Portage is more than just a convenience script that gives you a unified way to build any one project (program, library) from source. It will also fetch and install any necessary dependencies if you take care to specify these in your ebuild.

In the official ebuilds, all dependencies have already been specified, so when you issue emerge net-www/mozilla/mozilla-1.0, Portage will insure that all libraries necessary for Mozilla to build and run are properly installed before Mozilla itself is built.

Portage even distinguishes between build-time dependencies and run-time dependencies. (Caveat: Currently, Portage installs all build-time and run-time dependencies and leaves it at that. At a later stage, it will be possible to trim your installation so that only the run-time dependencies are left installed).

The DEPEND variable inside your foo-x.y.z.ebuild tells Portage about which packages are needed to build foo. The RDEPEND variable specifies which packages are needed for foo to run. RDEPEND should be set explicitly even if it's the same as DEPEND because in the future it defaulting to DEPEND is planned to be removed from Portage.

DEPEND="virtual/opengl
       dev-libs/libxml2"
RDEPEND="${DEPEND}

This tells Portage that to build foo-x.y.z, the packages virtual/opengl (more on virtuals in a bit) and dev-lib/libxml2 are needed. It does not say anything about which version of opengl or libxml2 that are needed, which means "anything goes".

The "anything goes" is of course a bit scary, and will not work in the general case. But for libraries, which strive very hard to be 100% binary compatible all the time, it actually works. For other libraries, we can of course specify version dependencies.

>=sys-apps/bar-1.2
=sys-apps/baz-1.0

>= and = do what you would expect; sys-apps/bar version 1.2 or newer is okay (this means that sys-apps/bar-2.0 is okay), while sys-apps/baz version 1.0 is the only version that is accepted. For more information on the version schema of packages, see the section above on Naming ebuild Files.

Other methods of specifying version dependencies are as follows:

~sys-apps/qux-1.0
=sys-apps/foo-1.2*
!sys-libs/gdbm

~sys-apps/qux-1.0 will select the newest portage revision of qux-1.0.

=sys-apps/foo-1.2* will select the newest member of the 1.2 series, but will ignore 1.3 and later/earlier series. That is, foo-1.2.3 and foo-1.2.0 are both valid, while foo-1.3.3, foo-1.3.0, and foo-1.1.0 are not.

!sys-libs/gdbm will prevent this package from being emerged while gdbm is already emerged.

There are many things that go wrong with the DEPEND and RDEPEND variables. Here are some important points to follow when you write the dependencies.

Additionally, it is important to ensure that all the dependencies are complete for your package:

For all the latest details about these DEPEND Atoms, please see the section 5 manpage on ebuilds: man 5 ebuild.