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? (http://ubuntuforums.org/showpost.php?postid=7891167) What Happened to +3? (http://ubuntuforums.org/showpost.php?p=7891167)
Read more: Wikipedia (http://en.wikipedia.org/wiki/Chmod). Man page (http://ss64.com/bash/chmod.html).

[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