Do you know about default-directory variable in Emacs Lisp?

(buffer-file-name) ;; "/home/tony/minibuffer/notes.el"
default-directory  ;; "/home/tony/minibuffer/"
(copy-directory "~/minibuffer/foo/" "./bar/")

(let ((default-directory "/tmp/"))
  (copy-directory "~/minibuffer/foo/" "./bar/" nil t))

◉ tony@tony:~/minibuffer:
$ tree
.
├── bar
├── foo
│   ├── foo-1
│   └── foo-2
├── #notes.el#
└── notes.el

2 directories, 4 files
◉ tony@tony:~/minibuffer:
$ tree
.
├── bar
│   └── foo
│       ├── foo-1
│       └── foo-2
├── foo
│   ├── foo-1
│   └── foo-2
├── #notes.el#
└── notes.el

3 directories, 6 files
◉ tony@tony:~/minibuffer:
$ rm -r bar/*
◉ tony@tony:~/minibuffer:
$ tree
.
├── bar
├── foo
│   ├── foo-1
│   └── foo-2
├── #notes.el#
└── notes.el

2 directories, 4 files
◉ tony@tony:~/minibuffer:
$ tree /tmp/bar/
/tmp/bar/
└── foo
    ├── foo-1
    └── foo-2

1 directory, 2 files
◉ tony@tony:~/minibuffer:
$