What is Umask in Linux?
Umask (short for user file-creation mode mask) is used by UNIX-based systems to set default permissions for newly created files and directories. It does this by masking or subtracting these permissions. For example, with the usual Umask being set to 022 on most systems all the new files we create will subtract the Umask value from full permissions (for files that would be 666 - 022 = 644). Umask can be expressed in octal or symbolic values.