3.5.2. Special bits

3.5.2.1. Setuid / Setgid

The 'x' (eXecute) bit for the User and the Group can be made setuid (SET User ID):

  -r-sr-xr-x  1 bob  users    12288 21 May 10:43 program

In the above example:

• Any user can run the program
• When the program runs, it will run with the privileges of user 'bob', since the access rights for User are r-s (setuid).

Other example:

  -r-sr-x---  1 root  wheel    12288 21 May 10:43 program

Here:

• Only users of the group 'wheel' and root itself can launch the program
• When it runs, it does with the privileges of the 'root' user

This makes it possible to restrict commands to a certain group of users.

This above is also true with the setgid (SET Group ID) bit:

  -r-xr-sr-x  1 bin  bin    12288 21 May 10:43 program

In this case:

• all users can run the program
• when the program runs, it does with the privileges of group 'bin'

3.5.2.2. The sticky bit

On a directory, it is possible to set what is called the 'sticky' bit:

  drwxrwxrwt  1 bin  staff      4 21 May 10:43 files/

This means that all users can create files in this directory, but only the owner of these files can modify / delete them. Example: /tmp