modify basic filestructure

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

shell/tree.md

@@ -0,0 +1,36 @@
+---
+title: "tree"
+tags: [ "basics", "tree", "markdown" ]
+---
+
+The `tree` utility outputs a full listing of everything in your current directory, and those below.
+
+- Just directories: `tree -d`
+- Output colour to `less`: `tree -C --info | less -re`
+- Ignore files in the `.gitignore` file: `tree --gitignore`
+
+You can place information about the files in a directory to use with the `tree --info` option, like this:
+
+```
+config
+	Config files.
+	This is a git submodule.
+README.md
+	Summary of the git.
+*.jpg
+	Little picture, does not display
+	words well.
+```
+
+Each description-line starts with a tab.
+
+## Markdown Conversion
+
+To represent a file structure as a nested series of markdown lists, you can try this horrifying `sed` one-liner:
+
+```sh
+tree -tf --dirsfirst --gitignore --noreport --charset ascii  | \
+    sed -e 's/| \+/  /g' \
+    -e 's/[|`]-\+/ */g' \
+    -e 's:\(* \)\(\(.*/\)\([^/]\+\)\):\1[\4](\2):g'
+```