How to automatically install Emacs packages by specifying a list of package names?

Question

; list the packages you want
(setq package-list '(package1 package2))

; list the repositories containing them
(setq package-archives '(("elpa" . "http://tromey.com/elpa/")
                         ("gnu" . "http://elpa.gnu.org/packages/")
                         ("marmalade" . "http://marmalade-repo.org/packages/")))

; activate all the packages (in particular autoloads)
(package-initialize)

; fetch the list of packages available 
(unless package-archive-contents
  (package-refresh-contents))

; install the missing packages
(dolist (package package-list)
  (unless (package-installed-p package)
    (package-install package)))

Based on comments by Profpatsch and answers below:

(defun ensure-package-installed (&rest packages)
  "Assure every package is installed, ask for installation if it’s not.

Return a list of installed packages or nil for every skipped package."
  (mapcar
   (lambda (package)
     ;; (package-installed-p 'evil)
     (if (package-installed-p package)
         nil
       (if (y-or-n-p (format "Package %s is missing. Install it? " package))
           (package-install package)
         package)))
   packages))

;; make sure to have downloaded archive description.
;; Or use package-archive-contents as suggested by Nicolas Dudebout
(or (file-exists-p package-user-dir)
    (package-refresh-contents))

(ensure-package-installed 'iedit 'magit) ;  --> (nil nil) if iedit and magit are already installed

;; activate installed packages
(package-initialize)

Emacs 25.1+ will automatically keep track of user-installed packages in the customizable package-selected-packages variable. package-install will update the customize variable, and you can install all selected packages with the package-install-selected-packages function.

Another convenient advantage of this approach is that you can use package-autoremove to automatically remove packages that are not included in package-selected-packages (though it will preserve dependencies).

(package-initialize)
(unless package-archive-contents
  (package-refresh-contents))
(package-install-selected-packages)

Source: http://endlessparentheses.com/new-in-package-el-in-emacs-25-1-user-selected-packages.html

Here's the code I use for Emacs Prelude:

(require 'package)
(require 'melpa)
(add-to-list 'package-archives
             '("melpa" . "http://melpa.milkbox.net/packages/") t)
(package-initialize)

(setq url-http-attempt-keepalives nil)

(defvar prelude-packages
  '(ack-and-a-half auctex clojure-mode coffee-mode deft expand-region
                   gist haml-mode haskell-mode helm helm-projectile inf-ruby
                   magit magithub markdown-mode paredit projectile
                   python sass-mode rainbow-mode scss-mode solarized-theme
                   volatile-highlights yaml-mode yari yasnippet zenburn-theme)
  "A list of packages to ensure are installed at launch.")

(defun prelude-packages-installed-p ()
  (loop for p in prelude-packages
        when (not (package-installed-p p)) do (return nil)
        finally (return t)))

(unless (prelude-packages-installed-p)
  ;; check for new packages (package versions)
  (message "%s" "Emacs Prelude is now refreshing its package database...")
  (package-refresh-contents)
  (message "%s" " done.")
  ;; install the missing packages
  (dolist (p prelude-packages)
    (when (not (package-installed-p p))
      (package-install p))))

(provide 'prelude-packages)

If you're not using MELPA you don't need to require it (and if you do melpa.el has got to be on your load-path (or installed via MELPA). The package db is not refreshed each time (as this would slow down the startup significantly) - only where there are uninstalled packages present.

No one has mentioned Cask yet, but it is quite suitable for this task.

Basically you create ~/.emacs.d/Cask listing the packages you want to install. For example:

(source melpa)
(depends-on "expand-region")
(depends-on "goto-last-change")
; ... etc

Running cask from the command line will install these packages for you, and any dependencies they need.

Also, you can automatically update installed packages using cask update.

Call package-install with the package name as a symbol. You can find the package names for your packages by calling package-install interactively and completing on the name. The function package-installed-p will let you know if it's already been installed.

For example:

(mapc
 (lambda (package)
   (or (package-installed-p package)
       (package-install package)))
 '(package1 package2 package3))