Understanding File Permissions (chmod, chown)

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

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!