Linux Chmod Permissions: A Complete Guide with Visual Examples

Linux file permissions are essential knowledge for every developer, but what do 777, 755, and 644 actually mean? How do you read rwxr-xr-x? This guide explains everything from basics to advanced tricks.

The Three-Tier Permission Structure

Every Linux file has three permission groups for three types of users:

Role	Symbol	Meaning
Owner	u (user)	The file creator
Group	g (group)	Members of the same user group
Others	o (others)	Everyone else

Each permission group has three operations:

Operation	Symbol	Number
Read	r	4
Write	w	2
Execute	x	1

How to Calculate Numeric Permissions

Add up the numbers for each group, then combine all three groups.

Example: What does 755 mean?

• Owner: 7 = 4(r) + 2(w) + 1(x) = Read, write, execute ✓
• Group: 5 = 4(r) + 0(w) + 1(x) = Read and execute ✓
• Others: 5 = 4(r) + 0(w) + 1(x) = Read and execute ✓

Example: What does 644 mean?

• Owner: 6 = 4(r) + 2(w) + 0(x) = Read and write ✓
• Group: 4 = 4(r) + 0(w) + 0(x) = Read only ✓
• Others: 4 = 4(r) + 0(w) + 0(x) = Read only ✓

Common Permission Reference Table

Numeric	Symbolic	Typical Use Case
777	rwxrwxrwx	Temp directories (not recommended for production)
755	rwxr-xr-x	Executable files, website directories
644	rw-r--r--	Regular files, config files
700	rwx------	Private scripts, SSH directories
600	rw-------	SSH private keys, sensitive configs
444	r--r--r--	Read-only files
000	---------	No permissions at all

Understanding Symbolic Permissions

Symbolic permissions use 9 characters, grouped in threes:

rwx r-x r-x
├──┤ ├──┤ ├──┤
 Owner Group Others

Symbolic Syntax with chmod

# Add execute permission for owner
chmod u+x script.sh

# Remove write permission for group and others
chmod go-w config.yml

# Set all permissions at once
chmod u=rwx,g=rx,o=rx script.sh

Special Permissions

Beyond rwx, there are three special permission bits:

Special	Numeric	Symbolic	Purpose
SUID	4	u+s	Execute as file owner
SGID	2	g+s	Execute as file group
Sticky	1	o+t	Only owner can delete files

Example: What is 4755?

• 4 = SUID bit set
• 755 = Owner has rwx, group and others have rx

Common in /usr/bin/passwd - lets regular users run it as root to change passwords.

Practical Tips

Quick Security Check

# Find files with overly permissive permissions
find /var/www -perm 777 -type f

# Find SUID files (potential security risk)
find / -perm -4000 -type f

Standard Web Directory Permissions

# Directories: 755
find /var/www -type d -exec chmod 755 {} \;

# Files: 644
find /var/www -type f -exec chmod 644 {} \;

Don't Want to Calculate Manually? Use an Online Tool

Converting between numeric and symbolic permissions is error-prone, especially with special bits. I use an online Chmod calculator - just check the boxes and it auto-generates the command:

👉 Chmod Permission Calculator - Jiebang Tools

Check permissions → Auto-generate chmod 755 file command → Copy and use directly.

Summary

What You Want	Command
Make script executable	chmod +x script.sh
Protect config file	chmod 600 config.yml
Set web directory	chmod 755 /var/www
Set SUID bit	chmod 4755 /usr/bin/cmd
Batch set directories	find . -type d -exec chmod 755 {} \;

Remember: Use minimum permissions necessary, never use 777 by default. Security first!