The mode bits
The special, user, group, and other permissions can be represented in a string of 12 binary bits, as shown in Figure 7-2.
It is common to refer to these bits in four sets of three, translated into four octal (base-8) digits. The first octal digit represents the special permissions SUID, SGID, and sticky. The other three represent the read, write, and execute permissions, respectively, in each of the user, group, and other user classes. Octal notation is used as shorthand for binary strings such as the access mode, and each group of three bits has 23 = 8 possible values, listed in Table 7-3.
The read permission by itself is
r--, which can be thought of as binary
100, or octal 4. Adding the write permission yields
rw-, or binary 110, which is octal 6.
Figure 7-2 shows
how to total bit values into the octal equivalents. Memorizing, or
even writing, the binary-to-octal equivalents may be easier on the
exam than adding bit values. Use the technique that works best for
To turn the mode bits
110111101001 into an octal representation,
first separate them into chunks of three bits:
110, 111, 101, and
001. The first group, representing the
special permissions, is
110. This can be thought of as 4 + 2 + 0 = 6. The second group, representing ...