Directory or File Permission in Linux

For Example---

[oracle@sbldb2 ~]$ ll
total 516
-rw-r--r-- 1 oracle oinstall 235340 Nov  2 11:50 02112015.lst
-rw-r--r-- 1 oracle oinstall 257702 Sep  8  2014 awrrpt_1_7407_7409.txt
drwxr-xr-x 2 oracle oinstall   4096 Mar 12  2014 Desktop
-rw-r--r-- 1 oracle oinstall    364 Apr  1  2014 Dhaka.bkp
drwxr-xr-x 3 oracle oinstall   4096 Mar 23  2014 oradiag_oracle
drwxr-xr-x 4 oracle oinstall   4096 Sep  2 15:45 rman
-rw-r--r-- 1 oracle oinstall   3965 Jan  5  2015 tuning.sql

Total 10 fields in prefix of every files and directory as above

Here
user ---> oracle
group ---> oinstall

-rwxr-xr-x

1st filed represents ---> Directory
2nd,3rd and 4th filed represent ---> Owner Permission
5th,6th and 7th filed represent ---> Group Member Permission
8th,9th and 10th field represent ---> Other Permission

You can setup following mode on each files. In a Linux and UNIX set of permissions is called as mode:

Read (r)
Write (w)
Execute (x)

Linux Read mode permissions

--Read access on a file allows you to view file
--Read access on a directory allows you to view directory contents with ls command
Write mode permissions

--Write access on a file allows you to write to file
--Write access on a directory allows you to remove or add new files
Execute mode permissions

--Execute access on a file allows to run program or script
--Execute access on a directory allows you access file in the directory

Octal numbers and permissions

You can use octal number to represent mode/permission:
r: 4
w: 2
x: 1

Use above method to calculate permission for group and others. Let us say you wish to give full permission to owner, read & execute permission to group, and read only permission to others, then you need to calculate permission as follows:

-rwx-rw--x-- 1 oracle oinstall   3965 Jan  5  2015 tuning.sql

User (oracle) = r+w+x = 4+2+1 = 7
Group (oinstall) = r+w = 4+2+0 = 6
Others = r= 4+0+0 = 4

Effective permission is 764.