Understanding Linux Filesystem: What Every User Should Know About /bin, /etc, /usr, and More

Top‑Level Directories and Their Roles

/bin : Essential binary executables needed for system boot and basic user commands.

/sbin : System binaries primarily used by the administrator; many are also essential for boot.

/etc : Configuration files for the system and installed services (e.g., /etc/passwd, /etc/rc.d, /etc/fstab).

/home : Home directories for regular users (e.g., /home/user).

/root : Home directory of the superuser.

/lib : Shared libraries required by binaries in /bin and /sbin.

/lib/modules : Kernel modules that can be loaded dynamically.

/dev : Device special files that provide access to hardware devices.

/tmp : Temporary files created by programs during execution.

/boot : Files used by the boot loader, including the kernel image.

/mnt : Mount points for temporarily attaching other filesystems.

/proc : Virtual filesystem exposing kernel and process information.

/usr : Large hierarchy containing most user‑land applications, libraries, documentation, and source code.

/var : Variable data such as logs, spool directories, and temporary files that change frequently.

Important Subdirectories of /etc

/etc/rc or /etc/rc.d – Startup scripts and run‑level configuration.

/etc/passwd – User account database.

/etc/fdprm – Floppy disk parameter table.

/etc/fstab – Filesystem mount table for automatic mounting.

/etc/group – Group information.

/etc/inittab – Init process configuration.

/etc/issue – Pre‑login message.

/etc/magic – File command configuration.

/etc/motd – Message of the day displayed after login.

/etc/mtab – Currently mounted filesystems (maintained by mount).

/etc/shadow – Secure password storage.

/etc/login.defs – Login command defaults.

/etc/printcap – Printer capabilities.

/etc/profile, /etc/csh.login, /etc/csh.cshrc – Global shell initialization files.

/etc/securetty – List of terminals allowed for root login.

/etc/shells – Available login shells.

/etc/termcap – Terminal capability database.

/dev Device Files Overview

/dev/console – System console.

/dev/hd* – IDE hard‑disk devices.

/dev/sd* – SCSI disk devices.

/dev/fd* – Floppy drives.

/dev/st* – SCSI tape devices.

/dev/tty* – Virtual consoles.

/dev/pty* – Pseudo‑terminals for remote logins.

/dev/ttys – Serial ports (e.g., COM1).

/dev/cua* – Serial devices for modems.

/dev/null – Null device that discards all written data.

/usr Hierarchy Highlights

/usr/x11r6 – X Window system binaries and support files.

/usr/bin – Most user commands.

/usr/sbin – System administration commands not needed for basic boot.

/usr/man, /usr/info, /usr/doc – Manual pages, GNU info documents, and other documentation.

/usr/include – C header files.

/usr/lib – Shared libraries and package configuration files.

/usr/local – Locally installed software, mirroring the /usr structure.

/var Directory Details

/var/catman – Pre‑formatted manual pages.

/var/lib – Variable state information for applications.

/var/local – Variable data for locally installed software.

/var/lock – Lock files used by programs.

/var/log – Log files for system and application events.

/var/run – Runtime information such as PID files.

/var/spool – Spool directories for mail, news, printers, etc.

/var/tmp – Larger temporary files that persist across reboots.

/proc Virtual Filesystem

The /proc directory provides a view into kernel and process information. Important entries include:

/proc/cpuinfo – CPU details.

/proc/meminfo – Memory usage.

/proc/uptime – System uptime.

/proc/version – Kernel version.

/proc/[pid]/ – Information about each running process.

Understanding this directory layout helps administrators manage Linux systems efficiently, keep the root filesystem minimal, and organize user data, applications, and system resources appropriately.