我正在使用package来管理我的Emacs扩展。为了在不同的计算机上同步我的Emacs设置，我想在.emacs文件中指定包名列表，然后package可以自动搜索和安装包，这样我就不需要手动安装它们了。叫M-x package-list-packages。怎么做？
; 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)))
这是我的，它更短:)
(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))
根据Profpatsch的评论和以下答案：
(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+将自动跟踪可自定义的package-selected-packages变量中的用户安装包。 package-install将更新自定义变量，您可以使用package-install-selected-packages函数安装所有选定的包。
这种方法的另一个方便的优点是，您可以使用package-autoremove自动删除package-selected-packages中未包含的包（尽管它将保留依赖项）。
(package-initialize)
(unless package-archive-contents
(package-refresh-contents))
(package-install-selected-packages)
资料来源：http://endlessparentheses.com/new-in-package-el-in-emacs-25-1-user-selected-packages.html
这是我用于Emacs Prelude的代码：
(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)
如果你没有使用MELPA，你不需要它（如果你做melpa.el必须在你的load-path上（或通过MELPA安装）。包db不会每次刷新（因为这会减慢启动显着） - 仅限存在已卸载软件包的地方。
还没有人提到Cask，但它非常适合这项任务。
基本上，您创建列出要安装的软件包的~/.emacs.d/Cask。例如：
(source melpa)
(depends-on "expand-region")
(depends-on "goto-last-change")
; ... etc
从命令行运行cask将为您安装这些软件包，以及它们所需的任何依赖项。
此外，您可以使用cask update自动更新已安装的软件包。
使用包名称作为符号调用package-install。您可以通过交互方式调用package-install并填写名称来查找软件包的软件包名称。功能package-installed-p会告诉您它是否已经安装。
例如：
(mapc
(lambda (package)
(or (package-installed-p package)
(package-install package)))
'(package1 package2 package3))
我喜欢检查用户是否想要首先安装包，就像在this answer中一样。在安装任何东西之前，我还要刷新包装内容。我不确定这是不是最好的方法，但我不认为最好的答案是为我做的。
(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)))
(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)
我遇到了一个问题，即在将(package-install 'org)添加到.emacs后没有任何反应。我想安装最新版本的org-mode，内置的org-mode已经很老了。
我从Emacs 25.3.1中挖出了package-install的源代码。函数self已经检查是否已安装软件包，如果已安装软件包，则拒绝安装软件包。所以来自答案(unless (package-installed-p package) ...)的检查10093312实际上是不必要的。
(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))))
内置的org-mode也算作安装，package-install拒绝安装ELPA的新版本。在花了一些时间阅读package.el之后，我提出了以下解决方案。
(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))
它起作用的原因是package-*族函数根据它是一个符号还是一个package-desc对象来不同地处理参数。您只能通过package-install对象为package-desc指定版本信息。