The umask command/utility is a bash builtin. It’s used to check and/or alter the file-creation mode mask. What’s a Mode Mask? A mode mask is just a little bit of information (an octal value) that defines what permissions a file or directory is set with when created. It’s generally stored/set in octal form and follows the same rules as the chmod command in terms of read/write/execute permissions are specified. The big difference though is that the octal form is a complement.

What in the heck is it? How does it work? The umask is a way to determine for a given user what the permissions of newly created files and directories have. Supposedly Linux wants 666 and 777 to be default perms for files and directories, respectively. Notice when you make a file, it’s set to 644, and directories to 755 though. That’s because this umask value is taking effect. On my machine anyway, the default umask is 022 (—-w–w-).