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Sreekanth Kuruba
Sreekanth Kuruba

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Linux File Permissions Explained Simply

#linux #devops #beginners #sysadmin

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
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Example output:

-rwxr-xr-- 1 user group  1024 Apr 10 12:34 script.sh
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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
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Numeric method (very common in servers)

chmod 755 script.sh
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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
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Owner → full access
Group → read + execute
Others → read + execute

Change Owner (chown) 

chown username file.txt
chown user:group file.txt
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Recursive:

chown -R user:group folder/
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Only the root user or sudo user can change ownership.

Changing Group (chgrp) 

chgrp developers project/
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Used when:

multiple developers work on same project
shared access is needed

⚠️ Dangerous Mistake Beginners Make 

chmod 777 file.txt     # ❌ Very Dangerous
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🚨 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
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And get:

Permission denied
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Fix:

chmod +x script.sh
./script.sh
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👉 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.

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