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File Systems

A structured way to store and manage files. Acts as a interface between the OS and the storage device.

Types of File Systems

• FAT(File Allocation Table): Used in older Windows systems. Simple and easy to implement, but lacks features like journaling and permissions.

• NTFS(New Technology File System): Used in modern Windows systems. Supports large file sizes, file permissions, journaling, compression and encryption.

• ext4: Used in Linux systems. Also supports large file sizes, journaling, and file permissions.

• HFS+(Hierarchical File System) / APFS(Apple File System): Used in macOS systems. Supports journaling, encryption, and file permissions.

File System Components

In Linux, the file system organizes files and directories into a hierarchical structure.

Directory	Description
/	Root Directory: The top-level directory in the file system hierarchy.
/bin	Binaries: Contains essential binary executables like ls, cp, mv.
/boot	Boot Files: Contains boot loader files and the kernel image.
/dev	Devices: Contains device files for hardware devices.
/etc	Configuration Files: Contains system-wide configuration files. passwd, fstab…
/home	Home Directories: Contains user home directories.
/lib	Libraries: Contains shared libraries used by binary executables in /bin and /sbin.
/media	Removable Media: Mount point for removable media like USBs.
/mnt	Mount Point: Temporary mount point, when manually mounting a storage.
/opt	Optional: Contains optional software packages.
/proc	Process Information: Virtual filesystem with information about the system processes and hardware.
/root	Root User: Home directory for the root user.
/sbin	System Binaries: Contains essential system binaries like init, shutdown.
/tmp	Temporary Files: Contains temporary files created by applications.
/usr	User Binaries: Contains user binaries, libraries, documentation.
/var	Variable Files: Contains variable data like logs, mail, cache.

File and Directory Permissions

In Linux, file permissions define who can read, write or execute a file.

Each file and directory has 3 permissions for groups, User(u) -> owner, Group(g) -> group memebers that share access, Others(o) -> all teh other users.

Permissions are represented by 3 characters:

• r -> Read, represented by 4.
• w -> Write, represented by 2.
• x -> Execute, represented by 1.

Permissions are represented by 3 sets of characters:

• rwxrw-r-- -> is divided into 3 sets, rwx for owner, rw- for group, r-- for others.

• rwxrw-r-- -> 764 in octal representation.

• rwx can be represented by 4 + 2 + 1 = 7.

Commands for File and Permission Manipulation

Basic File Commands

ls

List files and directories.

ls -l # Long listing format

ls -a # List all files including hidden files

ls -lh # Human-readable file sizes

touch

Create an empty file.

touch file.txt

touch file1.txt file2.txt

cat

Concatenate and display file content.

cat file.txt

cat file1.txt file2.txt

cat > file.txt # Create a file and write content

cat >> file.txt # Append content to a file

cat file.txt | less # Display file content page by page

cp

Copy files and directories.

cp file.txt newfile.txt

cp -r dir1 dir2 # Copy directories recursively

mv

Move or rename files and directories.

mv file.txt newfile.txt

mv file.txt dir/ # Move file to a directory

rm

Remove files and directories.

rm file.txt

rm -r dir/ # Remove directories recursively

Viewing and Modifying Permission

chmod

Change file or directory permissions.

Numeric Mode: chmod 755 file.txt -> rwxr-xr-x Symbolic Mode: chmod u+r file.txt -> Adds read permission for the owner.

Symbolic Mode Syntax:

• u -> User, g -> Group, o -> Others, a -> All.
• + -> Add permission, - -> Remove permission, = -> Set permission.

chmod u+x file.txt # Add execute permission for the owner

chmod g-w file.txt # Remove write permission for the group

chmod o=r file.txt # Set read permission for others

chmod 755 file.txt # Set permissions using numeric mode

Ownership Management

chown

Change file or directory owner.

chown user:group file.txt # Change owner and group

chown user file.txt # Change owner

chown :group file.txt # Change group

chgrp

Change file or directory group.

chgrp group file.txt # Change group

chgrp :group file.txt # Change group

umask

Sets the default file permissions when a file is created umask is reponsible to set its permissions.

• umask 022: sets default permissions to 755 for directories and 644 for files.

How to read umask?

• umask 022 -> 777 - 022 = 755 for directories. 777 is the default permission for directories.

• umask 022 -> 666 - 022 = 644 for files. 666 is the default permission for files.

If the umask is set to 022, the default permissions for files will be 644 and for directories will be 755. Which is by default in most of the systems.

stat

Displays detailed information about a file, including its permissions.

stat file.txt

stat -c %a file.txt # Display only the permissions