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

I am using package to manage my Emacs extensions. In order to synchronize my Emacs settings on different computers, I'd like a way to specify a list of package names in .emacs file and then package could automatically search and install the packages, so that I don't need to install them manually by calling M-x package-list-packages. How to do that?

; 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))

(require 'cl)
(require 'package)

(setq cfg-var:packages '(
       emmet-mode
       ergoemacs-mode
       flycheck
       flycheck-pyflakes
       monokai-theme
       py-autopep8
       py-isort
       rainbow-mode
       yafolding
       yasnippet))

(defun cfg:install-packages ()
    (let ((pkgs (remove-if #'package-installed-p cfg-var:packages)))
        (when pkgs
            (message "%s" "Emacs refresh packages database...")
            (package-refresh-contents)
            (message "%s" " done.")
            (dolist (p cfg-var:packages)
                (package-install p)))))

(add-to-list 'package-archives '("gnu" . "http://elpa.gnu.org/packages/") t)
(add-to-list 'package-archives '("melpa" . "http://melpa.org/packages/") t)
(add-to-list 'package-archives '("melpa-stable" . "http://stable.melpa.org/packages/") t)
(add-to-list 'package-archives '("org" . "http://orgmode.org/elpa/") t)
(package-initialize)

(cfg:install-packages)

I like checking if the user wants to install the packages first as done in this answer. Also I'm refreshing my package contents once before installing anything. I'm not sure if this is the best way, but I don't think the top answers were doing it for me.

(setq required-pkgs '(jedi flycheck cider clojure-mode paredit markdown-mode jsx-mode company))

(require 'cl)

(setq pkgs-to-install
      (let ((uninstalled-pkgs (remove-if 'package-installed-p required-pkgs)))
        (remove-if-not '(lambda (pkg) (y-or-n-p (format "Package %s is missing. Install it? " pkg))) uninstalled-pkgs)))

(when (> (length pkgs-to-install) 0)
  (package-refresh-contents)
  (dolist (pkg pkgs-to-install)
    (package-install pkg)))

Here's mine, it's shorter :)

(mapc
 (lambda (package)
   (unless (package-installed-p package)
     (progn (message "installing %s" package)
            (package-refresh-contents)
            (package-install package))))
 '(browse-kill-ring flycheck less-css-mode tabbar org auto-complete undo-tree clojure-mode markdown-mode yasnippet paredit paredit-menu php-mode haml-mode rainbow-mode fontawesome))

I ran into a problem that nothing happened after adding (package-install 'org) into .emacs. I wanted to install the up-to-date version of org-mode and the built-in org-mode is quite old.

I dug out the source code of package-install from Emacs 25.3.1. The function self already checks if a package is installed or not and refuses to install it if the package is already installed. So the check (unless (package-installed-p package) ...) from answer 10093312 is in fact uncalled for.

(defun package-install (pkg &optional dont-select)
  "Install the package PKG.
PKG can be a package-desc or a symbol naming one of the available packages
in an archive in `package-archives'.  Interactively, prompt for its name.

If called interactively or if DONT-SELECT nil, add PKG to
`package-selected-packages'.

If PKG is a package-desc and it is already installed, don't try
to install it but still mark it as selected."
  (interactive
   (progn
     ;; Initialize the package system to get the list of package
     ;; symbols for completion.
     (unless package--initialized
       (package-initialize t))
     (unless package-archive-contents
       (package-refresh-contents))
     (list (intern (completing-read
                    "Install package: "
                    (delq nil
                          (mapcar (lambda (elt)
                                    (unless (package-installed-p (car elt))
                                      (symbol-name (car elt))))
                                  package-archive-contents))
                    nil t))
           nil)))
  (add-hook 'post-command-hook #'package-menu--post-refresh)
  (let ((name (if (package-desc-p pkg)
                  (package-desc-name pkg)
                pkg)))
    (unless (or dont-select (package--user-selected-p name))
      (package--save-selected-packages
       (cons name package-selected-packages)))
    (if-let ((transaction
              (if (package-desc-p pkg)
                  (unless (package-installed-p pkg)
                    (package-compute-transaction (list pkg)
                                                 (package-desc-reqs pkg)))
                (package-compute-transaction () (list (list pkg))))))
        (package-download-transaction transaction)
      (message "`%s' is already installed" name))))

The built-in org-mode also counts as installed and package-install refuses to install the newer version from ELPA. After spending some time reading package.el, I came up with the following solution.

(dolist (package (package-compute-transaction
                  () (list (list 'python '(0 25 1))
                           (list 'org '(20171211)))))
  ;; package-download-transaction may be more suitable here and
  ;; I don't have time to check it
  (package-install package))

The reason why it works is that package-* family functions handle the arguments differently based on whether if it is a symbol or a package-desc object. You can only specify version info for package-install via a package-desc object.

Here's another way.

;; assure every package is installed
(defun ensure-package-installed (&rest packages)
  (let ((user-required-packages
         (seq-remove
          (lambda (package) (package-installed-p package))
          packages)))
    (when user-required-packages
      (package-refresh-contents)
      (dolist (package user-required-packages)
        (package-install package)))))

;; list of packages to install
(ensure-package-installed
 'try
 'which-key)