Linux File System

1. File System Overview

A file system is the way an operating system organizes and stores data. It defines how files are stored, named, accessed, and managed. In Linux, the file system is a hierarchical directory structure starting from the root directory (/) and extending downward with various subdirectories.

1.1 Features of Linux File System

• Hierarchical Structure: Linux uses a tree-like directory structure, extending downward from the root directory.
• Everything is a File: In Linux, almost everything is treated as a file, including devices, inter-process communication mechanisms, etc.
• File Permissions: Linux implements fine-grained file permission control to ensure file security.
• Multiple File System Support: Linux supports multiple file system types, such as ext4, XFS, Btrfs, etc.
• Mount Mechanism: Linux uses a mount mechanism to integrate different store devices into a unified file system tree.

2. Linux File System Hierarchy

Linux follows the File System Hierarchy Standard (FHS), which defines standards for directory structure and content. Here are the purposes of the main directories:

2.1 Main Directories and Their Purposes

• /: Root directory, the starting point for all files and directories.
• /bin: Contains basic command binary files, such as ls, cp, mv, etc.
• /sbin: Contains system administration command binary files, such as fdisk, ifconfig, etc.
• /etc: Contains system configuration files, such as network configuration, user account configuration, etc.
• /home: User home directories, each user has their own subdirectory here.
• /root: Root user's home directory.
• /var: Contains variable files, such as logs, mail, cache, etc.
• /tmp: Temporary file directory, contents are cleared after system reboot.
• /usr: Contains user programs and data, such as applications, library files, etc.
• /lib: Contains system library files, such as shared libraries.
• /mnt: Temporary mount points, used for mounting external devices.
• /media: Automatic mount points, used for mounting removable media.
• /dev: Contains device files, such as hard drives, optical drives, etc.
• /proc: Virtual file system, contains system and process information.
• /sys: Virtual file system, contains hardware device information.

2.2 Directory Structure Example

$ ls -la /
total 64
drwxr-xr-x  20 root root 4096 Apr 10 14:23 .
drwxr-xr-x  20 root root 4096 Apr 10 14:23 ..
drwxr-xr-x   2 root root 4096 Apr 10 14:24 bin
drwxr-xr-x   3 root root 4096 Apr 10 14:25 boot
drwxr-xr-x  18 root root 4608 Apr 10 14:26 dev
drwxr-xr-x 141 root root 4096 Apr 10 14:27 etc
drwxr-xr-x   4 root root 4096 Apr 10 14:28 home
drwxr-xr-x  14 root root 4096 Apr 10 14:24 lib
drwxr-xr-x   2 root root 4096 Apr 10 14:23 media
drwxr-xr-x   2 root root 4096 Apr 10 14:23 mnt
drwxr-xr-x   3 root root 4096 Apr 10 14:29 opt
dr-xr-xr-x 330 root root    0 Apr 10 14:26 proc
drwx------   6 root root 4096 Apr 10 14:30 root
drwxr-xr-x   2 root root 4096 Apr 10 14:24 run
drwxr-xr-x   2 root root 4096 Apr 10 14:24 sbin
drwxr-xr-x   2 root root 4096 Apr 10 14:23 srv
dr-xr-xr-x  13 root root    0 Apr 10 14:26 sys
drwxrwxrwt   8 root root 4096 Apr 10 14:31 tmp
drwxr-xr-x  12 root root 4096 Apr 10 14:24 usr
drwxr-xr-x  14 root root 4096 Apr 10 14:32 var

3. Linux File System Types

Linux supports multiple file system types, each with its own characteristics and applicable scenarios. Here are the commonly used file system types:

3.1 Local File Systems

3.1.1 ext4

ext4 is the default file system for Linux, an upgraded version of ext3. It supports larger files and partitions, providing better performance and reliability.

• Maximum file size: 16TB
• Maximum partition size: 1EB
• Features: Journaling file system, supports EXTENTS, delayed allocation, online defragmentation, etc.

3.1.2 XFS

XFS is a high-performance journaling file system developed by SGI, suitable for handling large files and mass store.

• Maximum file size: 8EB
• Maximum partition size: 8EB
• Features: High performance, supports online expansion, suitable for large servers

3.1.3 Btrfs

Btrfs (B-tree file system) is a modern file system that supports advanced features such as snapshots, cloning, and checksums.

• Maximum file size: 16EB
• Maximum partition size: 16EB
• Features: supports snapshots, cloning, checksums, RAID functionality, etc.

3.1.4 ZFS

ZFS is an advanced file system that supports data integrity verification, snapshots, cloning, compression, and other features.

• Maximum file size: 16EB
• Maximum partition size: 256ZB
• Features: Data integrity verification, supports RAID-Z, advanced snapshot functionality

3.2 Network File Systems

3.2.1 NFS

Network File System (NFS) allows different computers to share files over a network.

3.2.2 Samba

Samba allows Linux systems to share files and printers with Windows systems.

3.2.3 SSHFS

SSHFS mounts remote file systems via the SSH protocol, providing secure file sharing.

4. File System Mount managementment

Mounting is the process of connecting a file system to the Linux directory tree. In Linux, store devices need to be mounted to a mount point in the directory tree to access their files.

4.1 Mount Commands

4.1.1 mount Command

The mount command is used to mount file systems:

# Basic mount syntax
mount [options] device mount_point

# Mount USB device
mount /dev/sdb1 /mnt/usb

# Mount ISO file
mount -o loop /path/to/image.iso /mnt/iso

# View mounted file systems
mount

# View specific file system type mounts
mount -t ext4

4.1.2 umount Command

The umount command is used to unmount file systems:

# Unmount by mount point
umount /mnt/usb

# Unmount by device
umount /dev/sdb1

# Force unmount
umount -f /mnt/usb

# Lazy unmount (delayed unmount)
umount -l /mnt/usb

4.2 /etc/fstab File

The /etc/fstab file defines file systems that are automatically mounted at system startup. Each line contains the following fields:

# Device        Mount Point      File System Type  Mount Options     Dump  Pass
/dev/sda1    /           ext4          defaults    0     1
/dev/sda2    /home       ext4          defaults    0     2
/dev/sda3    swap        swap          defaults    0     0
/dev/sdb1    /mnt/data   ext4          defaults    0     2

4.3 Auto Mounting

The autofs service can automatically mount and unmount file systems as needed, reducing system resource using:

# Install autofs
sudo apt install autofs

# configuration auto mounting
# Edit /etc/auto.master file
# Add line: /mnt/autofs /etc/auto.misc

# Edit /etc/auto.misc file
# Add line: cd -fstype=iso9660,ro,nosuid,nodev :/dev/cdrom

# Restart autofs service
sudo systemctl restart autofs

5. File Operation Commands

Linux provides rich commands for file and directory operations:

5.1 Basic file operations

5.1.1 Creating and Deleting Files

# Create empty file
touch file.txt

# Create and write content
 echo "Hello World" > file.txt

# Append content
echo "Additional content" >> file.txt

# Delete file
rm file.txt

# Force delete
rm -f file.txt

# Confirm before delete
rm -i file.txt

5.1.2 Copying and Moving Files

# Copy file
cp source.txt destination.txt

# Copy directory
cp -r source_dir destination_dir

# Move file
mv source.txt destination.txt

# Rename file
mv oldname.txt newname.txt

5.1.3 Viewing File Content

# View file content
cat file.txt

# View with pagination
less file.txt

# View first n lines
head -n 10 file.txt

# View last n lines
tail -n 10 file.txt

# View in real-time
tail -f file.txt

5.2 Directory Operations

# Create directory
mkdir directory

# Create nested directories
mkdir -p dir1/dir2/dir3

# Delete empty directory
rmdir directory

# Delete non-empty directory
rm -r directory

# Change directory
cd directory

# Return to parent directory
cd ..

# Return to home directory
cd ~

# Show current directory
pwd

5.3 File Search

# Search by name
find /path -name "filename"

# Search by type
find /path -type f -name "*.txt"

# Search by size
find /path -size +10M

# Search by modification time
find /path -mtime -7

# Search for files containing specific content
grep -r "content" /path

6. File Permissions and Attributes

Linux uses a file permission system to control access to files and directories. Each file and directory has three sets of permissions: owner, group, and other users.

6.1 File Permission Representation

File permissions can be represented using symbols or numbers:

# Symbolic representation
-rw-r--r-- 1 user group 0 Apr 10 14:30 file.txt

# Numeric representation
# r=4, w=2, x=1
# 755 = rwxr-xr-x
# 644 = rw-r--r--

6.2 Modifying File Permissions

# Modify permissions using symbols
chmod u+x file.txt      # Add execute permission for owner
chmod g+w file.txt      # Add write permission for group
chmod o-r file.txt      # Remove read permission for others
chmod a+r file.txt      # Add read permission for all users

# Modify permissions using numbers
chmod 755 file.txt      # Set to rwxr-xr-x
chmod 644 file.txt      # Set to rw-r--r--

6.3 Modifying File Owner

# Modify owner
chown user file.txt

# Modify both owner and group
chown user:group file.txt

# Recursively modify directory and its contents
chown -R user:group directory

6.4 File Special Attributes

# Set SUID (Set User ID)
chmod u+s file

# Set SGID (Set Group ID)
chmod g+s file

# Set Sticky Bit
chmod +t directory

# View extended attributes
lsattr file

# Set extended attributes
chattr +i file          # Immutable
chattr +a file          # Append only

7. Linked Files

Linux supports two types of links: hard links and soft links (symbolic links).

7.1 Hard Links

A hard link is another reference to a file's inode, sharing the same inode and data with the original file.

# Create hard link
ln file.txt hardlink.txt

# View inode
ls -i file.txt hardlink.txt

7.2 Soft Links

A soft link is a special file that points to the path of another file or directory.

# Create soft link
ln -s file.txt softlink.txt

# View soft link
ls -l softlink.txt

# Link directory
ln -s directory link_directory

7.3 Differences Between Hard Links and Soft Links

• Hard Links: Share inode, deleting the original file does not affect hard links, cannot link directories, cannot cross file systems.
• Soft Links: Have their own inode, deleting the original file makes soft links invalid, can link directories, can cross file systems.

8. Practice Case: Managing File Systems

# View disk information
fdisk -l

# View partition information
df -h

# Enter fdisk tool
fdisk /dev/sdb

# Commands: 
n # Create new partition
p # Primary partition
1 # Partition number
# Press Enter to use default start sector
+10G # Partition size
w # Write and exit

# Format as ext4 file system
mkfs.ext4 /dev/sdb1

mkdir /mnt/data

mount /dev/sdb1 /mnt/data

# View UUID
blkid /dev/sdb1

# Edit /etc/fstab
# Add the following line
UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx /mnt/data ext4 defaults 0 2

# Unmount partition
umount /mnt/data

# Remount all partitions
mount -a

# View mount status
df -h

9. Interactive Exercises