Chapter 9: File and Folder Permissions

In this chapter, we’ll cover: 
- Understanding file permissions and ownership. 
- Viewing permissions with ls -l. 
- Changing permissions with chmod. 
- Changing ownership with chown. 
- Special permissions like setuid, setgid, and the sticky bit.

Why Learn About Permissions?

Permissions control who can read, write, and execute files and directories. Whether you’re protecting sensitive data, sharing files with others, or running scripts, understanding permissions is essential for maintaining security and functionality.

1. Understanding Permissions

Every file and directory in Linux has three types of permissions: 
1. Read (r): Allows viewing the contents of a file or listing the contents of a directory. 2. Write (w): Allows modifying a file or adding/removing files in a directory. 
3. Execute (x): Allows running a file as a program or entering a directory.

These permissions are assigned to three categories of users: 
1. Owner: The user who owns the file. 
2. Group: The group that owns the file.
 3. Others: Everyone else.

2. Viewing Permissions

Use the ls -l command to view permissions:

$ ls -l
-rw-r--r-- 1 user group  4096 Oct  4 12:34 example.txt
drwxr-xr-x 2 user group  4096 Oct  4 12:34 my_folder

Here’s how to interpret the output: 
- File Type: The first character (- for files, d for directories). 
- Permissions: The next 9 characters (e.g., rw-r--r--). 
- The first 3 characters: Owner permissions. 
 The next 3 characters: Group permissions. 
- The last 3 characters: Others permissions. 
- Owner: The user who owns the file (e.g., user). 
- Group: The group that owns the file (e.g., group).

3. Changing Permissions with chmod

The chmod command is used to change file permissions. You can use either symbolic or numericmode.

Symbolic Mode

Symbolic mode uses letters to represent users and permissions: 
- u: Owner 
- g: Group 
- o: Others 
- a: All (owner, group, and others) 
- +: Add a permission - -: Remove a permission 
- =: Set permissions explicitly

Examples: - Add execute permission for the owner:
$ chmod u+x example.txt 
- Remove write permission for others: 
$ chmod o-w example.txt
- Set read and write permissions for the owner and group: 
$ chmod ug=rw example.txt

Numeric Mode

Numeric mode uses a three-digit number to represent permissions: - 4: Read (r) - 2: Write (w) - 1: Execute (x)

Add the numbers to set permissions: - 7: Read + Write + Execute (4 + 2 + 1) - 6: Read + Write (4 + 2) - 5: Read + Execute (4 + 1) - 4: Read - 0: No permissions

Examples: - Set read, write, and execute permissions for the owner, and read-only for others:
$ chmod 744 example.txt 
- Set read and write for the owner, and read-only for the group and others:  
$ chmod 644 example.txt

4. Changing Ownership with chown

The chown command is used to change the owner and group of a file or directory.

Changing Owner

$ sudo chown new_owner example.txt

Changing Group

$ sudo chown :new_group example.txt

Changing Owner and Group

$ sudo chown new_owner:new_group example.txt

5. Special Permissions

Linux also has special permissions for advanced use cases:

Setuid (s)

The setuid bit allows a file to be executed with the permissions of its owner, rather than the user running it. This is often used for system programs like passwd.

$ sudo chmod u+s /usr/bin/passwd

Setgid (s)

The setgid bit ensures that files created in a directory inherit the group ownership of the directory, rather than the user’s default group.

$ sudo chmod g+s /shared_directory

Sticky Bit (t)

The sticky bit restricts file deletion in a directory. Only the file owner, directory owner, or root can delete files.

$ sudo chmod +t /shared_directory

Extra: Advanced Permission Management

Advanced permission management techniques like Access Control Lists (ACLs) and umask settings provide more granular control over file security. ACLs allow you to set permissions for specific users or groups beyond the standard owner-group-others model, while umask dictates the default permissions for newly created files and directories. 

By mastering these techniques, you can ensure proper access control in multi-user environments and establish sensible defaults for your system.

Set fine-grained permissions with ACL:

# Example usage:
$ setfacl -m u:user:rwx file.txt

Set default file permissions with umask:

$ umask 022

Practice Time!

Let’s put your new skills to the test: 
1. Use ls -l to view the permissions of files in your home directory. 
2. Change the permissions of a file to rw-r--r-- using both symbolic and numeric modes. 
3. Change the owner of a file to another user (requires sudo). 
4. Set the sticky bit on a directory and test its behavior.

That’s it for Chapter 9! You’ve now learned how to manage file permissions and ownership in Linux. In the next chapter, we’ll dive into processes—understanding and managing running programs. Until then, practice working with permissions to secure and organize your files.

Prev: Chapter 8 | Next: Chapter 10