Three Special Permission in Linux - SUID, SGID & Sticky Bit

Three Special Permission in Linux - SUID, SGID & Sticky Bit.

Three special permission in linux are SUID, SGID and Sticky Bit which is use to control the linux user in advance.

What is SUID?

The common explanation given for SUID is, it is an advance file permission SUID allows an user to execute a script as if the owner of the script is executing it.

Example for SUID

root@linuxforfreshers:~#ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 22984 Jan  2013 /usr/bin/passwd

SUID bit is set on an executable. It runs with the permission of its owner.

Command :

root@linuxforfreshers:~#chmod u+s  < file name >

                                  or

root@linuxforfreshers:~#chmod 4755  < file name >

What is SGID?

If the set gid bit on directory entry is set, file in that directory will have the group ownership as the directory, instead of than the group of the user that created the file.

Command :

root@linuxforfreshers:~#chmod g+s  < file name >

                                 or

root@linuxforfreshers:~#chmod 2755  <file name >

What is Sticky Bit?

Sticky Bit is mainly used on folders in order to avoid deletion of a folder and its content by other users though they having write permissions on the folder contents. If Sticky bit is enabled on a folder, the folder contents are deleted by only owner who created them and the root user. No one else can delete other users data in this folder(Where sticky bit is set). This is a security measure to avoid deletion of critical folders and their content(sub-folders and files), though other users have full permissions.

How can I setup Sticky Bit for a Folder?

Sticky Bit can be set in two ways

1.     Symbolic way (t,represents sticky bit)

2.     Numerical/octal way (1, Sticky Bit bit as value 1)

Symbolic way:

chmod o+t /opt/dump/
or
chmod +t /opt/dump/

Numerical way:

chmod 1757 /opt/dump/

Here in 1757, 1 indicates Sticky Bit set, 7 for full permissions for owner, 5 for read and execute permissions for group, and full permissions for others.

root@linuxforfreshers:~#chmod o+t  < Dir Name >

                                   or

root@linuxforfreshers:~#chmod 1777 < Dir Name >