Glossary
PATH
The ordered list of directories the shell searches for commands.
PATH is a colon-separated list of directories. When you type a command name, the shell walks that list in order and runs the first matching executable it finds — which is why order is precedence, and why prepending ~/.local/bin makes your own tools win over the system's.
"command not found" for something you know is installed almost always means its directory is not on PATH. Inspect yours with echo $PATH | tr : '\n', and see exactly which file would run with which or type -a.
See it in practice
$ echo $PATH | tr ":" "\n" && type -a python3First the search order, then every matching binary in it. Together they explain any "wrong version runs" mystery.
Where you'll meet it
Which binary actually runs when you type a name — and unmask aliases.
env & exportHow configuration flows into processes — PATH, export, and why edits "don't stick".
Environment variableA named value processes inherit when they start.