Advanced File Permissions in Linux

原文：http://www.techcuriosity.com/resources/linux/advanced_file_permissions_in_linux.php

Here we will discuss about the 3 special attributes other than the common read/write/execute.
Example:
drwxrwxrwt - Sticky Bits - chmod 1777
drwsrwxrwx - SUID set - chmod 4777
drwxrwsrwx - SGID set - chmod 2777

Sticky bit

Sticky bits are mainly set on directories.
If the sticky bit is set for a directory, only the owner of that directory or the owner of a file can delete or rename a file within that directory.

Example:
Consider you have a directory " test ".
chmod it to " 777 ". This gives permissions for all the users to read, write and execute.
chmod +t test

Example: ls -al
drwxrwxrwt 2 a1 a1 4096 Jun 13 2008 .
-rw-rw-r-- 1 a1 a1 0 Jun 11 17:30 1.txt
-rw-rw-r-- 1 b2 b2 0 Jun 11 22:52 2.txt

From the above example a1 is the owner of the test directory.
a1 can delete or rename the files 1.txt and 2.txt.
b2 can delete or rename the file 2.txt only.

SUID - [ Set User ID ]

SUID bit is set for files ( mainly for scripts ).
The SUID permission makes a script to run as the user who is the owner of the script, rather than the user who started it.

Example:
If a1 is the owner of the script and b2 tries to run the same script, the script runs with the ownership of a1.
If the root user wants to give permissions for some scripts to run by different users, he can set the SUID bit for that particular script.
So if any user on the system starts that script, it will run under the root ownership.

Note:
root user much be very carefull with this.

SGID - [ Set Group ID ]

If a file is SGID, it will run with the privileges of the files group owner, instead of the privileges of the person running the program.
This permission set also can make a similar impact. Here the script runs under the groups ownership.

You can also set SGID for directories.
Consider you have given 2777 permission for a directory. Any files created by any users under this directory will come as follows.

Example:
-rw-rw-r-- 1 b2 a1 0 Jun 11 17:30 1.txt

In the above example you can see that the owner of the file 1.txt is b2 and the group owner is a1.
So both b2 and a1 will have access to the file 1.txt.

Now lets make this more intresting and complicated.
Create a directory "test". Chmod it to 2777. Add sticky bit to it.

Example:
mkdir test
chmod 2777 test
chmod +t test

ls -al test
drwxrwsrwt 2 a1 a1 4096 Jun 13 2008 test

From the above permission set you can understand that SGID and sticky bit is set for the folder "test".
Now any user can create files under the test directory.

Example:
drwxrwsrwt 2 a1 a1 4096 Jun 13 2008 .
-rw-rw-r-- 1 b2 a1 0 Jun 11 17:30 1.txt
-rw-rw-r-- 1 c3 a1 0 Jun 11 17:30 2.txt
-rw-rw-r-- 1 d4 a1 0 Jun 11 17:30 3.txt

So all the a1 user has access to all the files under the test directory. He can edit, rename or remove the file.
b2 user has access to 1.txt only, c3 has access to 2.txt only...

If sticky bit was not set for the test directory, any user can delete any files from the test directory, since the test directory has 777 permissions.
But now it not possible.

Example:
If d4 tries to remove 1.txt
rm -f 1.txt
rm: cannot remove `1.txt': Operation not permitted