notes: add an interesting case of shell variables vs environment variables

NOTES.md

@@ -213,6 +213,17 @@ The form is $VAR, where VAR may only contain the following characters:
 * _
 * 0-9 (not in the first character)
 
+Just noticed an interesting case:
+```shell
+bash-5.2$ VAR=hello # we set a shell variable (NOT environment variable)
+bash-5.2$ VAR=hi env | grep VAR=; env | grep VAR= # here VAR is an environment variable, which is only valid for the next command (the second env returns nothing, confirming that it is not valid for that command)
+VAR=hi
+bash-5.2$ env | grep VAR= # var is not an environment variable
+bash-5.2$ echo $VAR # but it is a shell variable
+hello
+```
+Luckily for us, we don't have to handle shell variables, nor do we have to handle `VAR=value` or `VAR=value cmd`.
+
 #### Word Splitting
 cf. 3.5.7 Word Splitting