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39.4 Creating and Maintaining Package Archives

Via the Package Menu, users may download packages from package archives. Such archives are specified by the variable package-archives, whose default value contains a single entry: the archive hosted by the GNU project at http://elpa.gnu.org. This section describes how to set up and maintain a package archive.

— User Option: package-archives

The value of this variable is an alist of package archives recognized by the Emacs package manager.

Each alist element corresponds to one archive, and should have the form (id . location), where id is the name of the archive (a string) and location is its base location (a string).

If the base location starts with ‘http:’, it is treated as a HTTP URL, and packages are downloaded from this archive via HTTP (as is the case for the default GNU archive).

Otherwise, the base location should be a directory name. In this case, Emacs retrieves packages from this archive via ordinary file access. Such “local” archives are mainly useful for testing.

A package archive is simply a directory in which the package files, and associated files, are stored. If you want the archive to be reachable via HTTP, this directory must be accessible to a web server. How to accomplish this is beyond the scope of this manual.

A convenient way to set up and update a package archive is via the package-x library. This is included with Emacs, but not loaded by default; type M-x load-library <RET> package-x <RET> to load it, or add (require 'package-x) to your init file. See Lisp Libraries. Once loaded, you can make use of the following:

— User Option: package-archive-upload-base

The value of this variable is the base location of a package archive, as a directory name. The commands in the package-x library will use this base location.

The directory name should be absolute. You may specify a remote name, such as /ssh:foo@example.com:/var/www/packages/, if the package archive is on a different machine. See Remote Files.

— Command: package-upload-file filename

This command prompts for filename, a file name, and uploads that file to package-archive-upload-base. The file must be either a simple package (a .el file) or a multi-file package (a .tar file); otherwise, an error is raised. The package attributes are automatically extracted, and the archive's contents list is updated with this information.

If package-archive-upload-base does not specify a valid directory, the function prompts interactively for one. If the directory does not exist, it is created. The directory need not have any initial contents (i.e., you can use this command to populate an initially empty archive).

— Command: package-upload-buffer

This command is similar to package-upload-file, but instead of prompting for a package file, it uploads the contents of the current buffer. The current buffer must be visiting a simple package (a .el file) or a multi-file package (a .tar file); otherwise, an error is raised.

After you create an archive, remember that it is not accessible in the Package Menu interface unless it is in package-archives.

Maintaining a public package archive entails a degree of responsibility. When Emacs users install packages from your archive, those packages can cause Emacs to run arbitrary code with the permissions of the installing user. (This is true for Emacs code in general, not just for packages.) So you should ensure that your archive is well-maintained and keep the hosting system secure.

One way to increase the security of your packages is to sign them using a cryptographic key. If you have generated a private/public gpg key pair, you can use gpg to sign the package like this:

     gpg -ba -o file.sig file

For a single-file package, file is the package Lisp file; for a multi-file package, it is the package tar file. You can also sign the archive's contents file in the same way. Make the .sig files available in the same location as the packages. You should also make your public key available for people to download; e.g., by uploading it to a key server such as http://pgp.mit.edu/. When people install packages from your archive, they can use your public key to verify the signatures.

A full explanation of these matters is outside the scope of this manual. For more information on cryptographic keys and signing, see GnuPG. Emacs comes with an interface to GNU Privacy Guard, see EasyPG.