PL - 002 — Linux Filesystem

06 Jun, 2026

PL - 002 — Linux Filesystem

Linux Filesystem Hierarchy

Linux systems organize directories and resources under a unified root namespace (/). This structure is guided by the Filesystem Hierarchy Standard (FHS), ensuring consistent file locations across Unix-like operating systems.

Unix-like systems present a unified file-descriptor interface for interacting with system resources—including regular files, directories, device nodes, pipes, sockets, and virtual kernel interfaces. While summarized by the phrase “everything is a file,” processes and abstract kernel objects remain distinct internal entities managed by the kernel.

Filesystem Hierarchy

Directory	Purpose
`/`	Root of the entire filesystem hierarchy
`/boot`	Kernel and boot-related files
`/dev`	Device nodes and device interfaces
`/etc`	System-wide configuration
`/home`	Regular users’ home directories
`/media`	Removable-media mount points
`/mnt`	Manual/temporary mount point
`/opt`	Optional or third-party software
`/proc`	Process and kernel information
`/root`	Root user’s home directory
`/run`	Volatile runtime state
`/srv`	Data served by system services
`/sys`	Kernel objects, devices, and drivers
`/tmp`	Temporary files
`/usr`	User-space programs, libraries, and shared data
`/var`	Variable and persistent system/application data

A system may also contain additional directories such as /lost+found, /snap, or distribution-specific directories.

Linux File Types

Linux filesystems support several fundamental file types. The first character in the output of ls -l identifies the type.

Character	Type	Description
`-`	Regular file	Contains ordinary data such as text, programs, images, or scripts
`d`	Directory	Contains directory entries that organize files hierarchically
`l`	Symbolic link	Refers to another filesystem path
`c`	Character device	Provides a character-oriented interface to a device
`b`	Block device	Provides block-oriented access to a device such as a disk
`p`	Named pipe (FIFO)	Provides FIFO-style inter-process communication
`s`	Socket	Provides bidirectional communication between processes

Terminal Session

[aadarsha@mainserver ~]$ cat /etc/os-release 
NAME="CentOS Stream"
VERSION="10 (Coughlan)"
RELEASE_TYPE=stable
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="10"
PLATFORM_ID="platform:el10"
PRETTY_NAME="CentOS Stream 10 (Coughlan)"
ANSI_COLOR="0;31"
LOGO="fedora-logo-icon"
CPE_NAME="cpe:/o:centos:centos:10"
HOME_URL="https://centos.org/"
VENDOR_NAME="CentOS"
VENDOR_URL="https://centos.org/"
BUG_REPORT_URL="https://issues.redhat.com/"
REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux 10"
REDHAT_SUPPORT_PRODUCT_VERSION="CentOS Stream"

[aadarsha@mainserver ~]$ whoami
aadarsha

[aadarsha@mainserver ~]$ pwd
/home/aadarsha

[aadarsha@mainserver ~]$ cd /

[aadarsha@mainserver /]$ pwd
/

[aadarsha@mainserver /]$ ls
afs  boot  etc   lib    media  opt   root  sbin  sys  usr
bin  dev   home  lib64  mnt    proc  run   srv   tmp  var

[aadarsha@mainserver /]$ ls -l
total 20
dr-xr-xr-x.   2 root root    6 Apr  2  2025 afs
lrwxrwxrwx.   1 root root    7 Apr  2  2025 bin -> usr/bin
dr-xr-xr-x.   5 root root 4096 Jul 13 14:45 boot
drwxr-xr-x.  20 root root 3300 Aug 23 19:30 dev
drwxr-xr-x.  82 root root 8192 Aug 23 19:30 etc
drwxr-xr-x.   5 root root  100 Aug  5 21:47 home
lrwxrwxrwx.   1 root root    7 Apr  2  2025 lib -> usr/lib
lrwxrwxrwx.   1 root root    9 Apr  2  2025 lib64 -> usr/lib64
drwxr-xr-x.   2 root root    6 Apr  2  2025 media
drwxr-xr-x.   2 root root    6 Apr  2  2025 mnt
drwxr-xr-x.   2 root root    6 Apr  2  2025 opt
dr-xr-xr-x. 200 root root    0 Aug 23 19:29 proc
dr-xr-x---.   4 root root  186 Aug  5 21:46 root
drwxr-xr-x.  32 root root  880 Aug 23 19:30 run
lrwxrwxrwx.   1 root root    8 Apr  2  2025 sbin -> usr/sbin
drwxr-xr-x.   2 root root    6 Apr  2  2025 srv
dr-xr-xr-x.  13 root root    0 Aug 23 19:30 sys
drwxrwxrwt.  10 root root 4096 Aug 23 19:32 tmp
drwxr-xr-x.  12 root root  144 Jul 13 14:29 usr
drwxr-xr-x.  19 root root  264 Jul 13 14:45 var
[aadarsha@mainserver /]$ 

# here : 
  bin -> usr/bin,  lib -> usr/lib,  lib64 -> usr/lib64, sbin -> usr/sbin  are soft links

Primary Directories

Root & System Boot

/ (Root): Top-level directory; every absolute path originates here.
/boot: Contains files required to initialize the system during boot:
- vmlinuz-*: Compressed Linux kernel image.
- initramfs-*: Initial RAM filesystem used for early userspace initialization.
- grub2/ or loader/: Bootloader configurations and binary modules.

System Configuration & Hardware Interfaces

/etc: Contains global administrative system and service configuration files (e.g., fstab, hosts, passwd, sysctl.conf). Configuration in /etc may relate to:
- networking
- authentication
- users and groups
- SSH
- DNS
- package management
- boot configuration
- security
- system services
- installed applications
/dev: Exposes kernel device nodes managed dynamically (typically via udev).
- Block devices: /dev/sda
- Character interfaces: /dev/tty
- Pseudo-devices: /dev/null, /dev/zero, /dev/urandom

User Spaces & Mounting

/home: Houses persistent personal storage and configurations for regular users (/home/<username>).
/root: Dedicated home directory for the root administrative user, intentionally kept separate from /home on the primary root partition.
/media: Automated mount location for removable storage media (e.g., USB drives, optical disks).
/mnt: Standard temporary mount point for manual filesystem attachment by administrators.
/opt: Standard destination for standalone, third-party software packages installed outside system package management (e.g., Google, Oracle, VMware).
/srv: Holds site-specific data served by network daemons (e.g., /srv/www for web applications).

User-Space Programs (`/usr`)

The /usr directory serves as the primary secondary hierarchy for read-only user space data and binaries:

/usr/bin: Primary executable commands for user applications (bash, git, gcc, python).
/usr/sbin: Executable binaries for system administration and management tasks.
/usr/lib / /usr/lib64: Architecture-dependent shared libraries and kernel modules.
/usr/local: Primary destination for locally compiled or non-packaged custom software.
/usr/share: Platform-independent architecture-neutral assets (man pages, documentation, locales).
/usr/include: C/C++ header files required during source code compilation.
/usr/src: Kernel and application source code files.

UsrMerge Note: Modern distributions simplify this layout by converting legacy directories (/bin, /sbin, /lib, /lib64) into symbolic links pointing directly to their /usr counterparts (/usr/bin, /usr/sbin, /usr/lib, /usr/lib64).

Variable Application Data (`/var`)

Contains stateful data that continually changes during normal system runtime:

/var/cache: Cached application-level data intended to speed up operations.
/var/lib: Stateful persistent database engine files, container storage, and service states (do not delete).
/var/log: System and service logs (messages, journal/, dnf.log).
/var/spool: Queued tasks awaiting execution (print queues, cron schedules, mail queues).
/var/tmp: Temporary data designed to survive system reboots longer than /tmp.

Virtual & Runtime Filesystems

Linux implements pseudo-filesystems to expose dynamic kernel and hardware states as accessible file interfaces:

Path	Virtual Type	Primary Purpose	Persistent Disk Storage?
`/proc`	`procfs`	Exposes active process data (PID folders) and runtime kernel metrics (`/proc/meminfo`, `/proc/cpuinfo`)	No (In-Memory)
`/sys`	`sysfs`	Exposes structural kernel objects, hardware topology, device drivers, and subsystem tunables	No (In-Memory)
`/run`	`tmpfs`	Stores volatile runtime state, PID files, and IPC sockets generated during the current boot cycle	No (In-Memory)
`/tmp`	`tmpfs` / Disk	Short-lived temporary workspace cleared on reboot or periodic cleanup policies	Varies by distro

Structural Distinctions

/ vs /root: / is the global filesystem root; /root is the home directory for the administrative user.
/tmp vs /var/tmp: /tmp is for ephemeral scratch space; /var/tmp persists across reboots for longer-term temporary processing.
/proc vs /sys: /proc focuses on process status and kernel parameters; /sys focuses on hardware bus topology, drivers, and device representations.
/run vs /var: /run stores non-persistent volatile boot session data; /var stores persistent changing system logs and application databases.

Further Documentation & References

man hier — Local system manual page for directory structures
man proc — System documentation for /proc interfaces
man sysfs — Kernel object export subsystem documentation
man findmnt — Command tool for inspecting system mount trees
man file — Identify filesystem object types
man ls — List directory contents
Filesystem Hierarchy Standard (FHS) 3.0 Specification
Linux Kernel Documentation on procfs
Linux Kernel Documentation on sysfs