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Introduction

Understanding the Linux file system hierarchy and permission model is crucial for system administration and security. Every file and directory has an owner, a group, and a set of permissions that control access.

Directory Structure

/           # Root directory
/bin        # Essential binaries
/etc        # Configuration files
/home       # User home directories
/var        # Variable data (logs, cache)
/usr        # User programs
/tmp        # Temporary files
/opt        # Optional software

File Permissions

Every file has three permission sets: owner, group, and others. Each set has read (r), write (w), and execute (x) permissions.

# View permissions
ls -la
# -rwxr-xr-- 1 user group 4096 Jun 12 file.txt

# Change permissions (numeric)
chmod 755 script.sh    # rwxr-xr-x
chmod 644 config.txt   # rw-r--r--

# Change permissions (symbolic)
chmod u+x script.sh    # Add execute for owner
chmod g-w file.txt     # Remove write for group

Ownership

# Change owner
chown user:group file.txt

# Change ownership recursively
chown -R www-data:www-data /var/www/

# Change group
chgrp developers project/

Special Permissions

# Setuid - run as file owner
chmod u+s /usr/bin/program

# Setgid - inherit group
chmod g+s /shared/dir

# Sticky bit - only owner can delete
chmod +t /tmp

Summary

Linux permissions control access to files and directories. Understanding rwx notation, numeric codes, and special permissions is essential for securing your system.