Portage is a package management system used by Gentoo Linux. Gentoo is sometimes referred to as a "meta-distribution" due to the flexibility of Portage which allows it to manage other operating systems as well. In addition to Gentoo Linux there exist Portage implementations for FreeBSD, Mac OS X and Solaris.
Portage is similar to the BSD package management system called ports; in fact it was originally designed with FreeBSD's ports in mind. Gentoo's Portage system works similarly to Debian's APT. Portage is written in the Python programming language, and is the main utility that defines Gentoo. Although the system itself is known as Portage, it consists of two main parts, the ebuild system and emerge. These two have roughly the same relation as rpm and yum, or dpkg and apt, in that the ebuild system takes care of the actual work of building and installing packages, while emerge is a front-end that resolves dependencies and similar issues. A Qt-based GUI known as Kuroo, two GTK+-based GUIs: Porthole and Portato are also available for working with Portage.
There is an ongoing effort called the Package Manager Specification project (PMS) to standardise and document the behaviour of Portage, allowing the ebuild tree and Gentoo system packages to be used with alternate package managers such as Paludis and pkgcore. Its goal is to specify the behavior of package managers upon which Gentoo ebuilds can rely. There is a complete draft available.
The emerge command-line tool is the heart of Portage. The command is customizable with many options and modifiers. The emerge tool is the most important utility for accessing the features of Portage from the command line.
The program calculates and manages dependencies, executes ebuilds and maintains the local Portage tree and database of installed packages. The compilation settings used by ebuilds can be changed through the CFLAGS environment variable, based on the specifications of the individual computer and on the user's desire for optimization. The emerge utility executes ebuilds in a sandbox environment. This way the system is protected from software executed by the ebuild and resulting binaries are only merged after a successful build and sandboxed install.
What emerge installs as dependencies is affected by the USE flag-settings. They decide which optional features will be included when installing or upgrading an application. The emerge command can also be used to download and install binary files.
The Portage system offers the use of "USE flags," which allows users to indicate which software features they would like to include (and exclude) while building packages. For example, there is a USE flag to include DVD support, where available, in packages compiled with the flag enabled. The USE flags affect which dependencies are required, generally affecting which optional features will be built into a given program when it is compiled. For example, in packages which use a configure script, the USE flag
feature would translate to
The specification of USE flags is the usual way to configure programs on Gentoo. USE flags may be set manually, or via user-friendly tools such as 'ufed' (USE flag editor), which lists flags along with their description. A list of available USE flags is available in the "Gentoo Linux Use Variable Descriptions" page of the Gentoo website.
Gentoo does not, by default, use binary packages as other package management systems do (like RPM), employing instead a format known as the ebuild. Whereas RPMs are precompiled binaries, ebuilds are shell scripts with variables and functions which contain a description of the software, and instructions on how to obtain, configure, compile, and install it, more closely akin to (but more powerful than) the .spec files distributed in SRPMs. There are over 25,000 ebuilds available, the majority of which are distributed by the Gentoo mirrors. New and updated ebuilds can be obtained by synchronizing the local ebuild repository with the mirrors. This is done by executing the command emerge --sync. Recently, Gentoo has begun providing pre-compiled binary packages for many common programs, especially those which are lengthy to compile, such as Mozilla Firefox and OpenOffice.org. These are still installed with emerge, just by appending a "-bin" to the package name to instead install the binary version.
Gentoo does have a binary packaging format, which is a .tbz2 file (tar with bzip2 compression) with additional metadata. This feature enables the building of binary packages on one system (using Portage's buildpkg or quickpkg) followed by quick installation on other, identical systems (with Portage's getbinpkg or emerge -K). See Portage Features in the Gentoo Linux Handbook for more information.
Masking is how Gentoo determines which packages are suitable for a system. Ebuilds designed for different architectures or experimental software are usually masked in a way that will not allow a stable system to install them with no proper user intervention.
Packages that generally just require some testing but will often work fine are said to be keyword masked (i.e. they are available for systems with an ACCEPT_KEYWORDS make.conf entry starting with the character ~, such as ~x86, ~amd64, ~ppc). The standard way to unmask an individual keyword masked package is by adding a file with the full package name and keyword to /etc/portage/package.keywords/. Users can make subdirectories here as well, allowing for custom organization. For example, if a masked package had multiple masked dependencies, the user could make a directory with the name of the original masked package, and put all the mask files for the package and its dependencies in that directory. This scheme replaces the older scheme of having /etc/portage/package.keywords as a text file list.
Packages with known problems or not considered mature enough to be candidates for stable are hard masked by one of the various package.mask files in /usr/portage/profiles, and such entries are generally accompanied by a comment from developers explaining the reason for the mask.