The mode string

As mentioned earlier, the user, group, and other permissions are often spelled out in symbolic mode descriptions such as rwxr-xr-x. This notation is found in the output of the ls -l and stat commands. As you can see in the access mode for mount, this scheme is modified slightly in the presence of special permissions. Instead of adding three more bits to the left of rwxr-xr-x, the SUID permission is indicated in the string by changing the user execute position from x to s. SGID permission is handled the same way. The sticky permission is indicated by replacing x in the other execute position with T. For example, an executable program with mode 6755 would have the following equivalent symbolic mode:

rwsr-sr-x

A directory with mode 1774 would have this equivalent string:

rwxrwxr-T

While this layering of special permissions may appear to obscure the underlying execute permissions, it makes sense. The special permissions are relatively rare in the filesystem, so depicting the three extra bits would waste space on your terminal or terminal window. When the executable bits are set, the setuid and setgid bits are represented with s. When the executable bits are not set, the setuid and setgid bits are represented with S. Similarly, the sticky bit is represented with either t or T.

Get LPI Linux Certification in a Nutshell, 3rd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.