File and Directory Permissions (chmod, chown)

In Ubuntu (and Linux in general), file and directory permissions are crucial for security and access control. They define who can read, write, or execute a file or directory. The commands chmod and chown are used to change these permissions and ownership, respectively.

File and Directory Permissions

Each file and directory has three types of permissions for three categories of users:

• User (u): The owner of the file.
• Group (g): Other users who are in the same group as the owner.
• Others (o): All other users.

The permissions are represented as:

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

Permissions can be represented either symbolically (e.g., rwx) or numerically (e.g., 7).

Example 1: Viewing Permissions

ls -l

Output:

-rwxr-xr-- 1 user group 4096 Aug 10 10:00 myfile.txt

• -rwxr-xr-- shows the permissions:
  • rwx (user): The owner can read, write, and execute.
  • r-x (group): Group members can read and execute.
  • r-- (others): Others can only read.

chmod Command: Changing Permissions

You can change permissions using the chmod command.

Example 2: Changing Permissions Symbolically

chmod u+x myfile.txt

• Adds execute (x) permission for the user (owner).

Example 3: Changing Permissions Numerically

chmod 755 myfile.txt

• Sets permissions to:
  • 7 (rwx) for the user
  • 5 (r-x) for the group
  • 5 (r-x) for others

chown Command: Changing Ownership

The chown command changes the owner and group of a file or directory.

Example 4: Changing the Owner

sudo chown newuser myfile.txt

• Changes the owner of myfile.txt to newuser.

Example 5: Changing Owner and Group

sudo chown newuser:newgroup myfile.txt

• Changes the owner to newuser and the group to newgroup.

Example 6: Changing Ownership Recursively

sudo chown -R newuser:newgroup /mydirectory

• Changes the owner and group for all files and subdirectories within /mydirectory.

Summary

• Permissions determine what actions can be performed on a file/directory (read, write, execute).
• chmod modifies these permissions.
• chown changes the owner and group.

These tools are essential for managing access to files and directories in Ubuntu.