linux,file,unix,permissions,umask , Effect of umask on text files


Effect of umask on text files

Question:

Tag: linux,file,unix,permissions,umask

I understand how umask works, at least a basic level, when dealing with the permissions of an executable file or directory. However, I struggle when it comes to how umask applies its rules to text files.

For example, consider the umask 037. On newly created executables or directories I understand that the permissions would arise from a simple subtraction (777-037 = 740). My question though is then how does the apparent subtraction work for determining the permissions on say a text file who's default permissions would be 666.

To be clear how is the subtraction done (666-037 = 637?) obviously having an executable text file makes no sense, and according to some examples I have carried out I know that that the file permissions in such a case should work out to 640.

How is this subtraction done? How do we arrive at 666-037 = 640?


Answer:

It's not subtraction. It's masking: Boolean "AND".

Create mode    0666 (octal)   Binary = 110110110
Mask            037 (octal)   Binary = 000011111 &
                                       ---------
Result         0640                    110100000

See also http://en.wikipedia.org/wiki/Boolean_algebra#Basic_operations


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