The :ensure keyword makes use-package ask the Emacs package
manager to install a package if it is not already present on your
system.
For example:
(use-package magit :ensure t)
If you need to install a different package from the one named by
use-package, you can use a symbol:
(use-package tex :ensure auctex)
You can customize the user option use-package-always-ensure to
a non-nil value if you want this behavior to be global for all
packages:
(require 'use-package-ensure) (setq use-package-always-ensure t)
You can override the above setting for a single package by adding
:ensure nil to its declaration.