Knowing File Permissions in Linux

Not understanding about file permissions in Linux ? I'll give a simple explanation about that and hopefully it will be usefull.

Permissions
in Linux can be read=r, write=w and execute=x. There
are 3 categories of ownership for each file and directory:

u:
a valid user with an entry in /etc/passwd

g:
a valid group with an entry in /etc/group

o:
other

For example :

jafar@tebetreply:~$ ls -l

-rw-r--r-- 1 root  root     351 2007-09-27 12:39 symlink     
drwxr-xr-x 2 jafar jafar     48 2008-04-30 18:33 tes           
-rw-r--r-- 1 jafar jafar   7365 2007-10-29 13:13 tes.pl         

-rw-r--r--

The first character indicates the type of file, and the next three
indicate read, write, and execute permission for each of the three user
types, user, group and other. Since there are three types of permission
for three users, there are a total of nine permission bits. The table
below shows the syntax:

1	2	3	4	5	6	7	8	9	10
File	User Permissions			Group Permissions			Other Permissions
Type	Read	Write	Execute	Read	Write	Execute	Read	Write	Execute
d	r	w	e	r	w	e	r	w	e

• Character 1 is the type of file: - is ordinary, d is directory, l is link.
• Characters 2-4 show owner permissions. Character 2 indicates
  read permission, character 3 indicates write permission, and character
  4 indicates execute permission.
• Characters 5-7 show group permissions. Character 5=read, 6=write, 7=execute
• Characters 8-10 show permissions for all other users. Character 8=read, 9=write, 10=execute

So if we have something like this -rw-r--r--, we can say this file : for user (read+write), for group (read only) and for other (read only).

Octal
and symbolic permissions.

We can also use octal notation to give permission in a file or directory.

Symbolic	octal	binary
read	4	' 100'
write	2	' 010'
execute	1	' 001'

Permissions
apply to the user, the group and to others. An item has a set of  3
grouped permissions for each of these categories.

How
to translate -rw-r--r-- in octal notation

user	group	other
rw 4+2=6	r__ 4=4	r__ 4=4

-rw-r--r-- = 644 in octal notation

-rwxrwxrw- = 775 in octal notation

The
standard permission

UNIX
system create files and directories with standard permissions as
follows:

Standard
permission for:

Files 666 -rw-rw-rw-

Directories 777 -rwxrwxrwx

Not difficult isnt it.