Part II · Your system

23.3 PATH and command resolution

alsopath$pathwhichcommand -vtypehash -rcommand resolution.local/bin

TL;DR

PATH is a list of folders separated by :. The shell looks for a command in each, left to right, and runs the first match.
type NAME says what a name is: alias, function, builtin or file. type -a shows every match.
Your own scripts usually go in ~/.local/bin. Many distros add it to PATH when it exists; otherwise add it in a startup file (27.2).

echo "$PATH" | tr : '\n'
type -a python3
export PATH="$HOME/.local/bin:$PATH"

On your machine

alice@laptopexample

$ atfq 23.3
  PATH      /home/alice/.local/bin
            /usr/local/bin (doesn't exist)
            /usr/bin
            . ⚠ the current folder, wherever you are
  Repeated  /usr/bin (listed more than once: harmless, the first one wins)
  Libraries LD_LIBRARY_PATH=/opt/oracle/lib
  Runs      programs built for x86-64, glibc

An example: Alice's laptop, one of the test machines. In your terminal, this block shows your own machine, read live.

Pitfalls

bash remembers where it found each command: after installing or moving one, hash -r makes it search again.
An empty entry or . in PATH means "the current folder": a file named ls planted there would run instead of the real one.
which is a separate program that doesn't know your aliases and functions; type tells what your shell will actually run.
sudo uses its own PATH (secure_path in sudoers): a command your shell finds can be "not found" under sudo.

Deep dive 2 questions

In what order does the shell look?

For a name without a /: aliases first, then functions, then builtins (cd, echo), then the PATH folders. A name with a / in it, like ./build.sh or /usr/bin/python3, is a path: no search at all.

Why /usr/local/bin before /usr/bin?

So that a program you install by hand wins over the packaged program of the same name, without touching the files the package manager owns. The same goes for ~/.local/bin placed first: your version wins, for you only.

Commands: type which
Paths: /usr/local/bin ~/.local/bin
See also: 23.2 ELF and "command not found"
27.2 Shell startup files