Archive for September, 2006

Set user ID, set group ID, sticky bit

Posted on September 24, 2006. Filed under: Linux |

In addition to the basic permissions discussed above, there are also three bits of information defined for files in Linux:

  • SUID or setuid: change user ID on execution. If setuid bit is set, when the file will be executed by a user, the process will have the same rights as the owner of the file being executed.
  • SGID or setgid: change group ID on execution. Same as above, but inherits rights of the group of the owner of the file on execution. For directories it also may mean that when a new file is created in the directory it will inherit the group of the directory (and not of the user who created the file).
  • Sticky bit. It was used to trigger process to “stick” in memory after it is finished, now this usage is obsolete. Currently its use is system dependant and it is mostly used to suppress deletion of the files that belong to other users in the folder where you have “write” access to.
SUID If set, then replaces “x” in the owner permissions to “s”, if owner has execute permissions, or to “S” otherwise. Examples:
-rws------ both owner execute and SUID are set
-r-S------ SUID is set, but owner execute is not set
SGID If set, then replaces “x” in the group permissions to “s”, if group has execute permissions, or to “S” otherwise. Examples:
-rwxrws--- both group execute and SGID are set
-rwxr-S--- SGID is set, but group execute is not set
Sticky If set, then replaces “x” in the others permissions to “t”, if others have execute permissions, or to “T” otherwise. Examples:
-rwxrwxrwt both others execute and sticky bit are set
-rwxrwxr-T sticky bit is set, but others execute is not set

Reference:

http://www.zzee.com/solutions/linux-permissions.shtml

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