I typed a command and a program ran. Between my words and that program sits the shell, and it does work I did not ask for: it expands wildcards, splits words, runs redirects, and picks which file a name refers to. Most shell confusion is confusion about that work, not about the program.
About 2 min of reading
See the shell's own state.
Where am I
Before you run it, say out loud what it will print. Then open the answer.
What is in it
Before you run it, say out loud what it will print. Then open the answer.
Run the shortest command that shows the effect, before the one that makes it. ls before rm, cat before writing. The shell gives feedback fast enough that checking is cheaper than thinking ahead and being wrong.
No such file or directory means the name is wrong. Permission denied means the name is right and you cannot run or read it. Not a command means the shell did not find a program by that name. Three different problems, three different fixes.
The shell parses the line first: it expands globs, splits into words, sets up redirects. Then it finds the program and runs it with the arguments it decided on. The program never sees your original line.
A bare word is a path relative to the current directory. That is why the same command works in one folder and fails in another, and why an absolute path is the fix rather than more typing.
Current directory, environment variables, and exit status persist between commands. That state is why a session that worked once may not work again, and why a script sets its own state at the top.