Permissions Cheat Sheet

Understanding user management and file permissions is critical for security.

User and Group Management

Command	Description
useradd -m -s /bin/bash john	Create user 'john' with a home directory and bash shell.
usermod -aG sudo john	Add 'john' to the 'sudo' group.
passwd john	Change password for 'john'.
groupadd admins	Create a new group.

Octal Permissions Reference

• 4 = Read (r)
• 2 = Write (w)
• 1 = Execute (x)
• 0 = No permission (-)

Combine them:

• 7 = rwx
• 6 = rw-
• 5 = r-x

Chmod Examples

Command	Description
chmod 777 file	Everyone can read, write, and execute (Dangerous!).
chmod 755 dir	Owner has full access, others can read/execute. Standard for directories.
chmod 644 file	Owner can read/write, others can read. Standard for files.
chmod 600 file	Only owner can read/write. Standard for SSH keys.

Chown Examples

# Change owner to john
chown john file.txt

# Change owner and group
chown john:admins file.txt

# Recursively change owner of a directory
chown -R www-data:www-data /var/www/html