data
• required by udev
• a pseudo-filesystem stored in memory, called tmpfs