Chmod Explained

[Penguin Guy]

chmod is an abbreviation of change mode. There are two ways to chmod:

Symbolic (e.g. chmod +x)

chmod <people><+/-><permissions>
Example: chmod o-w (deny others from editing the file)
Example: chmod u+rwx (give the owner full control)
Exmaple: chmod +rwx (give everyone full control)
Example: chmod +x (allow anyone to execute the file)

Key:
r - Read
w - Write
x - Execute

u - The owner of the file
g - The group that the file belongs to
o - Anybody who is not one of the above
a - All users

+ - Add permissions
- - Remove permissions

Numerical (e.g. chmod 700):

                                : The first octet represents permissions for the owner.
        r w x  T                : The second octet represents permissions for the group.
Owner:  4 2 1  7                : The third octet represents permissions for everyone else.
Group:  0 0 0  0                : For each octet, start at 0 and:
Other:  0 0 0  0                : +4 for read permission.
                                : +2 for write permission.
Command: chmod 700              : +1 for execute permission.

What is an octet? What Happened to +3?
Read more: Wikipedia. Man page.

[Penguin Guy]

When I refer to 'octets' in my chmod guide, I am referring to these:
chmod 644 file

While digits go from 0-9, octets go from 0-7 (skipping 8 and 9). A number written using digits is called a decimal, a number written using octets is called an octal.

Counting to 20 in Decimal: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 
Counting to 20 in Octal: 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 15, 16, 17, 20, 21, 22, 23, 24

[credobyte]

Might be a little bit offtopic, but .. does anyone have an answer to the following question ?

Why there's no +3 ( +1, +2 ... +4 ) ?

[dondad]

Might be a little bit offtopic, but .. does anyone have an answer to the following question ?

Look at the place values. The first (rightmost) digit is the ones, the next is the 2's and the next is the 4's. To make 3 you would use the ones and the 2's place. so:
1 is 001
2 is 010
3 is 011
4 is 100
5 is 101
6 is 110
7 is 111.

There is no one place for the 3's

This is analogous to the base 10 system with the first place being the ones, the next the 10's etc.

[Penguin Guy]

Might be a little bit offtopic, but .. does anyone have an answer to the following question ?

Why there's no +3 ( +1, +2 ... +4 ) ?

Good question! The answer is simple; +3 is switched with +4 to avoid a duplicate number 3 in the sum octet:

+4 System      +3 System
r w x  Σ       r w x  Σ
4 2 1  7       3 2 1  6
4 2 0  6       3 2 0  5
4 0 1  5       3 0 1  4
4 0 0  4       3 0 0  3
0 2 1  3       0 2 1  3
0 2 0  2       0 2 0  2
0 0 1  1       0 0 1  1
0 0 0  0       0 0 0  0

Although this may seem confusing, it makes sense mathematically. This kind of code is based off binary.

[Fatjoint]

This is a subject of it's own, but here's a quick primer on counting in other number systems.

Counting in any base system is exactly the same as counting in decimal - representing, or writing down the count is what is different.

Base10 (Decimal) 1,110 has the following meaning:

10^3 10^2 10^1 10^0
------------------- = (1*10^3)+(1*10^2)+(1*10^1)+(0*10^0)
1    1    1    0


Base8 (Octal) 5477 has the following meaning:

8^3 8^2 8^1 8^0
--------------- = (5*8^3)+(4*8^2)+(7*8^1)+(7*8^0)
5   4   7   7


Base30 (Trigesimal) 11T has the following meaning:

30^2 30^1 30^0 
------------------- = (1*30^2)+(1*30^1)+(30*30^0)
1    1    T  (T represents 30)

How do we write these numbers? Each position has a representitive character or symbol whose count is equal to N(max)-1.

In Base8 (Octal), each position is counted from 0 (meaning none) to 7 (the maximum count in that position). When we want to say that we've counted past 7, we add an octet position and start counting again in the base^0 position again.

In Base10, we count from 0 to 9. When we want to say that we've counted past 9, we add a deimal position and start counting again in the base^0 position again.

In Base30, we count from 0 to T. When we want to say that we've counted past 30, we add a trigesimal position and start counting again in the base^0 position again.

Counting in a base 3 system would look like this:

3^3 3^2 3^1 3^0
---------------
1   1   1   1   = 40
    3   3   3   = 39
    3   3   2   = 38
    3   3   1   = 37
    3   2   3   = 36
    3   2   2   = 35
    3   2   1   = 34
    3   1   3   = 33
    3   1   2   = 32
    3   1   1   = 31
    2   3   3   = 30

....

....

    0   0   0   = 0