Access Control & OS: Unix 's File System
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Unix File Hierarchy. The Unix file system is organized
as a hierarchy with the root (/) directory at the highest level. Each
directory may contain subdirectories and files. Typically, some of
the directories that may occur under the root are usr, bin, sbin, home,
var, boot, dev, etc. In Figure 3, user1 and user2 are sub-directories
under home. hello.txt is a plain-text file and link_hello is a linking
file that points to hello.txt. In order to access the file /test/temp/hello.txt,
the system begins its search from the root(/) folder and then to test
and temp folders consecutively and then finally the file hello.txt.
Figure 3
Ownership and Permissions.
Ownership of files in UNIX can be viewed in one of three ways: owner
(creator), group or others. Using this simple notion of ownership access
to files can be controlled by associating unique user ID (UID) and group
ID (GID) with twelve permission bits for each file as shown below.
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Permission Bits |
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Extra |
Owner |
Group |
Others |
| su |
sg |
t |
r |
w |
x |
r |
w |
x |
r |
w |
x |
Typically these bits are divided into
three sets of three bits and three extra bits as shown in table below.
r, w and x bits stand for read, write and execute bits for each of the
owner, group and others permissions. su, sg and t stand for set_user_id,
set_group_id and sticky bits. These 4 sets of bits are often represented
in their octal digits. For example, “100 111 101 101” is represented as
“4755.” When the su bit is set, whosoever executes the file, the UID of
the process will be the owner of the file. Similarly, if sg is set, the
GID of the process will be the owner of the file.
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