File Permissions Overview

In Linux, every file and directory has a set of permissions associated with three types of users:

• Owner: The user who owns the file.
• Group: A set of users who share access.
• Others: Everyone else.

Understanding Permission Types

Each permission is represented by a character in a sequence:

• r (read): Permission to read the file or list directory contents.
• w (write): Permission to modify the file or directory contents.
• x (execute): Permission to run the file as a program or to access a directory.

Reading Permissions

You can view the permissions of a file or directory using the ls -l command:

ls -l filename

This command will output something like:

-rwxr-xr--

This string represents the permissions:

• The first character (-) indicates the type of file (e.g., - for a file, d for a directory).
• The next three characters (rwx) are the owner's permissions.
• The next three (r-x) are the group's permissions.
• The last three (r--) are the others' permissions.

Setting Permissions

You can change file and directory permissions using the chmod command.

Using Symbolic Mode

You can adjust permissions by adding or removing permissions:

chmod u+x filename  # Adds execute permission for the user
chmod g-w filename  # Removes write permission for the group
chmod o+r filename  # Adds read permission for others

Using Numeric Mode

Permissions can also be set using a numeric mode, where:

4 = read (r) 2 = write (w) 1 = execute (x)

For example, to set read, write, and execute permissions for the owner and read and execute for others:

chmod 755 filename

This sets:

7 for the owner (rwx) 5 for the group (r-x) 5 for others (r-x)

Changing Ownership and Groups

Changing the Owner

To change the owner of a file or directory, use the chown command:

sudo chown newowner filename

Changing the Group

To change the group associated with a file or directory, use:

sudo chown :newgroup filename

Or you can change both the owner and group simultaneously:

sudo chown newowner:newgroup filename