3. File System

3.1 Directory Structure Overview

Linux follows the everything is a file philosophy. All content (including hardware devices) exists as files. The entire file system starts from / (root directory), forming an inverted tree.

# View root directory structure
ls /

bin  boot  dev  etc  home  lib  lib64  lost+found  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

Directory	Full Name	Purpose
/	root	Root directory, the starting point of all directories
/bin	binaries	Basic user commands (ls, cp, mv, etc.)
/sbin	system binaries	System administration commands (fdisk, iptables, etc.)
/etc	etcetera	System configuration files (equivalent to Windows Control Panel)
/home	home	Home directories for regular users
/root	root home	Home directory for the root user
/var	variable	Variable data (logs, caches, mail, etc.)
/tmp	temporary	Temporary files (may be cleared on reboot)
/usr	Unix System Resources	User-level programs and data (the largest directory)
/opt	optional	Third-party software installation location
/proc	process	Virtual filesystem for process and kernel information
/dev	device	Device files (hard drives, terminals, etc.)
/boot	boot	Boot loader files (kernel, grub)
/mnt	mount	Temporary mount points
/lib	library	System shared library files

📝 Note: 📝 Memory trick: /etc can be understood as "Editable Text Configuration" — this is the easiest interpretation to remember.

3.2 Absolute Paths and Relative Paths

Absolute Path: a complete path starting from /, such as /home/user/Documents

Relative Path: a path relative to the current directory, not starting with /

Three special directory symbols:

• . — current directory
• .. — parent directory
• ~ — current user's home directory

# Assuming we're currently in /home/user, the following two methods are equivalent
cd /home/user/Documents        # Absolute path
cd Documents                   # Relative path

# Go to parent directory
cd ..

# Go up two directory levels
cd ../..

# Go to home directory (three equivalent methods)
cd ~
cd
cd $HOME

3.3 File Types

In Linux, everything is a file. File types are identified by the first character of ls -l output:

ls -l /

drwxr-xr-x   2 root root  4096 Dec 15 08:00 bin
drwxr-xr-x   3 root root  4096 Dec 15 08:00 boot
drwxr-xr-x  18 root root  3800 Jun 20 10:00 dev
drwxr-xr-x  96 root root  4096 Jun 20 10:00 etc
drwxr-xr-x   4 root root  4096 Jan 10 14:30 home
lrwxrwxrwx   1 root root     7 Dec 15 08:00 lib -> usr/lib
-rw-r--r--   1 root root    64 Jun 20 09:00 test.txt

Identifier	Type	Description
-	Regular file	Text, binary, images, etc.
d	Directory	Folder
l	Symbolic link	Similar to Windows shortcuts
c	Character device	Terminals, keyboards, etc.
b	Block device	Hard drives, USB drives, etc.
p	Named pipe	Inter-process communication
s	Socket	Network communication

Other Ways to View File Types:

file /bin/ls
file /etc/passwd
file /dev/sda

/bin/ls:        ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, ...
/etc/passwd:    ASCII text
/dev/sda:       block special (8/0)