Umask

umask is the way the UNIX operating system determines what default permissions that files and directories are created with.

The mask itself is applied as a bitwise AND operation then a bitwise NOT of that with the default 777 permisions.

The three popular umasks are 022, 002 and 000.

022

This as the example shows above that you will get full rwx for the user, r-x for the group and r-x for other. This is the default in almost all of our operating systems.

    user           group        other
  r   w    x        r     w    x      r    w    x
 111 111 111    111 111 111  111 111 111

        777
AND NOT 022
      = 755

002

This would give full rwx for the user, full rwx for the group and give only r-x for other. This is helpful when you want your default group (or a SetGID directory) to have full control over the files and directories you create while allowing everyone else read and execute permissions.

        777
AND NOT 002
      = 775

000

This is the most restrictive umask and gives no permissions

        777
AND NOT 022
      = 770