UNIX Articles : Access Rights and File Mode

The potential users of a file fall into three classes (syn1988@sdf.lonestar.org)

• The user who is the owner of the file
• The users who belong to the same group as the file, not including the owner
• All remaining users (others)

There are three types of access rights - Read, Write, and Execute - for each of these three classes. Thus, the set of access rights associated with a file consists of nine different binary flags, called "suid" --> Set User ID, "sgid" --> Set Group ID, and sticky, define the file mode. These flags have the following meanings when applied to executable files :

- suid

A process executing a file normally keeps the user ID (UID) of the process owner. However, if the executable file has the suid flag set, the
process gets the UID of the file owner.

A process executing a file keeps the group ID (GID) of the process group. However, if the executable file has the sgid flag set, the process 
gets the ID of the file group.

An executable file with sticky flag set corresponds to a request to a kernel to keep the program in memory after it's execution terminates.
Note, that this flag become obsolete; other approaches based on sharing of code pages are now used.