Controlling File and Directory Permissions
53
operating system, permissions
are represented in binary, so ON and OFF
switches are represented by 1 and 0, respectively. You can think of the
rwx
permissions as three ON/OFF switches, so when all permissions are
granted, this equates to 111 in binary.
A binary set like this is then easily represented
as one digit by convert-
ing it into
octal, an eight-digit number system that starts with 0 and ends
with 7. An octal digit represents a set of three binary digits, meaning we
can
represent an entire
rwx
set with one digit. Table 5-1 contains all possible
permission combinations and their octal and binary representatives.
Table 5-1:
Octal and Binary
Representations of Permissions
Binary
Octal
rwx
000
0
---
001
1
--x
010
2
-w-
011
3
-wx
100
4
r--
101
5
r-x
110
6
rw-
111
7
rwx
Using this information, let’s go through some examples. First, if we
want to
set only the read permission, we could consult Table 5-1 and locate
the value for read:
r w x
4 - -
Next, if we want to set the permission to
wx
, we could use the same
methodology and look for
what sets the
w
and what sets the
x
:
r w x
- 2 1
Notice in Table 5-1 that the octal representation for
-wx
is 3, which not
so coincidently happens to be the same value we get when we add the two
values for setting
w
and
x
individually: 2 + 1 = 3.
Finally, when
all three permissions are on, it looks like this:
r w x
4 2 1
And 4 + 2 + 1 = 7. Here, we see that in Linux, when all the permission
switches are on, they are represented by the octal equivalent of 7.
54
Chapter 5
So, if we wanted to represent all permissions for the owner, group, and
all users, we could write it as follows:
7 7 7
Here’s where the shortcut comes in. By passing
chmod
three octal digits
(one for each
rwx
set),
followed by a filename, we can change permissions on
that file for each type of user. Enter the following into your command line:
kali >
