Linux File Permissions: The Interview Question Everyone Gets Wrong

The Question That Ends Interviews

“What does 755 mean?”

I’ve seen candidates with impressive CVs stumble on this. They know Linux, they’ve used chmod, but they can’t explain what the numbers actually mean. Interview over.

File permissions are the foundation of Linux security. Every file, every directory, every script you run is governed by permissions. Get this wrong, and you create security vulnerabilities. Get this right, and you demonstrate the security thinking that senior roles require.

This isn’t going to be a dry manual page recitation. This is how permissions actually work, why they matter, and how to answer every interview question about them.

Reading Permission Strings

When you run ls -la, you see something like this:

-rwxr-xr-x 1 www-data www-data 4096 Dec 20 10:30 script.sh
drwxr-xr-x 2 root     root     4096 Dec 20 10:30 configs/

Let’s break down -rwxr-xr-x:

Position	Characters	Meaning
1	-	File type (-=file, d=directory, l=link)
2-4	rwx	Owner permissions
5-7	r-x	Group permissions
8-10	r-x	Others (everyone else) permissions

Each permission means:

Letter	For Files	For Directories
r	Read the file contents	List directory contents
w	Modify the file	Create/delete files in directory
x	Execute as program	Enter the directory (cd into it)
-	Permission not granted	Permission not granted

Key insight: Directory permissions work differently than file permissions. You need x on a directory just to cd into it, even if you have r.

Octal Notation: The Interview Favourite

Here’s the part everyone struggles with. Each permission has a numeric value:

Permission	Value
Read (r)	4
Write (w)	2
Execute (x)	1
None (-)	0

Add them up for each category:

Permission	Calculation	Result
rwx	4+2+1	7
rw-	4+2+0	6
r-x	4+0+1	5
r--	4+0+0	4
---	0+0+0	0

So 755 means:

• Owner: 7 (rwx) – full access
• Group: 5 (r-x) – read and execute
• Others: 5 (r-x) – read and execute

Memory trick: 4-2-1 is like binary. r=100, w=010, x=001. Add them together.

Common Permission Patterns

Memorise these—they come up constantly:

Octal	String	Use Case
755	rwxr-xr-x	Scripts, executables, directories (standard)
644	rw-r--r--	Regular files, configs (owner can edit, others can read)
600	rw-------	Private files (SSH keys, secrets)
700	rwx------	Private directories, scripts
777	rwxrwxrwx	Never use this. Ever. Security nightmare.
750	rwxr-x---	Group access, no public access
640	rw-r-----	Sensitive configs (group can read)

chmod: Changing Permissions

Octal Method

# Set exact permissions
chmod 755 script.sh
chmod 644 config.ini
chmod 600 private_key

Symbolic Method

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

# Remove write for group and others
chmod go-w file.txt

# Set read for everyone
chmod a+r public.txt

# Multiple changes
chmod u+rwx,go+rx script.sh

Symbol	Meaning
u	User (owner)
g	Group
o	Others
a	All (u+g+o)
+	Add permission
-	Remove permission
=	Set exactly

Recursive Changes

# Apply to directory and all contents
chmod -R 755 /var/www/html/

# Better: different perms for files and directories
find /var/www/html -type d -exec chmod 755 {} \;
find /var/www/html -type f -exec chmod 644 {} \;

chown: Changing Ownership

# Change owner
sudo chown nginx /var/www/html/index.html

# Change owner and group
sudo chown nginx:www-data /var/www/html/index.html

# Change group only
sudo chown :www-data /var/www/html/index.html

# Recursive
sudo chown -R nginx:www-data /var/www/html/

Ownership matters because permissions are evaluated based on who you are:

1. If you’re the owner, owner permissions apply
2. If you’re in the group, group permissions apply
3. Otherwise, other permissions apply

Real-World Scenarios

Scenario 1: Web Server Files

A web server needs to read files but shouldn’t be able to modify them:

# Files owned by deployer, readable by web server
sudo chown -R deployer:www-data /var/www/html/
sudo find /var/www/html -type d -exec chmod 750 {} \;
sudo find /var/www/html -type f -exec chmod 640 {} \;

# Upload directory where web server needs write access
sudo chmod 770 /var/www/html/uploads/

Scenario 2: SSH Key Permissions

SSH is very picky about permissions. Too loose and it refuses to work:

# .ssh directory
chmod 700 ~/.ssh

# Private keys
chmod 600 ~/.ssh/id_rsa

# Public keys
chmod 644 ~/.ssh/id_rsa.pub

# authorized_keys
chmod 600 ~/.ssh/authorized_keys

Scenario 3: Shared Project Directory

Multiple users need access to the same files:

# Create group
sudo groupadd developers

# Add users to group
sudo usermod -aG developers alice
sudo usermod -aG developers bob

# Set up directory
sudo mkdir /opt/project
sudo chown root:developers /opt/project
sudo chmod 775 /opt/project

# New files inherit group (setgid)
sudo chmod g+s /opt/project

Special Permissions

Beyond the basic rwx, there are special bits:

Setuid (4xxx)

# File runs as the file's owner, not the user running it
chmod 4755 /usr/bin/program
# Shows as: -rwsr-xr-x

The classic example is /usr/bin/passwd—it runs as root even when regular users execute it, because it needs to modify /etc/shadow.

Setgid (2xxx)

# Files in directory inherit the directory's group
chmod 2775 /shared/
# Shows as: drwxrwsr-x

Sticky Bit (1xxx)

# Only owner can delete their files (even with directory write access)
chmod 1777 /tmp
# Shows as: drwxrwxrwt

That’s why you can’t delete other users’ files in /tmp even though it’s world-writable.

Troubleshooting Permission Issues

Systematic Approach

# 1. Check the file permissions
ls -la /path/to/file

# 2. Check who you are
whoami
groups

# 3. Check directory permissions (you need x to traverse)
ls -la /path/to/
ls -la /path/

# 4. Check for ACLs (extended permissions)
getfacl /path/to/file

Common Fixes

Problem	Likely Cause	Fix
“Permission denied” on script	Missing execute bit	chmod +x script.sh
Can’t cd into directory	Missing execute on directory	chmod +x directory/
Web server can’t read files	Wrong owner or permissions	chown www-data:www-data
SSH refuses to authenticate	Key permissions too open	chmod 600 ~/.ssh/id_rsa
Can list files but not read them	Directory has r+x, files lack r	Check file permissions

Interview Questions

The Career Translation

Quick Reference

# View permissions
ls -la file

# Change permissions (octal)
chmod 755 script.sh      # rwxr-xr-x
chmod 644 file.txt       # rw-r--r--
chmod 600 secret.key     # rw-------

# Change permissions (symbolic)
chmod u+x script.sh      # Add execute for owner
chmod go-w file.txt      # Remove write for group/others
chmod a+r file.txt       # Add read for everyone

# Change ownership
chown user:group file
chown -R user:group dir/ # Recursive

# Common patterns
chmod 755 directories/    # Standard directory
chmod 644 files           # Standard file
chmod 600 private_keys    # SSH keys, secrets
chmod 700 ~/.ssh          # SSH directory

# Check effective permissions
getfacl file              # Show ACLs
namei -l /path/to/file    # Check path permissions

Next Steps

File permissions are step one of Linux security. To go deeper:

• ACLs – More granular permissions with setfacl/getfacl
• SELinux/AppArmor – Mandatory access control systems
• sudo configuration – Fine-grained privilege escalation
• umask – Default permissions for new files

Security is a mindset, not a checklist. Understanding permissions is how you start thinking like someone who protects systems, not just uses them.

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Part 4 of the Linux Fundamentals series. Next: network troubleshooting commands—because “is it the network?” is always the first question.

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