📂 Linux Directory Structure and Important Configuration Files

Linux Directory Structure (Filesystem Hierarchy Standard)

Root Directory (/)

The base of the filesystem hierarchy. All other directories and files are contained within it.

Essential System Directories

1. /bin - Essential command binaries

   • Contains fundamental user command binaries (like ls, cp, mv, rm, etc.)
   • Needed for single-user mode and system repair

2. /sbin - System binaries

   • Contains binaries for system administration (like fdisk, fsck, ifconfig, etc.)
   • Typically requires root privileges

3. /boot - Boot loader files

   • Contains files needed to boot the system (kernel, initramfs, GRUB files)
   • Important files:
     • vmlinuz - The Linux kernel
     • initrd.img - Initial RAM disk
     • grub/ - GRUB bootloader configuration

4. /dev - Device files

   • Contains special device files that represent hardware components
   • Examples: /dev/sda (first hard disk), /dev/tty (terminals), /dev/null (null device)

5. /etc - System configuration files

   • Contains host-specific system-wide configuration files
   • Most important configuration directory (covered in detail below)

6. /home - User home directories

   • Contains personal directories for each user
   • Each user has their own subdirectory (e.g., /home/username)

7. /lib - Essential shared libraries

   • Contains libraries needed by binaries in /bin and /sbin
   • /lib32 and /lib64 for 32-bit and 64-bit libraries respectively

8. /media - Removable media mount points

   • Automatic mount points for removable devices (USB drives, CDs, etc.)

9. /mnt - Temporary mount points

   • Traditionally used for temporarily mounting filesystems

10. /opt - Optional application software

    • Contains add-on applications from third parties

11. /proc - Process and kernel information

    • Virtual filesystem providing process and system information
    • Important files:
      • /proc/cpuinfo - CPU information
      • /proc/meminfo - Memory information
      • /proc/mounts - Mounted filesystems

12. /root - Root user’s home directory

    • Home directory for the root user (not in /home for recovery purposes)

13. /run - Runtime variable data

    • Contains system information since last boot (process IDs, lock files, etc.)

14. /srv - Service data

    • Contains data for services provided by the system (web, FTP, etc.)

15. /tmp - Temporary files

    • Stores temporary files that may be deleted between reboots

16. /usr - User utilities and applications

    • Secondary hierarchy containing most user utilities and applications
    • Important subdirectories:
      • /usr/bin - Non-essential command binaries
      • /usr/sbin - Non-essential system binaries
      • /usr/lib - Libraries for binaries in /usr/bin and /usr/sbin
      • /usr/local - Locally installed software
      • /usr/share - Architecture-independent data

17. /var - Variable data files

    • Contains files that change frequently (logs, spool files, caches)
    • Important subdirectories:
      • /var/log - System log files
      • /var/cache - Application cache data
      • /var/spool - Queued files (print jobs, mail, etc.)
      • /var/www - Default web server root (on some distributions)

Important Configuration Files

System Configuration Files (/etc)

1. /etc/passwd - User account information

   • Contains user account details (username, UID, GID, home directory, shell)
   • Format: username:x:UID:GID:comment:home_dir:shell

2. /etc/shadow - Secure user account information

   • Contains encrypted passwords and password aging information
   • Only readable by root

3. /etc/group - Group information

   • Defines system groups and their members

4. /etc/fstab - Filesystem table

   • Defines how disk partitions and other storage should be mounted
   • Used during system boot to mount filesystems

5. /etc/hosts - Static hostname resolution

   • Maps hostnames to IP addresses before DNS
   • Can override DNS lookups

6. /etc/resolv.conf - DNS resolver configuration

   • Configures DNS nameservers and search domains
   • Often managed dynamically by network services

7. /etc/hostname or /etc/hostname - System hostname

   • Contains the system’s hostname

8. /etc/network/interfaces or /etc/sysconfig/network-scripts/ - Network configuration

   • Configures network interfaces (Debian/Ubuntu vs RHEL/CentOS)

9. /etc/ssh/sshd_config - SSH server configuration

   • Configures the OpenSSH server daemon
   • Controls authentication methods, ports, etc.

10. /etc/sudoers - Sudo configuration

    • Defines who can run what commands as root via sudo
    • Should always be edited with visudo

11. /etc/crontab - System-wide cron jobs

    • Defines scheduled tasks run by the system
    • User-specific crontabs are in /var/spool/cron/

12. /etc/apt/sources.list (Debian/Ubuntu) or /etc/yum.repos.d/ (RHEL/CentOS) - Package repositories

    • Defines software repositories for package management

13. /etc/ld.so.conf - Shared library configuration

    • Lists directories to be included in the library search path

14. /etc/sysctl.conf - Kernel parameters

    • Configures kernel parameters at boot time

15. /etc/modprobe.d/ - Kernel module configuration

    • Configuration files for kernel modules

16. /etc/security/limits.conf - User process limits

    • Sets resource limits (number of processes, file handles, etc.)

17. /etc/default/grub - GRUB bootloader configuration

    • Main configuration file for GRUB bootloader
    • Changes require running update-grub (Debian) or grub2-mkconfig (RHEL)

Service-Specific Configuration Files

1. /etc/nginx/ - Nginx web server configuration

   • nginx.conf - Main configuration file
   • sites-available/ - Available website configurations
   • sites-enabled/ - Enabled website configurations

2. /etc/apache2/ (Debian) or /etc/httpd/ (RHEL) - Apache web server configuration

   • apache2.conf/httpd.conf - Main configuration file
   • sites-available/ and sites-enabled/ (Debian)
   • conf.d/ - Additional configuration files

3. /etc/mysql/ - MySQL configuration

   • my.cnf - Main MySQL configuration file

4. /etc/postgresql/ - PostgreSQL configuration

   • Contains configuration for PostgreSQL database server

5. /etc/php/ - PHP configuration

   • Contains PHP configuration files for different versions

Log Files (/var/log)

1. /var/log/syslog or /var/log/messages - General system messages

   • Primary system log file (name varies by distribution)

2. /var/log/auth.log - Authentication logs

   • Records authentication events (logins, sudo usage, etc.)

3. /var/log/kern.log - Kernel logs

   • Kernel-specific messages and errors

4. /var/log/dmesg - Kernel ring buffer

   • Contains messages from the kernel during boot

5. /var/log/apt/ or /var/log/yum.log - Package manager logs

   • Records package installation and updates

6. /var/log/nginx/ or /var/log/apache2/ - Web server logs

   • Contains access and error logs for web servers

7. /var/log/mysql/ - MySQL logs

   • Contains error logs and slow query logs for MySQL

User Configuration Files (in home directories)

1. ~/.bashrc - Bash shell configuration

   • Executed for interactive non-login shells
   • Contains aliases, functions, and shell options

2. ~/.bash_profile or ~/.profile - Login shell configuration

   • Executed for login shells
   • Typically sets environment variables and paths

3. ~/.ssh/ - SSH client configuration

   • config - SSH client configuration
   • known_hosts - Known SSH hosts
   • authorized_keys - Keys allowed for authentication

4. ~/.gitconfig - Git configuration

   • User-specific Git settings

5. ~/.vimrc - Vim configuration

   • Configuration for the Vim text editor

6. ~/.config/ - Application configuration

   • Modern location for user-specific application configurations

Special Files

1. /etc/motd - Message of the day

   • Displayed after successful login

2. /etc/issue - Pre-login message

   • Displayed before the login prompt

3. /etc/os-release - OS identification

   • Contains operating system identification data