UNIX for Dummies Questions & Answers

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

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.

Thanks for the reply. still I find it difficult to understand. do you have any standard steps or formula to derive it?

That's 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