Understanding File Permissions (chmod, chown)

Welcome! In Linux, file permissions are security settings that control who can access and modify files and directories. They are crucial for protecting your data and ensuring system stability.

Think of them as digital bouncers for your files, deciding who gets in and what they can do once inside.

There are three basic types of permissions:

• Read (r): Allows viewing the content of a file or listing the contents of a directory.
• Write (w): Allows modifying or deleting a file, or creating/deleting files within a directory.
• Execute (x): Allows running a file (if it's a script or program) or entering a directory.

These permissions are applied to three categories of users:

• User (u): The owner of the file or directory.
• Group (g): Users who belong to the file's primary group.
• Others (o): All other users on the system.
• Sometimes you'll see 'a' for 'all' (user, group, and others).

You can see a file's permissions using the ls -l command. Let's break down its output:

-rwxr-xr-- 1 user group 1024 Jan 1 10:00 myfile.txt

• The first character (-) indicates the file type (- for regular file, d for directory).
• The next nine characters (rwxr-xr--) represent the permissions.

The nine permission characters are grouped into three sets of three:

• 1st set (rwx): Permissions for the User (owner).
• 2nd set (r-x): Permissions for the Group.
• 3rd set (r--): Permissions for Others.

A hyphen (-) means that permission is not granted for that category.

The chmod command (change mode) is used to modify file and directory permissions. There are two main ways to use chmod:

1. Symbolic mode: Uses letters and symbols (e.g., u+x).
2. Octal (numeric) mode: Uses numbers (e.g., 755).

Symbolic mode is intuitive. You specify the category (u, g, o, a), the action (+ to add, - to remove, = to set exactly), and the permission (r, w, x).

• chmod u+x myscript.sh: Adds execute permission for the owner.
• chmod g-w myfile.txt: Removes write permission for the group.
• chmod o=r-x mydir: Sets read-only for others, removing write.

Octal mode uses numbers to represent permissions. Each permission type has a value:

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

You sum these values for each category (user, group, others). For example, rwx is 4+2+1=7, and r-x is 4+0+1=5.

You provide a three-digit number to chmod, representing user, group, and others permissions respectively.

• chmod 755 mydir: User gets rwx (7), Group gets r-x (5), Others get r-x (5). Common for directories.
• chmod 644 myfile.txt: User gets rw- (6), Group gets r-- (4), Others get r-- (4). Common for files.

The chown command (change owner) changes the user owner of a file or directory. This command usually requires superuser (root) privileges.

chown newuser myfile.txt

You can also change the group owner at the same time:

chown newuser:newgroup myfile.txt

You have a file named private_note.txt. You want to set its permissions so that:

• The owner can read and write.
• The group can only read.
• Others have no permissions at all.

Which of the following commands would achieve this?

Great job! You've mastered the fundamentals of Linux file permissions:

• Understood read, write, execute permissions.
• Learned about user, group, and others categories.
• Used ls -l to view permissions.
• Applied chmod in both symbolic and octal modes to change permissions.
• Discovered chown to change file ownership.

Protecting your files is a key skill in Linux. Next, you'll learn how to search for files and patterns!