Linux File Hierarchy Structure

The Linux File Hierarchy Structure or the Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix-like operating systems. It is maintained by the Linux Foundation. 

• In the FHS, all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices.
• Some of these directories only exist on a particular system if certain subsystems, such as the X Window System, are installed.
• Most of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS and are not considered authoritative for platforms other than Linux.

1. / (Root): 

At the top of every Linux file system is the root directory represented by a forward slash /. It’s the base point, and no directory exists above it. If you look at the file system graphically, you’ll see all other directories branching from this root directory.

• Every single file and directory start from the root directory.
• The only root user has the right to write under this directory.
• /root is the root user’s home directory, which is not the same as /

Only the root user has permission to modify contents inside this directory. Regular users cannot make changes here. For example, if you attempt to create a file in / as a non-root user, you’ll encounter a "Permission Denied" error.

2. /bin : 

The /bin directory contains essential commands and binaries needed by all users, including cp, ls, ssh, and kill. These commands are universally available across user types.

• Contains binary executables.
• Common linux commands you need to use in single-user modes are located under this directory.
• Commands used by all the users of the system are located here e.g. ps, ls, ping, grep, cp 

3. /boot :

This directory stores all files required for booting the system. It includes the GRUB bootloader configuration and essential kernel files that are loaded during startup. 

• Kernel initrd, vmlinux, grub files are located under /boot
• Example: initrd.img-2.6.32-24-generic, vmlinuz-2.6.32-24-generic

4. /dev :

Device files in Linux are stored in the /dev directory. These are special files that act as interfaces between hardware and software. Device files are of two types: block devices (e.g., hard drives) and character devices (e.g., microphones and speakers). Examples include /dev/sda1 for disk partitions.

• These include terminal devices, usb, or any device attached to the system.
• Example: /dev/tty1, /dev/usbmon0

5. /etc :

Short for "Editable Text Configuration," /etc contains configuration files for system applications, users, services, and tools or it contains the Host-specific system-wide configuration files. For example, user details like UID and local addresses are defined here.

• This also contains startup and shutdown shell scripts used to start/stop individual programs.
• Example: /etc/resolv.conf, /etc/logrotate.conf.

6. /home :

Every non-root user has a personal directory inside /home. For example, if your username is anshu, your personal directory would be /home/anshu. Each user can create, delete, or modify files only in their own home directory and cannot access others’ home directories.

• Home directories for all users to store their personal files, containing saved files, personal settings, etc..
• example: /home/kishlay, /home/kv

7. /lib:

Applications require shared libraries to run, which are stored in /lib. These include dynamic libraries needed during runtime. For example, Apache server libraries are available here.

• Library filenames are either ld* or lib*.so.*
• Example: ld-2.11.1.so, libncurses.so.5.7

8. /media:

Devices like USBs, CDs, and pen drives are mounted under /media. For example, when a CD-ROM is inserted (appeared in FHS-2.3), its details will appear here.

• Temporary mount directory for removable devices.
• Examples, /media/cdrom for CD-ROM; /media/floppy for floppy drives; /media/cdrecorder for CD writer

9. /mnt :

When external drives are connected, they are temporarily mounted in /mnt. This is where their contents become accessible to the system.

• Temporary mount directory where sysadmins can mount filesystems.

10. /opt : 

Third-party software and packages not part of the default system installation are stored in /opt. It includes their configuration and data files.

• Contains add-on applications from individual vendors.
• Add-on applications should be installed under either /opt/ or /opt/ sub-directory.

11. /sbin : 

Essential system binaries, e.g.,

This directory holds administrative binaries like iptables, firewall management tools, fsck, init, route etc. These binaries are primarily for system administrators and typically require root privileges to execute.

• Just like /bin, /sbin also contains binary executables.
• The linux commands located under this directory are used typically by system administrators, for system maintenance purposes.
• Example: iptables, reboot, fdisk, ifconfig, swapon

12. /srv : 

Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems.

• srv stands for service.
• Contains server specific services related data.
• Example, /srv/cvs contains CVS related data.

13. /tmp : 

Programs create temporary files during execution, and these are stored in /tmp. These files are deleted automatically after the program finishes or when the system is restarted.

• Directory that contains temporary files created by system and users.
• Files under this directory are deleted when the system is rebooted.

14. /usr : 

Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications. 

• Contains binaries, libraries, documentation, and source-code for second level programs.
• /usr/bin contains binary files for user programs. If you can’t find a user binary under /bin, look under /usr/bin. For example: at, awk, cc, less, scp
• /usr/sbin contains binary files for system administrators. If you can’t find a system binary under /sbin, look under /usr/sbin. For example: atd, cron, sshd, useradd, userdel
• /usr/lib contains libraries for /usr/bin and /usr/sbin
• /usr/local contains user's programs that you install from source. For example, when you install apache from source, it goes under /usr/local/apache2
• /usr/src holds the Linux kernel sources, header-files and documentation. 

15. /proc:

The /proc directory provides detailed information about system processes. Each process is assigned a unique ID and represented as a directory inside /proc. For example, /proc/meminfo gives real-time data about memory usage including total, free, buffer, and cache statistics.

• Contains information about system process.
• This is a pseudo filesystem that contains information about running processes. For example: /proc/{pid} directory contains information about the process with that particular pid.
• This is a virtual filesystem with text information about system resources. For example: /proc/uptime

Conclusion

The Linux File System (FHS) to store everything like programs and settings to your personal files. It starts at the root folder (/) and organizes files into special folders with specific jobs. Key folders include /home for your personal files, /etc for system settings, /bin for essential commands, and /tmp for temporary files. This organization keeps Linux systems tidy, secure, and consistent across different computers.