advanced file permissions in linux.

http://www.linuxserverworldadmin.blogspot.in/2015/03/advanced-file-permissions-in-linux.html

To view the location of a command use which 
[root@client24 myfolder]# which cat /bin/cat 
[root@client24 myfolder]# ls -ld /bin/cat 
-rwxr-xr-x 1 root root 23100 Nov 27 2006 /bin/cat 
[root@client24 myfolder]# su - tom 
[tom@client24 ~]$ cat > myfile 
hello 

Open another terminal as root user 
root@client24 ~]# ps -aux 
USER PID %CPU %MEM VSZ   RSS TTY STAT START   TIME
COMMAND 

tom 3972 0.0 0.1 3680 432 pts/1 S+ 22:01 0:00 cat


[root@client24 myfolder]# ls -ld /bin/cat 
-rwxr-xr-x 1 root root 23100 Nov 27 2006 /bin/cat 
Set SUID on the file 
[root@client24 myfolder]# chmod 4755 /bin/cat [root@client24 myfolder]# ls -ld /bin/cat 
-rwsr-xr-x 1 root root 23100 Nov 27 2006 /bin/cat 
[root@client24 myfolder]# su - tom 
[tom@client24 ~]$ cat >> myfile 
Open another terminal as root user 
root@client24 ~]# ps -aux 
USER PID %CPU %MEM VSZ   RSS TTY STAT START   TIME
COMMAND
root 4032 0.0 0.1 3680 436 pts/1 S+ 22:05 0:00 cat
[root@client24 ~]# mkdir /myfolder 
[root@client24 ~]# cd /myfolder 
[root@client24 myfolder]# ls 
[root@client24 myfolder]# cat > test 
Learning Linux is a wise man's decesion. 
[root@client24 myfolder]# ls -ld test 
-rw-r--r-- 1 root root 41 Jul 13 21:22 test 
[root@client24 myfolder]# ls -ld /myfolder 
drwxrwxrwx 2 root root 4096 Jul 13 21:21 /myfolder 
Set SGID on the folder 
[root@client24 myfolder]# chmod 2777 /myfolder 
[root@client24 myfolder]# ls -ld /myfolder 
drwxrwsrwx 2 root root 4096 Jul 13 21:21 /myfolder [root@client24 myfolder]# ls 
test 
[root@client24 myfolder]# su - tom 
[tom@client24 ~]$ cd /myfolder 
[tom@client24 myfolder]$ ls -l 
total 4 
-rw-r--r-- 1 root root 41 Jul 13 21:22 test 
[tom@client24 myfolder]$ cat > myfile file to test SGID 
[tom@client24 myfolder]$ ls -ld myfile 
-rw-r--r-- 1 tom root 18 Jul 13 22:14 myfile [root@client24 myfolder]# su - joy 
[joy@client24 ~]$ cd /myfolder 
[joy@client24 myfolder]$ ls -l 
total 8 
-rw-r--r-- 1 tom  root 18 Jul 13 22:14 myfile 
-rw-r--r-- 1 root root 41 Jul 13 21:22 test 

[joy@client24 myfolder]$ cat > joy_file 
hello this file is created by joy but because of SGID bit set its group ownership will be root 
[joy@client24 myfolder]$ ls -ld joy_file 
-rw-rw-r-- 1 joy root 95 Jul 13 22:15 joy_file 

joy and tom have the same primary group called joy 

[root@client24 ~]# mkdir /sticky 
[root@client24 ~]# chmod 777 /sticky 
[root@client24 ~]# su - tom 
[tom@client24 ~]$ cd /sticky 
[tom@client24 sticky]$ ls -l 
total 0 
[root@client24 sticky]# su - tom [tom@client24 ~]$ cd /sticky/ [tom@client24 sticky]$ ls 
[tom@client24 sticky]$ cat > tom_file 
hello am tom, am going to set sticky bit on the /sticky folder. [tom@client24 sticky]$ ls -l 
total 4 
-rw-r--r-- 1 tom joy 64 Jul 13 22:36 tom_file 
[tom@client24 sticky]$ chmod 664 tom_file 
[tom@client24 sticky]$ ls -ld tom_file 
-rw-rw-r-- 1 tom joy 64 Jul 13 22:36 tom_file [root@client24 sticky]# su - joy 
[joy@client24 ~]$ cd /sticky 
[joy@client24 sticky]$ ls -l 
total 4 
-rw-rw-r-- 1 tom joy 64 Jul 13 22:37 tom_file 
[joy@client24 sticky]$ rm tom_file 

[joy@client24 sticky]$ ls 
To avoid joy to del the file owned by the user tom apply sticky bit on the folder. 
[root@client24 sticky]# ls -ld /sticky 
drwxrwxrwx 2 root root 4096 Jul 13 22:38 /sticky 
Set Sticky Bit 
[root@client24 sticky]# chmod 1777 /sticky [root@client24 sticky]# ls -ld 
drwxrwxrwt 2 root root 4096 Jul 13 22:38 . [root@client24 sticky]# su - tom 
[tom@client24 ~]$ cd /sticky/ 
[tom@client24 sticky]$ ls 
[tom@client24 sticky]$ cat > tom_file 
this file is created by tom whose primary group is joy, but joy cannot delete this file. 
[tom@client24 sticky]$ ls -ld /sticky/tom_file 
-rw-r--r-- 1 tom joy 89 Jul 13 22:41 /sticky/tom_file [root@client24 sticky]# su - joy 
[joy@client24 ~]$ cd /sticky/ 
[joy@client24 sticky]$ ls -l 
total 4 
-rw-r--r-- 1 tom joy 89 Jul 13 22:41 tom_file 
[joy@client24 sticky]$ rm tom_file 

rm: remove write-protected regular file `tom_file'? y 
rm: cannot remove `tom_file': Operation not permitted 

[joy@client24 sticky]$ ls -l 
total 4 
-rw-r--r-- 1 tom joy 89 Jul 13 22:41 tom_file