Representation of file permissions

There are two notation schemes for file permission: symbolical or numerical. Different tools use either one, so it is necessary to understand both.

Symbolical representation

The symbolic representation lists the given access permissions in the order read, write, execute for each user class in the order user, group, other. A granted permission is indicated by its symbol a denied permission by a -. The special permissions are mixed within the symbolical representation by replacing the x-position in each access class by s or S (for set-uid and set-gid) and t or T (for sticky mode) .

This will be clear after a few examples:

permissions	numerical	user	group	other	special
rwxrw----	0750	read, write, execute	read, write,	none	none
r--r--r--	0444	read	read	read	none
rwxrwxrwx	0777	read, write, execute	read, write, execute	read, write, execute	none
r-x---r-x	0505	read, execute	none	read, execute	none
rwsr-s---	3750	read, write, execute	read, execute	none	set-uid, set-gid
r-Sr-sr-T	7455	read	read, execute	read	set-uid, sticky

Numerical representation

To obtain the numerical representation, the permissions are written as a bit field of length 12, divided in sections of three bits:

       Permissions
 ┌─┬─┬─┐ ┌─┬─┬─┐┌─┬─┬─┐┌─┬─┬─┐
 │u│g│t│ │r│w│x││r│w│x││r│w│x│
 └─┴─┴─┘ └─┴─┴─┘└─┴─┴─┘└─┴─┴─┘
 Special   User  Group  Other

A group of three bits may represent \(2^3=8\) different values. From the binary representation follows that r, w, x can be mapped to the numeric values \(2^2=4\), \(2^1=2\) and \(2^0=1\). All possibilities are shown in the table below. The same holds true for the special permissions.

permissions	binary	octal
---	000	0
--x	001	1
-w-	010	2
-wx	011	3
r--	100	4
r-x	101	5
rw-	110	6
rwx	111	7

To obtain the numerical representation the octal values of the special, user, group and other section are concatenated.

Note

If no special permissions are used, the leading 0 in the numerical representation can be omitted. So for example 0755 and 755 both represent rwxrw-rw- with no special permissions. The other sections cannot be omitted.