Ch 9 : Linux File and Folder Permissions

Permissions for Linux file and folder 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.

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 Linux File and Folder Permissions?

Mastering Linux file and folder permissions ensures secure and efficient system management. Permissions control who can read, write, or execute files, critical for file access control in multi-user environments. This chapter builds on Chapters 3 (ls), 4 (touch), and 6 (redirection), teaching beginners to manage access confidently.

Understanding Permissions

Basics of File Access Control

Every file and directory has three permission types:

Read (r): View file contents or list directory contents.
Write (w): Modify files or add/remove directory contents.
Execute (x): Run files as programs or enter directories.

Permissions apply to three user categories:

Owner: File owner.
Group: Group owning the file.
Others: All other users.

Viewing Permissions

Using ls to Check Permissions

$ ls -l displays permissions:
$ ls -l
Output: -rw-r--r-- 1 user group 4096 Jun 5 05:11 example.txt drwxr-xr-x 2 user group 4096 Jun 5 05:11 my_folder

Interpretation:

File Type: - (file), d (directory).
Permissions: rw-r--r-- (owner: read/write, group/others: read).
Owner/Group: user/group.

Changing Permissions with chmod

Modifying Linux File and Folder Permissions

$ chmod adjusts permissions using symbolic or numeric modes.

Symbolic Mode

Use letters: u (owner), g (group), o (others), a (all), + (add), - (remove), = (set).

Examples:
$ chmod u+x example.txt
Output: Adds execute for owner (-rwxr--r--).
$ chmod o-w example.txt
Output: Removes write for others.
$ chmod ug=rw example.txt
Output: Sets read/write for owner/group (-rw-rw-r--).

Numeric Mode

Use numbers: 4 (read), 2 (write), 1 (execute), summed per category.

Examples:
$ chmod 744 example.txt
Output: Owner: read/write/execute, others: read (-rwxr--r--).
$ chmod 644 example.txt
Output: Owner: read/write, others: read (-rw-r--r--).

Changing Ownership with chown

Managing File Ownership

$ chown changes owner/group, often requiring sudo. Caution: Use sudo carefully to avoid system issues.

Changing Owner

$ sudo chown new_owner example.txt
Output: Sets new_owner as owner.

Changing Group

$ sudo chown :new_group example.txt
Output: Sets new_group as group.

Changing Owner and Group

$ sudo chown new_owner:new_group example.txt
Output: Updates both owner and group.

Special Permissions

Advanced File Access Control

Special permissions enhance security.

Setuid (s)

Execute file as owner:
$ sudo chmod u+s /usr/bin/passwd
Output: Adds setuid (-rwsr-xr-x).

Setgid (s)

Inherit directory group:
$ sudo chmod g+s /shared_directory
Output: Adds setgid (drwxr-sr-x).

Sticky Bit (t)

Restrict deletions:
$ sudo chmod +t /shared_directory
Output: Adds sticky bit (drwxr-xr-t).

Advanced Permission Management

Granular Control with ACLs and umask

Access Control Lists (ACLs) and umask offer precise file access control.

ACLs:
$ setfacl -m u:user:rwx file.txt
Output: Grants user read/write/execute on file.txt.

umask:
$ umask 022
Output: Sets default permissions (files: 644, directories: 755).

Note: Compared to chmod (standard permissions, see Chapter 3), setfacl provides granular ACLs, allowing specific user permissions for complex setups.

Installation Note: setfacl requires the acl package, which may not be installed by default in Debian 12. Install with: $ sudo apt install acl. See Debian APT.

Glossary of Commands and Tools

Reference: For detailed command documentation, visit Linux Manpages. For package installation, search on Debian APT.

Command/Tool	Description
ls	Lists directory contents with permissions.
chmod	Changes file/directory permissions.
chown	Changes file/directory owner and group.
setfacl	Sets Access Control Lists for granular permissions.
umask	Sets default permissions for new files/directories.
setuid	Executes file with owner’s permissions.
setgid	Inherits directory group for new files.
sticky bit	Restricts file deletions in directories.

Practice Linux File and Folder Permissions

Test your skills:

$ ls -l ~: View file permissions in home directory.
$ chmod u=rw,g=r,o=r example.txt or $ chmod 644 example.txt: Set rw-r--r--.
$ sudo chown new_user example.txt: Change owner (use sudo cautiously).
$ sudo chmod +t /tmp: Set sticky bit on a directory, test deletion restrictions.

Conclusion

You’ve mastered Linux file and folder permissions, from chmod to setfacl, ensuring robust file access control. Practice these to secure your system. Next, we’ll explore process management! (See Chapters 3–6 for exploration and redirection.)

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.