emacs Package Management Automatic package installation on emacs start-up


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