Detailed Explanation of Linux File Attributes

🧱 Detailed Explanation of Linux File Attributes

📘 Understanding file attributes and permissions is the first step in mastering Linux system administration.

👥 1. Linux is a Multi-User System

Linux is a typical multi-user operating system. Different users have different system permissions. To ensure system security, Linux has strict rules regarding “who can access which files and what operations can be performed”.

Common modification commands include:

🧩 chown: Change the owner or group of a file.
🔐 chmod: Change the access permissions of a file.
🧱 chgrp: Change the group

🔍 2. Viewing File Attributes

Use the command:

ls -l
Example output:

dr-xr-xr-x 2 root root 4096 Dec 14 2012 bind
-r-xr-xr-x 4 root root 4096 Apr 19 2012 boot
Explanation as follows:

The first character d indicates the file type:

d: Directory
-: Regular file
l: Link file
b: Block device (random access)
c: Character device (e.g., keyboard, mouse)

The next 9 characters are divided into three groups (rwx rwx rwx):

r: Read (read) → Value is 4
w: Write (write) → Value is 2
x: Execute (execute) → Value is 1
If there is no permission, it is represented by -.

🧩 3. Structure of File Permissions

A file’s permissions consist of 10 characters, for example:

d rwx r-x r-x
Meaning explanation:
Position 0: Indicates file type (d for directory, - for regular file)
Positions 1–3: Owner's (owner) permissions
Positions 4–6: Group's (group) permissions
Positions 7–9: Other users' (others) permissions
Positions without permissions are displayed as -

👤 4. File Owner and Group

In Linux, each file has:

Owner (owner): The file’s owner;
Group (group): The user group to which the file belongs.

For example:

drwxr-xr-x 3 mysql mysql 4096 Apr 21 2014 mysql
Explanation:

Owner is mysql
Group is mysql
Owner has read, write, and execute permissions;
Group users have read and execute permissions;
Other users have read and execute permissions.

💡 Tip: root user is almost unrestricted and can access any file.

🔧 5. Changing File Attributes

1️⃣ Change Group: (`chgrp`)

Command format:

chgrp [-R] group_name file_name
-R: Recursively change the group of subdirectories and files.
Example:
chgrp users example.txt
2️⃣ Change Owner: (chown) Command format:
chown  [-R] username file_name
chown  [-R] username:group_name file_name
Example:

chown  bin install.log
chown  root:root install.log

🔧 6. Changing Permissions (`chmod`)

Linux permissions consist of three types of identities:

👤 owner (file owner)
👥 group (file group)
🌍 others (other users)

Each identity has r / w / x three permissions.

For example:

-rwxrwx---
Then:

owner = 7 (4+2+1)
group = 7 (4+2+1)
others = 0 → Permission number: 770

Command format:

chmod  [-R] 770 file_name
Example:

chmod  777 .bashrc
chmod  754 test.sh

🧠 7. Symbolic Representation

Identity representation:

u: Owner (user)
g: Group (group)
o: Other users (others)
a: All users (all)

Permission representation:

r: Read (read)
w: Write (write)
x: Execute (execute)

Example operations:

Set permissions:

chmod u=rwx,g=rx,o=r test1

Remove all execute permissions:

chmod a-x test1

🧰 8. Practical Case Illustrations

Example 1: Change in Numeric Permissions

# Initial state
-rw-r--r-- 1 root root 0 Nov 15 10:32 test.sh
# Change permissions to 755
chmod 755 test.sh
# State after change
-rwxr-xr-x 1 root root 0 Nov 15 10:32 test.sh
Change explanation:

Owner gains execute permission
Group and other users gain execute permission

Example 2: Change in Symbolic Permissions

# Initial state
-rwxrwxrwx 1 root root 0 Nov 15 10:32 test1
# Remove all execute permissions
chmod a-x test1
# State after change
-rw-rw-rw- 1 root root 0 Nov 15 10:32 test1
Change explanation:

Removed execute permission (x) for everyone

🔢 Common Permission Number Reference

777 → Everyone can read, write, and execute (⚠️ Lowest security)
755 → Owner can read, write, and execute; others can only read and execute (✅ Commonly used for scripts and directories)
700 → Only the owner can read, write, and execute (🔒 Commonly used for private files and key directories)
644 → Owner can read and write; others can only read (📄 Commonly used for text files)
600 → Only the owner can read and write (🔑 Commonly used for password files and configuration files)
400 → Only the owner can read (🧩 Critical configuration files to prevent accidental changes)
000 → No permissions for anyone (🚫 Access denied)

💡 Tips for Understanding Permissions

The larger the number, the higher the permissions.
The first number represents the owner, the second represents the group, and the third represents others.

Owner 7 = rwx
Group 5 = r-x
Others 4 = r--

Example:chmod 754

Recommended Practices:

Regular files: 644
Executable files: 755
Private files: 600
Private directories: 700

✅ Summary and Recommendations

The file permission mechanism in Linux is the cornerstone of system security. Understanding permission numbers and symbols, and proficiently using chown, chgrp, and chmod will enable you to:

✅ Quickly understand the file permission structure✅ Flexibly control access and security policies✅ Avoid system risks caused by accidental operations

Detailed Explanation of Linux File Attributes