Establishes the file-creation permissions mask
The umask builtin specifies a mask that the system uses to set up access permissions when you create a file. This builtin works slightly differently in each of the shells.
The mask can be a three-digit octal number or a symbolic value such as you would use with chmod (page 1103). The mask specifies the permissions that are not allowed.
When mask is an octal number, the digits correspond to permissions for the owner of the file, members of the group the file is associated ...