Example
;; package.el is available since emacs 24
(require 'package)
;; Add melpa package source when using package list
(add-to-list 'package-archives '("melpa" . "http://melpa.org/packages/") t)
;; Load emacs packages and activate them
;; This must come before configurations of installed packages.
;; Don't delete this line.
(package-initialize)
;; `package-initialize' call is required before any of the below
;; can happen
;; If you do not put the "(package-initialize)" in your ~/.emacs.d/init.el (or
;; ~/.emacs), package.el will do it for you starting emacs 25.1.
;; Below manual maintenance of packages should not be required starting emacs
;; 25.1 with the introduction of `package-selected-packages' variable. This
;; variable is automatically updated by emacs each time you install or delete a
;; package. After this variable is synced across multiple machines, you can
;; install the missing packages using the new
;; `package-install-selected-packages' command in emacs 25.1.
;; To clarify, below technique is useful on emacs 24.5 and older versions.
;; Request some packages:
(defconst my-package-list '()
"List of my favorite packages")
(defvar my-missing-packages '()
"List populated at each startup that contains the list of packages that need
to be installed.")
(dolist (p my-package-list)
(when (not (package-installed-p p))
(add-to-list 'my-missing-packages p)))
(when my-missing-packages
(message "Emacs is now refreshing its package database...")
(package-refresh-contents)
;; Install the missing packages
(dolist (p my-missing-packages)
(message "Installing `%s' .." p)
(package-install p))
(setq my-missing-packages '()))
References
