Ever ran a Linux command and saw this?
Permission denied
It feels random at first — but it’s not.
That error is Linux protecting your system using file permissions.
Once you understand this, Linux security suddenly starts making sense.
What Are Linux File Permissions?
Every file and directory has three types of permissions:
- Read (r) → View file content
- Write (w) → Modify or delete file
- Execute (x) → Run file as a program/script
👉 These decide what you are allowed to do with a file.
3 Types of Users in Linux
Permissions are not just for files — they are for users.
| User Type | Symbol | Meaning |
|---|---|---|
| Owner | u | The owner of the file |
| Group | g | Users in the same group |
| Others | o | Everyone else |
👉 Linux is a multi-user system, so access control is essential.
Understanding ls -l (Very Important)
Run this command:
ls -l
Example output:
-rwxr-xr-- 1 user group 1024 Apr 10 12:34 script.sh
Now it Breakdown:
- First character →
-= file,d= directory - Next 3 → Owner permissions
- Next 3 → Group permissions
- Last 3 → Others permissions
Permission Symbols
| Symbol | Meaning |
|---|---|
| r | Read |
| w | Write |
| x | Execute |
| - | No permission |
Changing Permissions chmod
Make script executable
chmod +x script.sh
Numeric method (very common in servers)
chmod 755 script.sh
Numeric permissions are a shortcut to set all three permission types at once.
Common Numeric Permissions:
| Number | Permission | Meaning |
|---|---|---|
| 7 | rwx | Read + Write + Execute |
| 6 | rw- | Read + Write |
| 5 | r-x | Read + Execute |
| 4 | r-- | Read only |
👉 So:
755 = rwx r-x r-x
Owner → full access
Group → read + execute
Others → read + execute
Change Owner (chown)
chown username file.txt
chown user:group file.txt
Recursive:
chown -R user:group folder/
Only the root user or sudo user can change ownership.
Changing Group (chgrp)
chgrp developers project/
Used when:
multiple developers work on same project
shared access is needed
⚠️ Dangerous Mistake Beginners Make
chmod 777 file.txt # ❌ Very Dangerous
🚨 This gives:
full access to everyone
no security control
breaks Linux permission model
👉 Never use this in real servers.
Real-Life Example
You create a script:
./script.sh
And get:
Permission denied
Fix:
chmod +x script.sh
./script.sh
👉 This is one of the most common Linux beginner issues.
Simple Mental Model
Think of Linux permissions like a building:
Owner → has master key
Group → shared access card
Others → guest access
And:
r → can look inside
w → can modify
x → can enter/run
Summary
You learned:
- What r, w, x mean
- User types (owner, group, others)
- How to read ls -l
- Using chmod
- Numeric permissions (755, 644, etc.)
- Changing ownership with chown
- Changing groups with chgrp
- Why chmod 777 is dangerous
Why This Matters
File permissions are not just theory.
They are used in:
Linux servers
DevOps pipelines
Docker containers
Cloud systems
Production deployments
👉 If you understand this, you understand Linux security basics.
Next Post:
Linux User & Group Management Explained Simply,useradd, usermod, groups
Which permission concept confused you the most when you first used Linux?
I’ll simplify it even further in Part 4.