Ebuild repository

An overlay is an additional repository that Portage takes into account when dealing with software.

Within Gentoo Linux, users already have one "main" package repository, called the Portage tree. This main repository contains all the software packages (called ebuilds) maintained by Gentoo developers. But users can add additional repositories to the tree that are "layed over" the main tree - hence the name, overlays.

Since package repositories are nothing more (or less) than a set of files (ebuilds, metadata files, ChangeLog entries ...) these repositories can be pulled in from public repositories (git, cvs, svn ...) or downloaded as tarballs and extracted manually onto the system. It is advised to use managed repositories by trusted third parties; any installed overlay will cause Portage to look through the overlayed files when deciding which software to install. If compromised code is in the overlay, then compromised packages could be installed on the system.

Treatment of overlays
Portage uses the  variable to link to the installed overlays on the system. This variable uses a space-delimited list of paths on the system where Portage can find the roots of the additional repositories.

Manually setting overlay locations
To make a personal overlay, create a location ( for example) in which the packages Portage is to look for are located. Then add the  variable to the  file containing the path to the location of the custom overlay.

An example of an overlay being located in the a user's home directory:

Add user to Portage group
To add a user to the portage group, use the gpasswd command like so:

See also adding a user to a group.

Using crossdev
crossdev will automatically place the ebuilds/categories it generates into the first overlay found in. Most users will want to prevent crossdev from disturbing layman's overlays or the user's personal per-machine overlay (normally created at ). Create an overlay specifically for crossdev's use:

Then instruct Portage and crossdev to use this overlay:

Using layman
To make management of multiple overlays simple, a tool called layman is developed. This tool knows about popular user- and developer managed overlays and is able to install & synchronize them as well as add them to the  location.

Local overlay
For instructions on setting up a local overlay, see Overlay/Local_overlay.

Overlay priorities
Each overlay has its unique priority. This makes sure that in the case of a specific version being found in several overlays, the resolution is unambiguous. Ebuilds from overlays with higher priorities take precedence over ebuilds from overlays with lower priorities.

The list of overlays with their priorities can be obtained through the output of the following command

Unless the  variable has been modified as described below, the default Gentoo Portage tree will have a priority of -1000. That means that all other overlays take precedence. That is the default behavior, because overlays are designed to "lay over/on top" of the portage tree.

Setting overlay priorities
The overlay priority is calculated from the order of overlay entries in the  variable. Portage "walks" through the variable from left to right and increments the priority on the way. The leftmost entry starts with a priority of 1, the next entry has a priority of 2 and so on.

Unless the  contains the portage tree entry, the portage tree will always be assigned a priority of -1000. This can be easily changed by putting  in the   variable:

In the example above the user overlay will be assigned a priority of 1 and the portage tree will be assigned a priority of 2.

Using unsafe overlays
When using huge overlays or those with unknown/low quality it is best practice to hardmask the whole overlay.

After that unmask the packages that will be installed.

This way nothing weird will happen on updates and it is safer than using priorities.

Cache generation
When large overlays are installed, portage may take a long time to perform operations like dependency resolution. This is because overlays do not usually contain a metadata cache.

Generate a local metadata cache by running emerge --regen after syncing the overlays:

Be careful, because emerge --regen takes a lot of time and it's not recommended for rsync users as rsync updates the cache using server-side caches (most of users of portage are rsync users). Rsync users should simply run emerge --sync (or eix-sync) to regenerate the cache. It's probably only users of very large overlays should try emerge --regen.

eix-sync
eix-sync can run emerge --regen</tt> after syncing the overlays and portage tree.

eix-update
eix-update</tt> can utilize the metadata cache generated by emerge --regen</tt> for a speedup and better accuracy. To enable this, set the  to   in.