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### 50.2 Package Installation

Packages are most conveniently installed using the package menu (see Package Menu), but you can also use the command `M-x package-install`. This prompts for the name of a package with the ‘`available`’ status, then downloads and installs it.

A package may require certain other packages to be installed, because it relies on functionality provided by them. When Emacs installs such a package, it also automatically downloads and installs any required package that is not already installed. (If a required package is somehow unavailable, Emacs signals an error and stops installation.) A package’s requirements list is shown in its help buffer.

By default, packages are downloaded from a single package archive maintained by the Emacs developers. This is controlled by the variable `package-archives`, whose value is a list of package archives known to Emacs. Each list element must have the form `(id . location)`, where id is the name of a package archive and location is the HTTP address or directory name of the package archive. You can alter this list if you wish to use third party package archives—but do so at your own risk, and use only third parties that you think you can trust!

The maintainers of package archives can increase the trust that you can have in their packages by signing them. They generate a private/public pair of cryptographic keys, and use the private key to create a signature file for each package. With the public key, you can use the signature files to verify who created the package, and that it has not been modified. A valid signature is not a cast-iron guarantee that a package is not malicious, so you should still exercise caution. Package archives should provide instructions on how you can obtain their public key. One way is to download the key from a server such as http://pgp.mit.edu/. Use `M-x package-import-keyring` to import the key into Emacs. Emacs stores package keys in the `gnupg` subdirectory of `package-user-dir`. The public key for the GNU package archive is distributed with Emacs, in the `etc/package-keyring.gpg`. Emacs uses it automatically.

If the user option `package-check-signature` is non-`nil`, Emacs attempts to verify signatures when you install packages. If the option has the value `allow-unsigned`, you can still install a package that is not signed. If you use some archives that do not sign their packages, you can add them to the list `package-unsigned-archives`.

For more information on cryptographic keys and signing, see Top in The GNU Privacy Guard Manual. Emacs comes with an interface to GNU Privacy Guard, see EasyPG in Emacs EasyPG Assistant Manual.

If you have more than one package archive enabled, and some of them offer different versions of the same package, you may find the option `package-pinned-packages` useful. You can add package/archive pairs to this list, to ensure that the specified package is only ever downloaded from the specified archive.

Another option that is useful when you have several package archives enabled is `package-archive-priorities`. It specifies the priority of each archive (higher numbers specify higher priority archives). By default, archives have the priority of zero, unless specified otherwise by this option’s value. Packages from lower-priority archives will not be shown in the menu, if the same package is available from a higher-priority archive. (This is controlled by the value of `package-menu-hide-low-priority`.)

Once a package is downloaded and installed, it is loaded into the current Emacs session. Loading a package is not quite the same as loading a Lisp library (see Lisp Libraries); loading a package adds its directory to `load-path` and loads its autoloads. The effect of a package’s autoloads varies from package to package. Most packages just make some new commands available, while others have more wide-ranging effects on the Emacs session. For such information, consult the package’s help buffer.

By default, Emacs also automatically loads all installed packages in subsequent Emacs sessions. This happens at startup, after processing the init file (see Init File). As an exception, Emacs does not load packages at startup if invoked with the ‘`-q`’ or ‘`--no-init-file`’ options (see Initial Options).

To disable automatic package loading, change the variable `package-enable-at-startup` to `nil`.

The reason automatic package loading occurs after loading the init file is that user options only receive their customized values after loading the init file, including user options which affect the packaging system. In some circumstances, you may want to load packages explicitly in your init file (usually because some other code in your init file depends on a package). In that case, your init file should call the function `package-initialize`. It is up to you to ensure that relevant user options, such as `package-load-list` (see below), are set up prior to the `package-initialize` call. This will automatically set `package-enable-at-startup` to `nil`, to avoid loading the packages again after processing the init file. Alternatively, you may choose to completely inhibit package loading at startup, and invoke the command `M-x package-initialize` to load your packages manually.

For finer control over package loading, you can use the variable `package-load-list`. Its value should be a list. A list element of the form `(name version)` tells Emacs to load version version of the package named name. Here, version should be a version string (corresponding to a specific version of the package), or `t` (which means to load any installed version), or `nil` (which means no version; this disables the package, preventing it from being loaded). A list element can also be the symbol `all`, which means to load the latest installed version of any package not named by the other list elements. The default value is just `'(all)`.

For example, if you set `package-load-list` to `'((muse "3.20") all)`, then Emacs only loads version 3.20 of the ‘`muse`’ package, plus any installed version of packages other than ‘`muse`’. Any other version of ‘`muse`’ that happens to be installed will be ignored. The ‘`muse`’ package will be listed in the package menu with the ‘`held`’ status.

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