chmod Command Cheatsheet

The chmod command changes file and directory permissions in Unix/Linux.

🧑 User Classes

🔐 Permission Types

📝 Symbolic Mode

Use +, -, or = to modify permissions:

chmod u+x file      # Add execute to user
chmod go-w file     # Remove write from group and others
chmod a=r file      # Set read-only for all

🔢 Numeric (Octal) Mode

Combine values for user, group, and others:

chmod 755 script.sh
chmod 644 notes.txt

🧪 Special Permission Flags

These flags go beyond basic read/write/execute and control execution context and deletion behavior.

🔸 setuid (Set User ID on Execution)

• Octal prefix: 4
• Applies to: Executable files
• Effect: When executed, the process runs with the file owner’s privileges (not the user’s).
• Use case: Programs like passwd need elevated privileges to modify system files.

chmod 4755 /usr/bin/somebinary
# -rwsr-xr-x → 's' replaces 'x' in user field

⚠️ Security risk if misused — avoid setting on scripts or user-controlled binaries.

🔸 setgid (Set Group ID)

• Octal prefix: 2
• Applies to:
  • Files: Runs with the file group’s privileges.
  • Directories: New files inherit the directory’s group.
• Use case: Shared project folders where all files should belong to the same group.

chmod 2755 /shared/folder
# drwxr-sr-x → 's' in group field

📁 On directories, this ensures group consistency for collaborative environments.

🔸 sticky Bit

• Octal prefix: 1
• Applies to: Directories only
• Effect: Only the file owner (or root) can delete or rename files inside.
• Use case: /tmp directory — prevents users from deleting each other’s files.

chmod 1777 /tmp
# drwxrwxrwt → 't' in others field

🧷 Think of it as a “delete lock” for shared spaces.

📂 Execute (x) Flag: Files vs Directories

🔍 Directory Access Matrix

🧠 Without x on a directory, you can’t cd into it — even if you can list its contents.

🔍 Quick Reference

💡 Tip: Use ls -l to view current permissions.