Complete Guide to Linux File System Structure and User Management

Before delving into Linux system administration, it is crucial to understand the file system structure and user management mechanisms. This guide will detail the Linux File System Hierarchy Standard (FHS) and user permission management.

Core Concepts of the Linux File System

Basic Features of the File System

There are several important features of the Linux file system to remember:

1. Filename Length Limit: Directory and file names cannot exceed 255 characters

2. Case Sensitivity: File.txt and file.txt are two different files

3. Everything is a File: In Linux, devices, processes, network connections, etc., all exist in the form of files

Path Representation Methods

There are two path representation methods in Linux:

Absolute Path

The complete path starting from the root directory /:

/home/user1/file1          /etc/passwd          /usr/local/bin/program

Relative Path

The path relative to the current working directory:

# Currently in /home/user1 directory          ./file1          # file1 in the current directory          file1            # shorthand for ./          ../user2/file2   # file2 in the parent directory user2          # Currently in /home/user2 directory          ../user1/file1   # Accessing file1 in user1 directory

Detailed Explanation of the File System Hierarchy Standard (FHS)

Linux follows the File System Hierarchy Standard, with the main directory structure as follows:

Core System Directories

/                    # Root directory, the starting point of the entire file system          ├── bin/            # Basic command binaries (for regular users)          ├── sbin/           # System management commands (for administrators)          ├── boot/           # System boot files, kernel, and boot programs          ├── dev/            # Device file directory          │   └── null        # System "trash can", discards output data          ├── etc/            # System and software configuration files          ├── proc/           # Virtual file system, kernel and process information          └── tmp/            # Temporary file storage

User-Related Directories

/root/              # Home directory for the root user          /home/              # Home directories for regular users          └── username/       # Personal directory for a specific user

System Program Directories

/usr/               # System programs and files          ├── bin/           # User program binaries          ├── sbin/          # System management programs          ├── local/         # Locally installed software          ├── lib/           # 32-bit library files          └── lib64/         # 64-bit library files

Variable Data Directories

/var/               # Stores frequently changing files          ├── log/           # System and application logs          ├── mail/          # Mail storage          └── lib/           # Application database files          /media/           # Automatically mounted removable devices          /mnt/               # Manual mount points

Basic File Management Operations

File and Directory Operations

# Create a file          touch filename.txt          # Copy files/directories          cp source_file destination_file          cp -r source_dir destination_dir    # Recursively copy directories          # Move/Rename          mv old_name new_name          mv file_path new_location/          # Create a directory          mkdir directory_name          mkdir -p parent/child/grandchild    # Create multi-level directories          # Delete files/directories          rm filename          rm -rf directory_name               # Force delete directory and contents

File Type Identification

Use ls -la to view detailed file information; the file type is indicated by the first character:

-rw-r--r--  # - indicates a regular file          drwxr-xr-x  # d indicates a directory          brw-rw----  # b indicates a block device file          crw-rw-rw-  # c indicates a character device file          srw-rw-rw-  # s indicates a socket file          prw-r--r--  # p indicates a pipe file          lrwxrwxrwx  # l indicates a symbolic link

View detailed file type information:

file /etc/hosts          # Output: /etc/hosts: ASCII text

Text Editor Usage Guide

Vi/Vim Editor

Vi is the default text editor in Linux systems, mastering basic operations is very important:

Basic Mode Switching

vi filename         # Open file and enter command mode          i                   # Enter insert mode          Esc                 # Return to command mode

Cursor Movement Commands

# Basic directional movement          k                   # Move up          j                   # Move down          h                   # Move left          l                   # Move right          # Word-level movement          w                   # Next word's beginning          e                   # Current/next word's end          b                   # Previous word's beginning          # In-line movement          ^                   # Line's beginning          $                   # Line's end

Edit and Save Commands

dd                  # Delete current line          u                   # Undo last operation          :line_number       # Jump to specified line          :wq                 # Save and exit          :q!                 # Force exit without saving

Nano Editor

For beginners, nano provides a more user-friendly interface:

# Install nano          yum -y install nano          # Use nano to edit a file          nano filename

Nano’s common shortcut keys are displayed at the bottom of the screen, such as Ctrl+O to save, Ctrl+X to exit.

Linux User Management System

Viewing User Information

# View current user information          id          # Output: uid=1000(username) gid=1000(username) groups=1000(username)          # View specified user information          id username          # View currently logged-in users          who          # View system processes          ps aux | less

User Account Files

User information in Linux is stored in the following files:

1. /etc/passwd – Basic user information

2. /etc/shadow – User password information (stored encrypted)

User Creation and Management

Adding Users

# Basic user creation          useradd username          # Advanced user creation (specifying detailed parameters)          useradd -u 1020 \
           # Specify user ID                    -g 1001 \
           # Specify primary group ID                    -c "Test User" \
           # Add comment                    -d /home/test1 \
           # Specify home directory                    -s /bin/bash \
           # Specify default shell                    -G games \
           # Specify additional groups                    test1               # Username

Parameter Explanation:

• -u : Specify User ID (UID)

• -g : Specify Primary Group ID (GID)

• -c : Add user description

• -d : Specify user home directory path

• -s : Specify user default shell

• -G : Specify user additional groups

• -r : Create a system user

Privilege Escalation and Switching

# Temporarily elevate privileges to execute a command          sudo command          # Switch to root user          sudo su          # Switch to specified user          su username

Configuring Sudo Privileges

Edit the sudoers file to configure user sudo privileges:

# Use visudo command for safe editing          visudo          # Or directly edit the file (not recommended)          vi /etc/sudoers

Add configurations in the sudoers file:

# Allow user to execute all commands          username ALL=(ALL) ALL          # Allow user group to execute specific commands          %groupname ALL=(ALL) /bin/ls, /bin/cat

Special Nature of the Root User

The root user is the super administrator of the Linux system:

• UID of 0: This is the unique identifier for the root user

• Unlimited Privileges: Can perform any system operation

• System Security: Use root privileges cautiously to avoid accidental operations

Practical Recommendations and Summary

1. Principle of Least Privilege: Only grant users the minimum necessary privileges

2. Avoid Direct Use of Root: Use sudo to elevate privileges for administrative tasks

3. Regularly Review User Accounts: Remove unnecessary accounts and update permission settings

4. Possible Professional Directions for Further Study: System Administration, Network Security, CTF Competitions, DevOps

5. Continue learning to achieve proficiency in navigating the Linux file system and effectively managing files and directories

6. Understand and configure user permissions to safely perform system administration tasks

The structure of the Linux file system and user management is a core foundation of system administration. The file system hierarchy standard provides a clear framework for system organization, while the user management system ensures the security of the system and the orderly operation of a multi-user environment.