mjhjmtu

Trying to understand what is umask in Bash-scripting, I went to man bash and searched "umask". I found these entries, for starters:

the file creation mode mask as set by umask or inherited from the shell's parent

and:

umask [-p] [-S] [mode]

The user file-creation mask is set to mode. If mode begins with a digit, it is interpreted as an octal number; otherwise
it is interpreted as a symbolic mode mask similar to that accepted by
chmod(1).

I understand that umask as a shell built-in command that changes some particular default permissions mode of newly created files by executing some predefined bit mask (or just "mask") somehow.

I executed umask and got 0000. I tried to execute umask 0600 and then umask returned 0600 but I can't see how it effects my system, hence I'm not sure what "mode" means here.

Before I try to understand how umask works technically, it is important to me to understand the concept of umask in Linux in general.

What is the "mode" that is changed for newly created files? In other words, it doesn't seem to me to be their permissions I change with chmod (dirs and regular files are created with their default permissions either way - umask 0000 or umask 0600) so what default permissions change exactly?

In Linux the default file permission is 0644, default directory permission is 0755 and default umask is 0022.

Now, as the full permission of a file is 0666, the default permission came from (0666 - 0022 = 0644) and for directory it is (0777 - 0022 = 0755).

You can set the mask using #umask 0000, and it will create files and directories with full 666/777 permission (which is very insecure though).

umask is used to set the mask.