Linux File Permissions

2.1 Key Concepts

Every file and directory in Linux has associated permissions that determine how they can be accessed. These permissions are represented as three sets of attributes:

• Owner (User)
• Group
• Others

2.2 Permission Types

Each of the above sets has three types of permissions:

• Read (r): Permission to read the file or list the directory.
• Write (w): Permission to modify the file or directory.
• Execute (x): Permission to execute the file (if it’s a script or binary) or access the directory.

These permissions can be viewed using the command:

ls -l filename

The output will show permissions in the format: -rwxr-xr--, where:

• - indicates it's a file; d indicates a directory.
• Next three characters are for the owner.
• Next three for the group.
• Last three for others.

3.1 Symbolic Method

The symbolic method uses letters to represent users and permissions:

To add a permission:

chmod u+x filename

To remove a permission:

chmod g-w filename

3.2 Numeric Method

In the numeric method, permissions are represented by numbers:

• Read = 4
• Write = 2
• Execute = 1

Thus, to set permissions:

chmod 755 filename

This means: Owner can read, write and execute; Group and Others can read and execute.