modify basic filestructure

It's unclear what's 'basic', so `basic/` notes have been mostly moved.
The remainder became `shell/`.

shell/eval.md

@@ -0,0 +1,22 @@
+---
+title: "eval"
+tags: [ "basics" ]
+---
+
+Compose a statement for execution.
+
+```sh
+x='echo $y'
+echo $x
+y=dragon
+eval "$x"
+```
+
+The results remain in the current shell, unlike sub-shells.
+
+```sh
+b=basilisk
+sh -c 'echo $b'
+eval "g=goblin"
+echo $g
+```