Files

One line: A name in a directory has a type before it has permissions. ls -l shows the type as one letter, and the type decides what opening the name does: read bytes, wait for a writer, or refuse. Dots have rules of their own: . and .. are entries every directory lists, and a dot at the start of a name only hides it from ls and *.

Lesson	Level	What it settles
The first letter of ls -l is the type	101	the seven letters on real files, the same types in test, find -type, zsh's glob qualifiers and fish's path filter, what opening a FIFO and a socket file does, and one Mac disk under two letters
What a dot means to the shell	101	. and .. as entries every directory lists, a leading dot that hides a name from ls and * and from nothing else, the . builtin and where it looks for a file, and ./, in bash 5.2 and 3.2, zsh and fish

Planned

Rough order, not a promise:

• Nine permission bits, and three more: rwx, umask, setuid, setgid and the sticky bit, and the @ or + a Mac prints after them
• A hard link is a second name; a symlink is a file holding one: the link count, ln against ln -s, and what rm removes
• stat and the inode: sizes, blocks and three timestamps, through GNU and BSD stat's different flags