Welcome back, young explorer!
Today we’re going to unpack something that confuses almost every Linux beginner:

👉 File permissions and ownership
Or in Linux language:

• chmod

• chown

• rwx

• 755, 644, etc.

Don’t worry — we’re going to break this down until it feels natural and easy.
Think of this as learning how locks and keys work in your digital house.

Let’s begin.

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🧩 Step 1: What Are File Permissions?

In Linux, every file has rules controlling:

1. Who can read it

2. Who can modify it

3. Who can run it

These are known as permissions, represented by:

r = read  
w = write  
x = execute  

And they apply to three groups:

user    (owner of the file)
group   (members of the file’s group)
others  (everyone else)

Let’s see an example.

Run:

ls -l

You might see something like:

-rw-r--r-- 1 root root  4096 Jan 10 index.html

Breakdown:

Let’s decode the permission part:

-rw-r--r--
 ^ ^  ^  ^
 | |  |  |
type |  |  others
     |  group
     user

This means:

• user: read + write

• group: read

• others: read

Simple so far, right?

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🛠️ Step 2: Changing Permissions with chmod

chmod means change mode — the tool used to change file permissions.

There are two methods:

• Letter (r, w, x)

• Number (755, 644)

Let’s start with the beginner-friendly number system.

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🔢 Step 3: Understanding Permission Numbers (chmod)

Each permission has a number:

To assign permissions, you add up the values.

Example:

Read + Write:

4 + 2 = 6

→ permission = 6

Read Only:

4 = 4

→ permission = 4

Read + Execute:

4 + 1 = 5

→ permission = 5

Full Permissions:

4 + 2 + 1 = 7

→ permission = 7

So when you see:

chmod 755 file

Break it down:

• user: 7 → rwx

• group: 5 → r-x

• others: 5 → r-x

This is very common for web apps.

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🧪 Step 4: Common chmod Values You Will Use Daily

Setting 755 on a folder:

chmod 755 /var/www/myapp

Setting 644 on a file:

chmod 644 /var/www/myapp/index.html

Setting 600 on private key:

chmod 600 ~/.ssh/id_rsa

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👑 Step 5: Changing Ownership with chown

chown means change owner of a file.

Format:

chown user:group filename

Example: assigning ownership to user www-data:

sudo chown www-data:www-data /var/www/myapp -R

-R = recursively (apply to all files inside)

When do you use chown?

✔ When installing web apps
✔ When uploading files via FTP
✔ When fixing "Permission denied" errors
✔ When Apache/Nginx cannot read a file

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⚠️ Step 6: Fixing Common Permission Problems

❌ Error: Permission denied

Cause: You don’t have permission.
Fix: Allow read/write:

chmod 644 filename

Or change owner:

sudo chown youruser:youruser filename

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❌ Nginx/Apache cannot access your site

Give correct web user permissions:

Ubuntu/Debian uses:

sudo chown -R www-data:www-data /var/www/html

CentOS/RHEL uses:

sudo chown -R nginx:nginx /usr/share/nginx/html

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❌ SSH key not working

Your permissions are too open.

Fix:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
chmod 600 ~/.ssh/id_rsa

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🎯 Step 7: What Permissions Should You Never Use?

❌ Never use chmod 777

This gives EVERYONE full control.

Hackers LOVE 777.
Don’t give it to them.

❌ Never give write access to “others”

Example:

-wx-wx-wx

This is a recipe for disaster.

Stick to safe numbers:

• 644 for files

• 755 for folders

• 600 for private files

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🔐 Bonus: Permission Cheat Sheet (Copy & Save!)

chmod 755 folder/            # Allow read+execute for everyone, write only for owner  
chmod 644 file.txt           # Normal file permissions  
chmod 600 id_rsa             # Private SSH key  
chown user:user file.txt     # Change file ownership  
chown -R www-data:www-data /var/www  # Fix web server ownership  

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🎉 Conclusion

You just learned:

✔ How Linux permissions work
✔ How to understand rwx
✔ How chmod numbers like 755 and 644 work
✔ How to change ownership with chown
✔ How to fix common permission errors
✔ What to avoid (like 777!)

File permissions are fundamental.
After reading this guide, you are now far more comfortable managing secure Linux servers.