Chmod Cheat Sheet

Table of contents

1. chmod
2. 1. Understanding Permissions
   1. Permission Symbols and Numeric Codes
3. 2. Basic Syntax of chmod
4. 3. Using Symbolic Permissions
5. 4. Using Numeric (Octal) Permissions
6. 5. Recursive Permissions
7. 6. Examples of Common chmod Commands
8. 7. Special Permissions
   1. Setuid (Set User ID)
   2. Setgid (Set Group ID)
   3. Sticky Bit
9. Combined Special Permissions

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chmod

chmod is a command used in Unix-based systems (like Linux and macOS) to modify the access permissions of files and directories. Permissions define who can read, write, or execute a file. This guideline provides an overview of how to use chmod effectively.

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1. Understanding Permissions

Permissions on Unix-like systems are split into three categories:

• User (u): The owner of the file.
• Group (g): Users who are part of the file’s group.
• Others (o): All other users.

Each file or directory has three types of permissions:

• Read (r): View the contents of the file.
• Write (w): Modify or delete the contents.
• Execute (x): Run the file (if it’s an executable or script), or access a directory’s contents.

Permission Symbols and Numeric Codes

Permissions are represented by symbols or octal (numeric) codes:

Permission	Symbol	Octal
Read	r	4
Write	w	2
Execute	x	1
No Access	-	0

Each permission group (user, group, others) has an octal value. For example, rwx translates to 7 (4 + 2 + 1).

2. Basic Syntax of chmod

The chmod command follows this syntax:

chmod [options] mode file

• mode: Permissions to set (in symbolic or numeric format).
• file: The target file(s) or directory(ies).

Example:

chmod 755 filename

3. Using Symbolic Permissions

Symbolic permissions allow you to modify permissions by adding (+), removing (-), or setting (=) specific permissions for user categories.

Syntax:

chmod [user class][operator][permissions] filename

• User Classes:
  • u: User/Owner
  • g: Group
  • o: Others
  • a: All (user, group, and others)
• Operators:
  • +: Adds a permission
  • -: Removes a permission
  • =: Sets a permission

Examples:

chmod u+x filename     # Add execute permission for the user
chmod g-w filename      # Remove write permission for the group
chmod o=r filename      # Set read-only permission for others
chmod a+x filename      # Add execute permission for all
chmod u=rwx,g=rx,o=r filename # Set user to rwx, group to rx, others to r

4. Using Numeric (Octal) Permissions

Numeric (or octal) permissions use a three-digit number to represent permissions for user, group, and others.

Permission	Octal
—	0
–x	1
-w-	2
-wx	3
r–	4
r-x	5
rw-	6
rwx	7

The format is:

chmod XYZ filename

Where:

• X = User permission
• Y = Group permission
• Z = Others permission

Examples:

chmod 644 filename     # rw-r--r-- (User: rw, Group: r, Others: r)
chmod 755 filename     # rwxr-xr-x (User: rwx, Group: rx, Others: rx)
chmod 700 filename     # rwx------ (User: rwx, no access for group and others)

5. Recursive Permissions

Use the -R option to apply permissions recursively to all files and directories within a directory.

Example:

chmod -R 755 /path/to/directory

6. Examples of Common chmod Commands

Command	Description
chmod 644 file	Sets file to be read/write for user, read-only for others.
chmod 755 file	Allows everyone to read/execute, only user can write.
chmod u+x file	Adds execute permission for the user.
chmod g-w file	Removes write permission from the group.
chmod o=r file	Sets read-only permission for others.
chmod -R 700 dir	Gives full access to user, no access for others, recursively.

7. Special Permissions

Special permissions modify the default behavior of files and directories.

Setuid (Set User ID)

• Represented as 4 in front of regular permissions (e.g., 4755).
• When applied to executables, it allows users to run the executable with the owner’s privileges.

Example:

chmod 4755 file    # Sets the setuid bit on a file

Setgid (Set Group ID)

• Represented as 2 in front of regular permissions (e.g., 2755).
• For executables, it allows users to run the executable with the group’s privileges.
• For directories, new files created within inherit the directory’s group.

Example:

chmod 2755 directory    # Sets the setgid bit on a directory

Sticky Bit

• Represented as 1 in front of regular permissions (e.g., 1755).
• Applied to directories, it restricts file deletion within the directory: only the file’s owner, the directory owner, or root can delete files.

Example:

chmod 1755 directory    # Sets the sticky bit on a directory

Combined Special Permissions

Special permissions are combined by summing values (setuid=4, setgid=2, sticky=1).

Command	Meaning
chmod 4755 file	Setuid on executable
chmod 2755 dir	Setgid on directory
chmod 1755 dir	Sticky bit on directory
chmod 7755 dir	Setuid + Setgid

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This guide should help you understand and use chmod effectively to manage permissions on Unix-like systems. Proper use of chmod enhances both security and functionality.