Linux Filesystem Hierarchy Standard (FHS)
What is the Filesystem Hierarchy Standard?
The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux distributions. It's maintained by the Linux Foundation and provides a common layout that ensures consistency across different Linux systems, making it easier for users, administrators, and software developers to locate files and understand system organization.
Purpose of FHS: - Consistency: Uniform directory structure across distributions - Predictability: Users and software know where to find files - Compatibility: Applications work across different Linux systems - Administration: Simplified system management and troubleshooting
Root Directory (/)
The root directory is the top-level directory from which all other directories branch. Everything in a Linux system is located under this directory.
/ (root)
├── bin/ # Essential user command binaries
├── boot/ # Boot loader files
├── dev/ # Device files
├── etc/ # System configuration files
├── home/ # User home directories
├── lib/ # Essential shared libraries
├── media/ # Mount points for removable media
├── mnt/ # Temporary mount points
├── opt/ # Optional software packages
├── proc/ # Virtual filesystem (process information)
├── root/ # Root user's home directory
├── run/ # Runtime data
├── sbin/ # Essential system binaries
├── srv/ # Service data
├── sys/ # Virtual filesystem (system information)
├── tmp/ # Temporary files
├── usr/ # User programs and data
└── var/ # Variable data files
Essential Directories
/bin (Essential User Binaries)
Contains essential command binaries that need to be available in single-user mode and for all users.
Contents:
- Basic commands: ls, cp, mv, rm, cat, grep
- Shell programs: bash, sh
- System utilities: ps, kill, mount, umount
# View contents
ls -la /bin
# Common binaries
/bin/bash # Bash shell
/bin/ls # List directory contents
/bin/cp # Copy files
/bin/mv # Move/rename files
/bin/rm # Remove files
/sbin (Essential System Binaries)
Contains essential system administration binaries, typically used by the system administrator.
Contents:
- System management tools: fdisk, fsck, mkfs
- Network configuration: ifconfig, route
- System control: init, shutdown, reboot
# View contents (may require root)
ls -la /sbin
# Common system binaries
/sbin/fdisk # Disk partitioning
/sbin/fsck # Filesystem check
/sbin/iptables # Firewall configuration
/sbin/init # System initialization
/etc (Configuration Files)
Contains system-wide configuration files and shell scripts used during boot.
Important subdirectories and files:
/etc/passwd # User account information
/etc/shadow # Encrypted passwords
/etc/group # Group information
/etc/fstab # Filesystem mount information
/etc/hosts # Static hostname to IP mappings
/etc/hostname # System hostname
/etc/resolv.conf # DNS resolver configuration
/etc/crontab # System cron jobs
/etc/ssh/ # SSH server configuration
/etc/nginx/ # Nginx web server configuration
/etc/apache2/ # Apache web server configuration
/usr (User Programs)
Contains user programs and data. This is often the largest directory on the system.
Structure:
/usr/bin/ # Non-essential user binaries
/usr/sbin/ # Non-essential system binaries
/usr/lib/ # Libraries for /usr/bin and /usr/sbin
/usr/local/ # Locally installed software
/usr/share/ # Architecture-independent data
/usr/include/ # Header files for C programming
/usr/src/ # Source code
/var (Variable Data)
Contains variable data files that change during system operation.
Subdirectories:
/var/log/ # Log files
/var/cache/ # Application cache data
/var/lib/ # State information for applications
/var/spool/ # Spool directories (mail, print queues)
/var/tmp/ # Temporary files preserved between reboots
/var/www/ # Web server document root (common)
/var/mail/ # User mailboxes
/home (User Home Directories)
Contains home directories for regular users.
/home/username/ # Individual user directories
/home/john/ # John's home directory
/home/jane/ # Jane's home directory
# User subdirectories (common)
~/Documents/ # User documents
~/Downloads/ # Downloaded files
~/Desktop/ # Desktop files
~/Pictures/ # Images
~/Videos/ # Video files
/tmp (Temporary Files)
Contains temporary files created by applications and users. Files here may be deleted on reboot.
# Temporary files
/tmp/ # World-writable temporary directory
# Often mounted as tmpfs (RAM-based filesystem)
/dev (Device Files)
Contains device files that represent hardware devices and special files.
Common device files:
/dev/sda # First SATA drive
/dev/sda1 # First partition of first SATA drive
/dev/null # Null device (discards all data)
/dev/zero # Produces null characters
/dev/random # Random number generator
/dev/urandom # Pseudo-random number generator
/dev/tty # Current terminal
/proc (Process Information)
Virtual filesystem that provides information about running processes and kernel parameters.
Important files and directories:
/proc/cpuinfo # CPU information
/proc/meminfo # Memory information
/proc/version # Kernel version
/proc/uptime # System uptime
/proc/loadavg # System load average
/proc/[PID]/ # Process-specific information
/sys (System Information)
Virtual filesystem that exposes kernel objects, their attributes, and relationships.
/sys/class/ # Device classes
/sys/devices/ # Device hierarchy
/sys/kernel/ # Kernel information
/sys/module/ # Loaded kernel modules
Optional Directories
/opt (Optional Software)
Contains optional software packages, typically commercial software or large software suites.
/opt/google/ # Google software
/opt/teamviewer/ # TeamViewer installation
/opt/sublime/ # Sublime Text editor
/srv (Service Data)
Contains data for services provided by the system.
/srv/www/ # Web server data
/srv/ftp/ # FTP server data
/srv/git/ # Git repositories
/media (Removable Media)
Mount points for removable media devices.
/media/username/ # User-specific removable media
/media/usb/ # USB drives
/media/cdrom/ # CD/DVD drives
/mnt (Mount Points)
Temporary mount points for administrators to mount filesystems.
/mnt/backup/ # Backup drive mount point
/mnt/nfs/ # Network filesystem mount
/mnt/windows/ # Windows partition mount
Special Directories
/root (Root Home)
Home directory for the root user (system administrator).
/root/ # Root user's home directory
/root/.bashrc # Root's bash configuration
/root/.ssh/ # Root's SSH keys
/boot (Boot Files)
Contains files needed for system boot process.
/boot/vmlinuz # Linux kernel
/boot/initrd.img # Initial RAM disk
/boot/grub/ # GRUB bootloader configuration
/lib (Essential Libraries)
Contains essential shared libraries needed by binaries in /bin and /sbin.
/lib/modules/ # Kernel modules
/lib/firmware/ # Firmware files
/lib64/ # 64-bit libraries (on 64-bit systems)
/run (Runtime Data)
Contains runtime data for system services, only valid until next reboot.
/run/user/ # User runtime directories
/run/systemd/ # systemd runtime data
/run/lock/ # Lock files
Navigation Commands
Basic Navigation
# Show current directory
pwd
# Change to different directories
cd / # Root directory
cd /home/username # User home
cd /etc # Configuration files
cd /var/log # Log files
cd ~ # Home directory (shortcut)
cd - # Previous directory
# List directory contents
ls / # Root directory contents
ls -la /etc # Detailed listing of /etc
ls -la /home # List home directories
# Find files in the hierarchy
find / -name "filename" 2>/dev/null
locate filename
which command # Find command location
whereis command # Find binary, source, manual
Directory Information
# Show directory sizes
du -sh /var/log # Size of log directory
du -sh /home/* # Size of each user directory
du -sh /* # Size of root-level directories
# Show filesystem usage
df -h # Disk space usage
df -i # Inode usage
# Show directory tree structure
tree /etc -L 2 # Show /etc tree, 2 levels deep
tree /home -d # Show only directories
File System Information
# Show mounted filesystems
mount | column -t
findmnt
# Show filesystem hierarchy
lsblk # Block device tree
lsblk -f # With filesystem info
# Show file types
file /bin/ls # File type information
file /etc/passwd # Check file type
Directory Permissions and Ownership
Standard Permissions
# View permissions
ls -ld /tmp # Sticky bit directory
ls -ld /etc # Configuration directory
ls -ld /home # Home directories
# Common permission patterns
drwxr-xr-x /home # 755 - directories
-rw-r--r-- /etc/passwd # 644 - readable config files
-rw------- /etc/shadow # 600 - sensitive files
drwxrwxrwt /tmp # 1777 - sticky bit temp directory
Special Directory Permissions
# Sticky bit (typically on /tmp)
ls -ld /tmp # Shows 't' at end: drwxrwxrwt
# SGID on directories (group inheritance)
chmod g+s /shared/dir # New files inherit group ownership
# Check special permissions
find /usr/bin -perm -4000 # SUID binaries
find /usr/bin -perm -2000 # SGID binaries
Modern Additions to FHS
systemd Directories
/etc/systemd/ # systemd configuration
/usr/lib/systemd/ # systemd unit files
/run/systemd/ # systemd runtime data
/var/lib/systemd/ # systemd state data
Container and Virtualization
/var/lib/docker/ # Docker data
/var/lib/libvirt/ # Libvirt virtual machines
/etc/containers/ # Container configuration
Snap and Flatpak
/snap/ # Snap packages
/var/lib/snapd/ # Snap daemon data
/var/lib/flatpak/ # Flatpak applications
Best Practices
For System Administrators
- Keep user data in
/home - Store logs in
/var/log - Mount external storage under
/mntor/media - Install optional software in
/opt - Use
/srvfor service-specific data
For Developers
- Install from source in
/usr/local - Store application data in
/var/lib/appname - Create logs in
/var/log/appname - Use
/etc/appnamefor configuration files
For Users
- Keep personal files in home directory (
~) - Use subdirectories like
~/Documents,~/Downloads - Don't modify system directories without proper permissions
- Understand the purpose of each directory before making changes