Set User Identification Attribute

The file permissions bits include an execute permission bit for file owner, group and other. When the execute bit for the owner is set to "s" the set user ID bit is set. This causes any persons or processes that run the file to have access to system resources as though they are the owner of the file. When the execute bit for the group is set to "s", the set group ID bit is set and the user running the program is given access based on access permission for the group the file belongs to. The following command:  

  chmod +s myfile 

sets the user ID bit on the file "myfile". The command: 

 chmod g+s myfile 

sets the group ID bit on the file "myfile".

The listing below shows a listing of two files that have the group or user ID bit set. 

-rws--x--x   1 root    root    14024 Sep  9 1999 chfn
-rwxr-sr-x   1 root   mail    12072 Aug 16 1999 lockfile

The files chfn and lockfile are located in the directory "/usr/bin".  
The "s" takes the place of the normal location of the execute bit in the
 file listings above.  This special permission mode has no meaning 
unless the file has execute permission set for either the group or other
 as well.   This means that in the case of the lockfile, if the other 
users (world execute) bit is not set with permission to execute, then 
the user ID bit set would be meaningless since only that same group 
could run the program anyhow.  In both files, everyone can execute the 
binary.  The first program, when run is executed as though the program 
is the root user.  The second program is run as though the group "mail" 
is the user's group.



For system security reasons it is not a good idea to set many program's 
set user or group ID bits any more than necessary, since this can allow 
an unauthorized user privileges in sensitive system areas.  If the 
program has a flaw that allows the user to break out of the intended use
 of the program, then the system can be compromised.