The chmod (Change Mode) command is used in Linux to modify file and directory permissions. It controls who can read (r), write (w), and execute (x) a file or directory.

Understanding Permissions in Linux

Each file and directory in Linux has three types of users:

1. Owner (u) – The person who created the file.
2. Group (g) – Users who belong to the same group.
3. Others (o) – Everyone else.

Each file has permission settings like:

-rwxr-xr-- 1 user group  1234 Jan 30 12:00 example.sh

• r → Read
• w → Write
• x → Execute
• - → No permission

Now, let’s explore 10 powerful chmod commands with real-world explanations.

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1. Give Read, Write, and Execute Permissions to the Owner

chmod u+rwx file.txt

✔ What it does:

• Grants read (r), write (w), and execute (x) permissions to the owner (u).
• Does not affect group (g) or others (o).

📌 Example Before (ls -l file.txt):

-r--r--r-- 1 user group  1234 Jan 30 12:00 file.txt

📌 Example After:

-rwxr--r-- 1 user group  1234 Jan 30 12:00 file.txt

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2. Give Execute Permission to Everyone

chmod +x script.sh

✔ What it does:

• Grants execute (x) permission to the owner, group, and others.

📌 Before:

-rw-r--r-- script.sh

📌 After:

-rwxr-xr-x script.sh

💡 Now you can run ./script.sh instead of bash script.sh.

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3. Remove Write Permission from Others

chmod o-w file.txt

✔ What it does:

• Removes write (w) permission from others (o).

📌 Before:

-rw-rw-rw- file.txt

📌 After:

-rw-rw-r-- file.txt

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4. Set Full Permissions for Owner and Read-Only for Others

chmod 744 file.txt

✔ What it does:

• 7 (Owner) → Read (4) + Write (2) + Execute (1) = rwx
• 4 (Group) → Read (4) = r–
• 4 (Others) → Read (4) = r–

📌 Before:

-rw-rw-rw- file.txt

📌 After:

-rwxr--r-- file.txt

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5. Give All Permissions to Everyone

chmod 777 file.txt

✔ What it does:

• 7 (Owner) → rwx
• 7 (Group) → rwx
• 7 (Others) → rwx

📌 Before:

-rw-r--r-- file.txt

📌 After:

-rwxrwxrwx file.txt

💡 Warning: This makes the file executable by anyone, which is risky!

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6. Remove All Permissions from Group and Others

chmod 700 file.txt

✔ What it does:

• 7 (Owner) → rwx
• 0 (Group) → No permissions
• 0 (Others) → No permissions

📌 Before:

-rw-r--r-- file.txt

📌 After:

-rwx------ file.txt

💡 Best for private files!

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7. Make a Directory Accessible by Everyone

chmod 755 mydir

✔ What it does:

• 7 (Owner) → rwx (Full control)
• 5 (Group) → r-x (Read & Execute)
• 5 (Others) → r-x (Read & Execute)

📌 Before:

drwxr-x--- mydir

📌 After:

drwxr-xr-x mydir

💡 Great for public directories, allowing others to enter (x).

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8. Remove Execute Permission from a Directory

chmod a-x mydir

✔ What it does:

• Removes x (execute permission) from all users (a = ugo).
• Now, users cannot enter the directory.

📌 Before:

drwxr-xr-x mydir

📌 After:

drw-r--r-- mydir

💡 Useful for restricting access while keeping files readable.

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9. Apply Permissions Recursively to All Files and Folders

chmod -R 755 /var/www/html/

✔ What it does:

• -R applies changes recursively to all files and subdirectories in /var/www/html/.

📌 Example: Before:

-rw-r--r-- index.html
-rw------- config.php
drwx------ private/

After:

-rwxr-xr-x index.html
-rwxr-xr-x config.php
drwxr-xr-x private/

💡 Commonly used for web server files.

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10. Set a File to be Read-Only for Everyone

chmod 444 file.txt

✔ What it does:

• 4 (Owner) → Read only (r–)
• 4 (Group) → Read only (r–)
• 4 (Others) → Read only (r–)

📌 Before:

-rw-rw-rw- file.txt

📌 After:

-r--r--r-- file.txt

💡 Useful for protecting important system files.

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Final Thoughts

• chmod is essential for Linux security.
• Use symbolic (u, g, o, a) and numeric (777, 755, 644) modes wisely.
• Never use chmod 777 on sensitive files unless absolutely necessary!