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Function Package management systems are charged with the task of organising all of the packages installed on a system and maintaining their usability. These systems meet these goals using various combinations of the following techniques: Several of the widely used package management systems take advantage of simple backends for actually installing the packages. For instance, yum relies on rpm as a backend, and apt relies on dpkg. Challenges with shared libraries On systems where applications share pieces of machine instructions (i.e. packages' binaries are dynamic, as opposed to static), such as most Linux distributions, dependency checking becomes a necessity when installing and uninstalling packages. Some of the more advanced package management features are recursive and cascading package removal (c.f. DLL hell), in which all packages that depend on the target package and all packages that only the target package depends on, are also removed, respectively. Front-ends for locally compiled packages It is common for local administrators to install software not available in the repositories available through the package management. An example would be a newer version of a software application than that supplied with a distribution, or an alternative to that chosen by the distribution. If the additional software is distributed in source-only form, this approach requires local compilation. However, if additional software is locally added, the state of the local system may fall out of synchronization with the state of the package manager's database. If so, the local administrator user will be required to take additional measures to ensure the entire system is kept up to date. The package manager may no longer be able to do so automatically. There are tools available to ensure that locally compiled packages are integrated with the package management. For distributions based on .deb and .rpm files as well as Slackware Linux, there is checkinstall, and for recipe-based systems such as Gentoo Linux and hybrid systems such as Arch Linux, it is usually easy to write a recipe first, which then ensures that the package fits into the local package database. Conversion of binary packages Alien is a program that converts between different Linux package formats. It supports conversion between Linux Standard Base, RPM, deb, Stampede (.slp) and Slackware (tgz) packages. Maintenance of configuration Particularly troublesome with software upgrades are upgrades of configuration files. Since package management systems, at least on Unix systems, originated as extensions of file archiving utilities, they can usually only either overwrite or retain configuration files, rather than applying rules to them. There are exceptions to this that usually apply to kernel configuration (which, if broken, will render the computer unusable after a restart). Problems can be caused if the format of configuration files changes. For instance, if the old configuration file does not explicitly disable new options that should be disabled. Some package management systems, such as Debian's dpkg, allow configuration during installation. In other situations, it is desirable to install packages with the default configuration and then overwrite this configuration, for instance, in headless installations to a large number of computers. (This kind of pre-configured installation is also supported by dpkg.) Repositories In order to give users easy control over the kinds of software that they are allowing to be installed on their system (and sometimes due to legal or convenience reasons on the distributors' side), software is often downloaded from a number of repositories.• Upgrade suppression When a user interacts with the package management software to bring about an upgrade, it is customary to present the user with the list of things to be done (usually the list of packages to be upgraded, and possibly giving the old and new version numbers), and allow him to either accept the upgrade in bulk, or select individual packages for upgrades. Many package management systems can be configured to never upgrade certain packages, or only upgrade them when critical vulnerabilities or instabilities are found in the previous version, as defined by the packager of the software. This process is sometimes called version pinning. For instance, yum supports this with the syntax exclude=openoffice Free software systems By the nature of free software, packages under similar and compatible licenses are available for use on a number of operating systems. These packages can be easily combined and distributed using configurable and internally complex packaging systems to handle many permutations of software and manage version-specific dependencies and conflicts. Some packaging systems of free software are also themselves released as free software. For binary packages For installing from a recipe Hybrid systems Meta package managers The following unify package management for several or all Linux and sometimes Unix variants. These, too, are based on the concept of a recipe file. Proprietary software systems A wide variety of package management systems are in common use today by proprietary software operating systems, handling the installation of both proprietary and free packages. Package management built into applications Some package management systems are not part of the native operating system, such as fink for Mac OS X or Cygwin's Unix-like environment for Windows. Several dynamic programming languages have their own package management systems used for language modules. Other software may also come with its own system for managing modules, as do the programming languages Perl (see CPAN) and PHP (see PHP Extension and Application Repository). See also | |||||||||
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