Linux Fundamentals

In Ubuntu (and Linux in general), file permissions are represented by three characters: r (read), w (write), and x (execute). These permissions are assigned to three different categories of users: the owner of the file, the group associated with the file, and others (everyone else).

Each of these characters has a numeric equivalent, which is used when setting permissions via the chmod command.

Here are the values:

Permission Characters and Their Values

Permission	Symbol	Numeric Value
Read	r	4
Write	w	2
Execute	x	1
No Permission	-	0

Understanding the Numeric Representation

The permissions are represented by a 3-digit number (for user, group, and others). The number is a sum of the individual permissions, where:

• Owner: First digit (User permissions)

• Group: Second digit (Group permissions)

• Others: Third digit (Other users' permissions)

Common Permission Sets

Here are some common examples of permissions with their numeric equivalents:

Permission	Symbol	Numeric Value
rwxrwxrwx	777	Full permissions for everyone (owner, group, others)
rwxr-xr-x	755	Owner can read/write/execute; group/others can read/execute
rw-r--r--	644	Owner can read/write; group/others can read
r--------	400	Owner can read only
rwx------	700	Owner can read/write/execute; group/others have no permissions

Get the permissions back as in human readable format with :

stat -c "%a" <filename>