A package manager, or package management system, is software that supports installing, upgrading, configuring, and removing software for the host system in a consistent manner.[1]
A package manager deals with packages, distributions of software and data in archive files. Packages contain metadata, such as the software's name, description of its purpose, version number, vendor, checksum (usually a cryptographic hash function), and a list of dependencies necessary for the software to run properly. Upon installation, metadata is stored in a local package database. Package managers typically maintain a database of software dependencies and version information to prevent software mismatches and missing prerequisites. They work closely with software repositories, binary repository managers, and app stores.
Package managers are designed to eliminate the need for manual installs and updates. This can be particularly useful for large enterprises whose operating systems (OSes) typically consist of hundreds or even tens of thousands of distinct packages.[2]
An early package manager was SMIT (and its backend install) from IBM AIX. SMIT was introduced with AIX 3.0 in 1989. Package managers like dpkg have existed as early as 1994.[3] Early package managers, from around 1994, had no automatic dependency resolution[4] but could already simplify the process of adding and removing software from a running system.[5]
By around 1995, beginning with CPAN, package managers began handling package repository downloads, and dependency resolution and installation as needed, making it easier to install, uninstall and update software.[6]
A software package is an archive file containing a computer program as well as necessary metadata for its deployment. The program may be in source code that has to be compiled and built first.[7] Package metadata include the software's description, version number, and dependencies.
Package managers are charged with the task of finding, installing, maintaining or uninstalling packages upon the user's command. Typical functions of a package management system include:
Computer systems that rely on dynamic library linking, instead of static library linking, share executable libraries of machine instructions across packages and applications. In these systems, conflicting relationships between different packages requiring different versions of libraries results in a situation called dependency hell. On Microsoft Windows systems, this is also called DLL hell when working with dynamically-linked libraries.[8]
Modern package managers have mostly solved these problems by allowing parallel installation of multiple library versions (e.g. OPENSTEP's Framework system), a dependency of any kind (e.g. slots in Gentoo Portage), and even packages compiled with different compiler versions (e.g. dynamic libraries built by the Glasgow Haskell Compiler, where a stable ABI does not exist), in order to enable other packages to specify which version they are linked or installed against.
System administrators may install and maintain software using tools other than package managers. For example, a local administrator may download unpackaged source code, compile it, and install it. This may cause the local system to fall out of synchronization with the package manager's database. The local administrator will be required to take additional measures, such as manually managing some dependencies or integrating the changes into the package manager.
There are tools to ensure that locally-compiled packages are integrated with the package management. For operating systems 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 possible to write a recipe first, which then ensures that the package fits into the local package database.
Upgrades of configuration files may be particularly troublesome. Since package managers, at least on Unix systems, originated as extensions of file archivers, they can usually only overwrite or retain configuration files, rather than apply 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 configuration file format changes; for instance, if the old file does not explicitly disable new options that should be disabled. Some package managers, such as Debian's dpkg, allow configuration during installation. In other situations, packages are installed with the default configuration, which is then overwritten, for example in headless installations to a large number of computers. This kind of pre-configured installation is also supported by dpkg.
To give users more control over the software they allow to be installed on their systems, and sometimes due to legal or convenience reasons on the distributors' side, packages are often downloaded from software repositories.[9]
When a user is upgrading a package with package management software, it is customary for them to be presented with the actions to be executed (usually the list of packages to be upgraded, and possibly the old and new version numbers), and allow them to either accept the upgrade in bulk, or select individual packages for upgrading. Many package managers can be configured to never upgrade certain packages, or to upgrade them only when critical vulnerabilities or instabilities are found in the previous version, as defined by the packager of the software. This process is called upgrade suppression, or version pinning. For instance, to prevent upgrades to the OpenOffice program:
exclude=openoffice*[10]IgnorePkg= openoffice (to suppress upgrading openoffice in both cases)Some of the more advanced package management features offer cascading package removal,[13] in which all packages that depend on the target package and all packages that only it depends on are also removed.
Although commands are specific to each package manager, they are translatable to a large extent, as most package managers offer similar functions. The Arch Linux wiki offers an extensive overview of commands.[14]
| zypper[15] | Nix | XBPS[16] | swupd[17] | WinGet | |||||
|---|---|---|---|---|---|---|---|---|---|
| scope=row | Install package |
