#HOWTO #Linux: File-System

Filesystem Structure:

Each file has its own permissions for read, write and execute access for every type of user trying to access the file and combined from three main groups: owner, group and any.

File permissions overview:

OWNER			GROUP			ANY
R	W	X	R	W	X	R	W	X

Each cell in the RWX group is a flag that is enabled or not.

Let’s take a look at /etc/passwd file and it’s permissions:

$ ls -la /etc/passwd

 -rw-r--r-- 1 root root 2829 Jul 29 15:46 /etc/passwd

 ^^^^^^^^^^      |    |    |        |               |

 RWX Access      |    |    |        |           File Name

                 |    |    |   Date Modified

             Owner    |    |

                  Group    |

                        Size

Let’s try to understand this shit

OWNER			GROUP			ANY
R	W	X	R	W	X	R	W	X

This means, that OWNER (root) can read and write, GROUP (root) can read, and ANY other can read the file. That means that ONLY the root user can modify this file and any other user does not can not write or execute this file.

Command chmod is used to modify file permissions:

chmod +x somefile   # will enable EXECUTE flag for all groups

chmod a-x somefile # will disable EXECUTE flag for ANY group only

Base-2 numeral system or binary numeral system:

A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" (zero) and "1" (one).

Each digit in such number is referred to as a bit,

Since you can’t use other digits other than 0 and 1 following will explain incremental counting:

Decimal (base-10) -> Binary (base-2)

                0 ->     0

                1 ->     1

                2 ->    10

                3 ->    11

                4 ->   100

                5 ->   101

                6 ->   110

                7 ->   111

                8 ->  1000

                etc...

Each additional enabled bit adds 2**(bit_number) to the resulting number, where 0b1001 <- this is the starting bit and it’s bit number is 0

(0b - prefix to identify binary number)

0b0001 == 2**0 == 1 (decimal)

0b0010 == 2**1 == 2 (decimal)

0b0100 == 2**2 == 4 (decimal)

0b1000 == 2**3 == 8 (decimal)

So practically, we can add above numbers in binary:

--------------------------------

0001  (1) | 0011 (3) | 0111 (7)

   +      |    +     |    +

0010  (2) | 0100 (4) | 1000 (8)

   =      |    =     |    =

0011  (3) | 0111 (7) | 1111 (15)

4 enabled bits in number 0b1111 could be converted to decimal with this formula: 0b1111 == 2**(bit_number+1)-1 == 15, where RED bit number is 3 (starting from 0)

Back to file permissions:

Let’s issue another command to check file permissions:

$ stat /etc/passwd

  File: /etc/passwd

  Size: 2829      Blocks: 8          IO Block: 4096   regular file

Device: fd00h/64768d Inode: 30937510    Links: 1

Access: (0644/-rw-r--r--)  Uid: (0/root)   Gid: (0/root)

Access: 2021-08-07 15:47:01.513224885 +0300

Modify: 2021-07-29 15:46:25.903259562 +0300

Change: 2021-07-29 15:46:25.903259562 +0300

 Birth: -

Here we can see some number (in red) representing file permissions: 0644

0644 is the numeric representation of -rw-r--r--.

OWNER			GROUP			ANY
R	W	X	R	W	X	R	W	X
1	1	0	1	0	0	1	0	0
6			4			4

Each group represents 3 bits that are Read (R) Write (W) Execute(X) which may be in enabled (1) or in disabled (0) state.

Each 3 bit forms a binary number containing 3 bits only.

It is possible to change file permissions with the chmod command using these numbers.

For example:

We want following permissions:

1. Owner is allowed to READ, WRITE, EXECUTE

2. Group is allowed to EXECUTE

3. ANY is allowed to EXECUTE

For the OWNER part, we enable all bits: 111 which is 7 in decimal

For the GROUP part, we enable only Execute bit 001 and it’s 1 in decimal

For the ANY part, we enable again Execute bit only 001 and it’s 1 in decimal

As a resulting number we will have: 711

chmod 711 somefile #will set our desired permissions to somefile