Chapter 9 . Running Debian (Post office web site) GNU/Linux 301 .
Chapter 9 . Running Debian GNU/Linux 301 . Optional preinst, postinst, prerm, and postrm files can instruct the package management tools to perform functions before or after package installation or removal. For example, most packages containing daemons (such as Apache HTTPD) include a postinst script that starts the daemon automatically after installation. . A conffiles file can designate specific files in the package as configuration files, which are not automatically overwritten during upgrades. By default, all files under the /etc/ directory are configuration files. Two special package types, meta and virtual, also exist. Meta packages are standard binary packages that do not contain any files, but depend on a number of other packages. Installation of a meta package results in the automatic installation of all packages that they depend on. These can be used as a convenient method for installing a set of related packages. Virtual packages do not actually exist as files but can be referenced in the package interrelationship fields. They are most commonly used in cases where more than one package fulfills a specific requirement. Packages with this requirement can reference the virtual package in their Depends field, and packages that satisfy this dependency reference it in their Provides field. Because most programs providing a virtual package are mutually exclusive, they also include the virtual package in their Conflicts field to prevent the installation of conflicting packages. An example of this is the mail-transport-agent virtual package, which is required by most system programs in order to send mail. An easy way to browse the list of available packages is through the Debian Web site at www.debian.org/distrib/packages. Debian Package Management Tools Perhaps the most interesting and well-known part of the Debian package management system is APT, the Advanced Package Tool. APT, through the apt-get utility, maintains a database of packages available in the repositories that it is configured to check and can handle automatically downloading new or upgraded packages. When installing or upgrading packages, APT downloads the necessary files to a local cache directory and then instructs the dpkg tool to take the appropriate actions. Among other things, this allows the user to select programs for addition or removal without having to manually instruct the system to handle any package dependencies. Most basic package management functions are performed by dpkg, although not always at the direct request of the user. This tool handles medium-level package installation and removal and also manages the package status database. That database contains information about every package known to dpkg, including the package meta information and two other important fields: the package state and selection state. Note
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