Advance File permissions

Advance File permissions

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 Sticky bit: It was used to trigger process to "stick" in memory after it is finished, now this usage is obsolete.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 dependent 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.

Numeric representation
Octal digit Binary value Meaning
0 000 setuid, setgid, sticky bits are cleared
1 001 sticky bit is set
2 010 setgid bit is set
3 011 setgid and sticky bits are set
4 100 setuid bit is set
5 101 setuid and sticky bits are set
6 110 setuid and setgid bits are set
7 111 setuid, setgid, sticky bits are set
Textual representation
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
Setting the sticky bit on a directory : chmod +t
Setting the SGID attribute on a directory : chmod g+s
If the SGID (Set Group Identification) attribute is set on a directory, files created in that directory inherit its group ownership. If the SGID is not set the file's group ownership corresponds to the user's default group.
In order to set the SGID on a directory or to remove it, use the following commands:
# chmod g+s directory
# chmod g-s directory
When set, the SGID attribute is represented by the letter "s" which replaces the "x" in the group permissions:
# ls -l directory
drwxrwsr-x 10 raju student 4096 2012-12-05 12:50 directory
Setting SUID and SGID attributes on executable files : chmod u+s, chmod g+s
By default, when a user executes a file, the process which results in this execution has the same permissions as those of the user. In fact, the process inherits his default group and user identification.
If you set the SUID attribute on an executable file, the process resulting in its execution doesn't use the user's identification but the user identification of the file owner.
By setting SUID and SGID attributes the owner makes it possible for other users to execute the file as if they were him or members of his default group.
The SUID and GUID are represented by a "s" which replaces the "x" character respectively in the user and group permissions:

# chmod u+s directory
# ls -l
drwxrwsr-x 10 raju student 4096 2012-12-05 12:50 directory

# chmod u-s directory

# chmod g+s directory
# ls -l
-rwxrwsrwx 10 raju student 4096 2012-12-05 12:50 directory