How to Use Linux Chmod Command for File Permissions?

Quick Answer: Use chmod 755 filename to set read/write/execute for owner and read/execute for group/others, or chmod u+x filename to add execute permission for owner. Chmod controls file and directory access permissions in Linux.

Frequently Asked Questions

Q: What does chmod 755 mean in Linux? A: chmod 755 sets permissions to rwxr-xr-x: full access for owner (read/write/execute), read/execute only for group and others. It’s the standard permission for executable files.

Q: How do I make a file executable in Linux? A: Use chmod +x filename to add execute permission for all users, or chmod u+x filename to add execute permission only for the file owner.

Q: What’s the difference between chmod 644 and 755? A: chmod 644 (rw-r–r–) gives read/write to owner, read-only to others – used for data files. chmod 755 (rwxr-xr-x) adds execute permission – used for scripts and programs.

Q: How do I remove all permissions from a file? A: Use chmod 000 filename to remove all permissions, or chmod -rwx filename to remove read, write, and execute permissions for all users.

Q: What does chmod -R do? A: The -R flag applies chmod recursively to directories and all their contents. Example: chmod -R 755 /var/www sets permissions for the directory and all files/subdirectories inside.

Essential Steps to Use Chmod Command

1. Check current permissions: Use ls -l filename to see current permission settings before making changes
2. Choose permission method: Use octal numbers (755) for speed or symbolic notation (u+x) for precision
3. Set basic file permissions: Use chmod 644 for data files and chmod 755 for executable files
4. Test permissions: Verify changes work by attempting the intended file operations
5. Apply to directories: Use chmod -R 755 directory/ to set permissions recursively for web directories
6. Secure sensitive files: Use chmod 600 for private keys and confidential files (owner access only)

Most Common Chmod Commands

Understanding Linux File Permissions

Permission Structure

Linux file permissions use a 3-digit octal system or symbolic notation:

# View current permissions
ls -l filename
# Output: -rw-r--r-- 1 user group 1024 Jan 1 12:00 filename
#         ^^^^^^^^^
#         Permission bits: owner|group|others

Octal Permission Numbers

# Each digit represents: owner, group, others
# Each position adds: 4(read) + 2(write) + 1(execute)

# Common combinations:
7 = 4+2+1 = rwx (read, write, execute)
6 = 4+2+0 = rw- (read, write)
5 = 4+0+1 = r-x (read, execute)
4 = 4+0+0 = r-- (read only)
0 = 0+0+0 = --- (no permissions)

Step-by-Step Permission Setting

1. Basic File Permissions

# Standard data file permissions
chmod 644 document.txt

# Standard executable permissions
chmod 755 script.sh

# Private file permissions
chmod 600 ~/.ssh/id_rsa

2. Symbolic Permission Changes

# Add execute permission for owner
chmod u+x filename

# Remove write permission for group and others
chmod go-w filename

# Set exact permissions using symbolic notation
chmod u=rwx,g=rx,o=r filename

3. Directory Permissions

# Standard directory permissions
chmod 755 /home/user/public

# Recursive permission setting
chmod -R 644 /var/www/html/*.html
chmod -R 755 /var/www/html/scripts/

Advanced Chmod Techniques

Conditional Permission Setting

# Set different permissions based on file type
find /var/www -type f -exec chmod 644 {} \;  # Files
find /var/www -type d -exec chmod 755 {} \;  # Directories

# Make all .sh files executable
chmod +x *.sh

Special Permission Bits

# SUID - Run as file owner
chmod 4755 /usr/bin/program

# SGID - Run as group owner
chmod 2755 /usr/bin/program

# Sticky bit - Prevent deletion by others
chmod 1777 /tmp/shared-directory

Bulk Permission Management

# Web server permissions
chmod 644 *.html *.css *.js
chmod 755 *.cgi *.sh *.php

# Secure configuration files
chmod 600 /etc/ssl/private/*
chmod 644 /etc/ssl/certs/*

Permission Reference Tables

Octal Permission Chart

Common Permission Combinations

Step-by-Step Security Implementation

1. Web Server Security

# Secure web directory structure
chmod 755 /var/www/html                    # Directory accessible
chmod 644 /var/www/html/*.html             # Files readable
chmod 600 /var/www/html/config.php         # Config files private
chmod 755 /var/www/html/cgi-bin/*.cgi      # Scripts executable

2. SSH Key Security

# Secure SSH configuration
chmod 700 ~/.ssh                           # SSH directory private
chmod 600 ~/.ssh/id_rsa                    # Private key owner-only
chmod 644 ~/.ssh/id_rsa.pub               # Public key readable
chmod 600 ~/.ssh/authorized_keys           # Authorized keys secure

3. System File Security

# Secure system configurations
chmod 644 /etc/passwd                      # User database readable
chmod 600 /etc/shadow                      # Password hashes private
chmod 755 /etc/init.d/*                    # Init scripts executable
chmod 600 /etc/ssl/private/*               # SSL keys private

Symbolic Notation Reference

User Classes

• u = User (owner)
• g = Group
• o = Others
• a = All (user + group + others)

Operations

• + = Add permission
• - = Remove permission
• = = Set exact permission

Permissions

• r = Read (4)
• w = Write (2)
• x = Execute (1)

Symbolic Examples

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

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

# Set read-only for all
chmod a=r readonly.txt

# Multiple operations
chmod u+rwx,g+rx,o+r filename

Common Use Cases and Solutions

Making Scripts Executable

# Single script
chmod +x script.sh

# All scripts in directory
chmod +x *.sh

# Recursive for script directories
find /usr/local/bin -name "*.sh" -exec chmod +x {} \;

Securing Configuration Files

# Database configuration
chmod 600 /etc/mysql/my.cnf

# Web server configuration
chmod 644 /etc/apache2/apache2.conf

# Application secrets
chmod 600 /var/www/app/.env

Directory Permission Management

# Public directory (like web root)
chmod 755 /var/www/html

# User home directories
chmod 755 /home/username

# Shared directories with sticky bit
chmod 1777 /tmp/shared

Troubleshooting Permission Issues

Check Current Permissions

# Detailed file information
ls -l filename

# Octal permission display
stat -c "%a %n" filename

# Directory contents with permissions
ls -la /path/to/directory

Fix Common Permission Problems

# Fix "Permission denied" for scripts
chmod +x script.sh

# Fix web server 403 errors
chmod 755 /var/www/html
chmod 644 /var/www/html/index.html

# Fix SSH key permissions
chmod 600 ~/.ssh/id_rsa

Restore Default Permissions

# Reset home directory permissions
chmod 755 $HOME
chmod 644 $HOME/.*

# Reset web directory permissions
find /var/www/html -type d -exec chmod 755 {} \;
find /var/www/html -type f -exec chmod 644 {} \;

Security Best Practices

Essential Security Rules:

• Never use 777 permissions except for temporary debugging
• Apply principle of least privilege – minimum permissions needed
• Use 600 for private keys and sensitive configuration files
• Set 755 for directories that need to be browsable
• Regularly audit permissions with find commands

Permission Auditing:

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

# Find SUID/SGID files
find /usr -type f \( -perm -4000 -o -perm -2000 \)

# Find world-writable files
find /home -type f -perm -002

Related Linux Permission Commands

• chown – Change file ownership (user and group)
• chgrp – Change group ownership only
• umask – Set default permissions for new files
• ls -l – Display detailed file permissions
• stat – Show detailed file information including permissions
• getfacl/setfacl – Manage Access Control Lists (ACLs)
• sudo – Execute commands with elevated privileges

Mastering the chmod command provides the foundation for Linux security, enabling precise control over file access while maintaining system integrity and preventing unauthorized access to sensitive data.