Set user ID, set group ID, sticky bit

Source taken from: http://wiki.e107.org/?title=Unix_Permissions

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

• 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. For directories it also may mean that when a new file is created in the directory it
• 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 It was used to trigger process to "stick" in memory after it is finished, now this usage is obsolete.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.

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