How to calculate umask values?


 
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# 1  
How to calculate umask values?

Hi,

I was trying to understand how to calculate umask value but couldnt get the right way to calculate it. can some one please give me a small formula or easy method to do it?

Thanks,
Waseem
# 2  
The permissions you see when you ls -l a file are derived from a 4 digit octal number. Ignoring the first digit for the moment (setuid,setgid and the sicky bit) we have 3 octal digits these digits represent the rights of, in order, owner, group and world to the resource. since 8 is the 3rd power of 2 we can represent 3 states with each octal digit (since 4+2+1=7) depending on which values are set (this is a bitmask ie based on powers of 2, handy to manipulate with bitwise operations)

Code:
   1 = executable
   2 = writable
   4 = readable

The umask defines the permissions that are NOT available to each user group.
thus a umask of 027 means that owner has all rights, group has read and execute rights and everyone else has no rights.

Last edited by Scrutinizer; 06-01-2012 at 09:55 AM..
# 3  
Thanks for the reply. still I find it difficult to understand. do you have any standard steps or formula to derive it?
# 4  
  1. What permissions do you want to assign to owner, group and world by default
  2. Add the index above (1, 2 or 4) of all other permissions together and put in that position in the umask
  3. set umask.
That's it.
# 5  
Quote:
Originally Posted by ahmedwaseem2000
Thanks for the reply. still I find it difficult to understand. do you have any standard steps or formula to derive it?
It's nothing but an ordinary, boolean AND. Convert the octal numbers into bits, AND the permissions with the (reversed) mask, and there you have it.

Code:
      rwxrwxrwx
027 = 000010111 (umask)
!027= 111101000 (reversed umask)
666 = 110110110 (default permissions)

rwxrwxrwx
110110110 &
111101000
=========
110100000


Last edited by Corona688; 06-01-2012 at 02:16 PM..
 

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