Chmod Permissions Explained: The Complete Linux Guide

What Is chmod?

chmod (change mode) is a Unix and Linux command that controls who can read, write, and execute files and directories. Every file on a Linux system has three sets of permissions for three types of users:

Each user type can have three permission types: read (r) to view file contents, write (w) to modify or delete, and execute (x) to run as a program or enter a directory. Understanding chmod is essential for every Linux system administrator, DevOps engineer, and web developer. Use our interactive Chmod Calculator to visualize permissions as you learn.

Understanding the Permission Number System

The chmod number system assigns a value to each permission type. These values are added together to create a single digit for each user role:

Add the values together to get each digit: read (4) + write (2) + execute (1) = 7. Three digits represent owner, group, and others — so 755 means owner=7 (rwx), group=5 (r-x), others=5 (r-x). This math is much easier with our Chmod Calculator — just toggle checkboxes and see the result.

The octal number system (base 8) is used because 3 permission bits map perfectly to one octal digit. If you want to understand how octal relates to binary and decimal, see our Number Base Converter or the octal to decimal conversion page.

Common Permission Values Explained

Symbolic vs Numeric Notation

There are two ways to use the chmod command. Numeric mode sets all permissions at once with three digits. Symbolic mode uses letters and operators to add or remove specific permissions:

# Numeric mode — set all permissions at once
chmod 755 script.sh          # rwxr-xr-x
chmod 644 index.html         # rw-r--r--
chmod 600 .env               # rw-------

# Symbolic mode — add/remove specific permissions
chmod +x script.sh           # Add execute for everyone
chmod u+x script.sh          # Add execute for owner only
chmod g-w file.txt           # Remove write from group
chmod o-rwx private.key      # Remove all permissions from others
chmod u=rwx,g=rx,o=rx dir/   # Set exact permissions (same as 755)

# Recursive — apply to directory and all contents
chmod -R 755 ./public/       # Set 755 for entire directory tree

When to use which: Numeric mode is faster when you know the exact permissions. Symbolic mode is safer for making small changes (like adding execute) without affecting other permissions. Our Chmod Calculator shows both formats simultaneously.

chmod Examples for Common Scenarios

# Web server files
chmod 644 *.html *.css *.js    # Static files: owner read/write, everyone read
chmod 755 /var/www/html/       # Web root directory

# Scripts and programs
chmod 755 deploy.sh            # Deployment script
chmod 755 /usr/local/bin/app   # Installed binary

# Security-sensitive files
chmod 600 ~/.ssh/id_rsa        # SSH private key (REQUIRED by SSH)
chmod 600 .env                 # Environment variables with secrets
chmod 600 /etc/shadow          # Password hashes

# Shared project directory
chmod 775 /project/shared/     # Owner + group: full access
chmod 664 /project/shared/*    # Files: owner + group read/write

# Docker volumes
chmod -R 755 ./docker-data/    # Container needs read/execute access

# CI/CD pipeline
chmod +x ./scripts/*.sh        # Make all shell scripts executable

For automated permission changes on a schedule, combine chmod with our Cron Expression Generator to create scheduled tasks. Verify file integrity after permission changes using our Hash Generator to compute checksums.

Security Best Practices

Protect your credentials with strong passwords generated by our Password Generator — use 20+ characters with all character types for maximum security.

Special Permissions (SUID, SGID, Sticky Bit)

Beyond the basic rwx permissions, Linux has three special permission bits that modify execution behavior:

# Set SUID (runs as owner)
chmod 4755 /usr/local/bin/app    # 4 = SUID + 755

# Set SGID on directory (new files inherit group)
chmod 2775 /project/shared/      # 2 = SGID + 775

# Set Sticky Bit (only owners can delete)
chmod 1777 /tmp/                 # 1 = Sticky + 777

Security warning: SUID binaries are a common attack vector. Only set SUID on programs that absolutely need it. Audit SUID files with find / -perm -4000 -ls.

Frequently Asked Questions

Related Tools & Articles