permissions.md

Linux Permissions (Level 1) 🔒

Understanding Permissions (ls -l)

Format: drwxr-xr-x

• d — Directory (or - for file).
• rwx (User/Owner) — Read, Write, Execute.
• r-x (Group) — Read, Execute.
• r-x (Others) — Read, Execute.

Numeric Mode (Octal)

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

Common Examples:

• 777 (rwxrwxrwx) — Full access for everyone (Dangerous!).
• 755 (rwxr-xr-x) — Owner can do everything, others can read/execute (Standard for scripts/directories).
• 644 (rw-r--r--) — Owner can read/write, others can read (Standard for files).
• 600 (rw-------) — Owner read/write only (Private keys/passwords).

Commands

• chmod <mode> <file> — Change file mode bits.
  • chmod 755 script.sh
  • chmod +x script.sh (Add execute permission).
  • chmod u+w file.txt (Add write permission for user).
• chown <user>:<group> <file> — Change file owner and group.
  • chown root:root /etc/file
  • chown -R user:user /var/www (Recursive).
• chgrp <group> <file> — Change group ownership.

Special Permissions & Umask

• umask — Sets default permissions for new files/directories (e.g., 022 results in 755 dirs and 644 files).
• sudo — Run command as superuser (root).