Software >> OS >> Unix >> What are the special permissions setuid, setgid and sticky bit

Setuid Permission 

When set-user identification (setuid) permission is set on an executable file, a process that runs this file is granted access based on the owner of the file (usually root), rather than the user who is running the executable file. This special permission allows a user to access files and directories that are normally only available to the owner. For example, the setuid permission on the passwd command makes it possible for a user to change passwords, assuming the permissions of the root ID
 
#ls -l /usr/bin/passwd 
-r-sr-sr-x 3 root sys 96796 Jul 15 21:23 /usr/bin/passwd
 

Setuid  permissions can be set by using the chmod command to assign the octal value 4 as the first number in a series of four octal values.
 
#chmod 4555 file1
 
 
Setgid Permission

The set-group identification (setgid) permission is similar to setuid, except that the process's effective group ID (GID) is changed to the group owner of the file, and a user is granted access based on permissions granted to that group. ex /usr/bin/mail has setgid permission set.
 
When setgid permission is applied to a directory, files that were created in this directory belong to the group to which the directory belongs, not the group to which the creating process belongs. Any user who has write and execute permissions in the directory can create a file there. However, the file belongs to the group that owns the directory, not to the user's group ownership.   
Setgid  permissions can be set by using the chmod command to assign the octal value 2 as the first number in a series of four octal values.
 
#chmod 2551 file1
#ls -ltr 
-r-xr-s—x   1 dj  dj  26876 Jul 15 21:23 file1

Sticky Bit

The sticky bit is a permission bit that protects the files within a directory. If the directory has the sticky bit set, a file can be deleted only by the owner of the file, the owner of the directory, or by root. This special permission prevents a user from deleting other users' files from public directories such as /tmp.

sticky bits permissions can be set by using the chmod command to assign the octal value 1 as the first number in a series of four octal values. 

Typically set on directory

# chmod 1777 dir1
or
# chmod +t dir1

# ls -l dir1
drwxrwxrwt 2 dj dj 512 Jul 15 21:23 dir1

 

Note: In Linux, if the execute permission (x) was not set, sticky bit will be shown as uppercase T. When x was set, sticky bit will be shown as lowercase t