The shell environment

Up to now you have thought that what you type goes to the command as it is. It does not, in fact.

Between you and Linux there is an interpreter called the shell, and it rewrites your letters the moment you press ⏎. Spreading *.txt into a list of file names, putting the real place in for $HOME — that is the shell's work, not the command's.

• environment variables — echo $HOME export env
• PATH — the very mechanism by which a command is found
• quoting and escaping — the difference between ' and " and \
• the order of expansion — when * ? ~ $ widen out
• aliases and history — alias history
• .profile — the settings read every time you start

This is the chapter with the most "ah, I see" in it. Why typing ls runs ls (PATH), why a space in a file name breaks things (expansion) and why you choose between the quotes (whether $ widens out or not) all join up and come into view.

Once you have this, the way you read an error message changes too, because you can tell apart "the command is at fault" from "the shell handed it something else".