Basic guide to write Gentoo Ebuilds

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This is a guide to getting started writing ebuilds, to harness the power of Portage, to install and manage even more software.

Write an ebuild to install a piece of software on Gentoo, when there are no suitable preexisting ebuilds. It's a relatively straight forward task, and is the only way to cleanly install most "third party" software system-wide. The ebuild will allow the package manager to track every file installed to the system, to allow clean updates and removal.

Tip
From the ebuild article: An ebuild file is a text file, usually stored in a repository, which identifies a specific software package and tells the Gentoo package manager how to handle it. Ebuilds use a bash-like syntax style and are standardized through the Package Manager Specification, by adhering to a specific EAPI version.

Ebuilds contain metadata about each version of a piece of available software (name, version number, license, home page address...), dependency information (both build-time and run-time), and instructions on how to build and install the software (configure, compile, build, install, test...).

Once an ebuild is working, it can be shared by submitting it in a pull request or in a separate ebuild repository and making it accessible publicly. With a little effort, ebuilds can be proposed and maintained in the GURU repository.

Note
See the dev manual on writing ebuilds for full reference documentation. See the quick start to writing ebuilds in the dev manual for further examples of how to write ebuilds. See the Ebuild article for explanations about ebuilds themselves, the ebuild repository article about what an ebuild repository is, and the creating an ebuild repository article on how to create them.

Ebuild repositories

In order for ebuilds to be available to Portage, they are placed in an ebuild repository that is configured for Portage through /etc/portage/repos.conf (see the section on repository management for general information about working with ebuild repositories).

Create an ebuild repository to experiment in, while following on with this guide. The rest of the article will consider a repository in /var/db/repos/example_repository.

eselect repository makes creating a repository simple:

root #eselect repository create example_repository
Note
Ebuilds can be installed with the ebuild command, however this is not recommended - this command is for development purposes only. This article will use the ebuild command with the ebuild file for testing during development, but be sure to use the emerge command with an ebuild in a repository otherwise.

How to create an ebuild

Ebuilds are simply text files, in their most basic form. All that is needed to start writing ebuilds is a text editor, to provide installable software packages for Gentoo.

Note
In this section, {CATEGORY}, {PN}, and {P} represent package category, package name, and package name and version, and are standard variables used in ebuilds. Together, these variables can represent a version specifier.

Some editors have optional ebuild functionality, in that case there skip to the appropriate section. Otherwise a skeleton ("template") may be used to get started quicker.

Start with the skeleton

If the editor does not have integrated ebuild functionality to help to start off, there is a skeleton ebuild file (skel.ebuild) located in the Gentoo ebuild repository. To start with that file as a base, simply copy it to an appropriate location (nano is used as the text editor in this example):

user $mkdir --parents /var/db/repos/example_repository/{CATEGORY}/{PN}
user $cp /var/db/repos/gentoo/skel.ebuild /var/db/repos/example_repository/{CATEGORY}/{PN}
user $cd /var/db/repos/example_repository/{CATEGORY}/{PN}
user $nano {P}.ebuild

Vim

There is a vim plugin to automatically start from a skeleton when creating an empty ebuild file.

After installing app-vim/gentoo-syntax, create the appropriate directory for the ebuild, then launch vim with a new "{P}.ebuild" filename provided on the command line, to be automatically met with a basic skeleton that can be modified and saved:

user $mkdir --parents /var/db/repos/example_repository/{CATEGORY}/{PN}
user $cd /var/db/repos/example_repository/{CATEGORY}/{PN}
user $vim {P}.ebuild

Emacs

A similar tool is available for users of Emacs, provided by app-emacs/ebuild-mode or app-xemacs/ebuild-mode, depending on Emacs distribution.

Demonstration by example

This example will create an ebuild for scrub, version 2.6.1 (if it didn't already exist), to show how a typical process might go.

Create a directory to house the ebuild, in the ebuild repository created earlier:

root #mkdir -p /var/db/repos/example_repository/app-misc/scrub

Change the shell working directory to the new path:

root #cd /var/db/repos/example_repository/app-misc/scrub
Tip
Some shells, such as Bash, provide the last parameter of the previous command in the "$_" variable. This can be used to call the newly created directory without specifying the path on the command line, as long as this is used as the directly next command.
root #cd $_

This example will use Vim to create the ebuild file and provide a skeleton to serve as a basis to write the ebuild on, but use editor of choice (see previous section about using Emacs, or the skeleton file):

user $vim ./scrub-2.6.1.ebuild

Add important information about the new package by setting the ebuild-defined variables: DESCRIPTION, HOMEPAGE, SRC_URI, LICENSE:

FILE scrub-2.6.1.ebuildvim editing a new file from template
# Copyright 1999-2022 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
 
EAPI=8
 
DESCRIPTION="Some words here"
HOMEPAGE="https://github.com/chaos/scrub"
SRC_URI="https://github.com/chaos/scrub/releases/download/2.6.1/scrub-2.6.1.tar.gz"
 
LICENSE="GPL-2"
SLOT="0"
KEYWORDS="~amd64 ~x86"
IUSE=""
 
DEPEND=""
RDEPEND="${DEPEND}"
BDEPEND=""

This - after removal of those lines containing ="" - is the minimum information necessary to get something that will work. Ebuilds inheriting certain eclasses might come with a different set of minimal information, e.g. ant-jsch-1.10.9.ebuild. Save the file - voila an ebuild, in it's most basic form,it's that simple!

Note
Using the ${PN} variable inside SRC_URI is allowed, though this is not necessarily best practice. While it may be shorter to type, there is some reasoning on why not to use it that may be worth consideration.
CODE avoid using PN as such
SRC_URI="https://github.com/gentoo/${PN}/releases/download/${PV}/${P}.tar.gz"
# Reads better as, e.g.
SRC_URI="https://github.com/gentoo/gentoo/releases/download/${PV}/${P}.tar.gz"
See this ebuild file format policy guide page for more recommendations.

It is possible to test fetching and unpacking the upstream sources by the new ebuild, using the ebuild command:

root #ebuild ./scrub-2.6.1.ebuild manifest clean unpack
Appending /var/db/repos/example_repository to PORTDIR_OVERLAY...
>>> Downloading 'https://ftp.halifax.rwth-aachen.de/gentoo/distfiles/scrub-2.6.1.tar.gz'
--2019-06-03 16:42:57--  https://ftp.halifax.rwth-aachen.de/gentoo/distfiles/scrub-2.6.1.tar.gz
Resolving ftp.halifax.rwth-aachen.de... 137.226.34.46, 2a00:8a60:e012:a00::21
Connecting to ftp.halifax.rwth-aachen.de|137.226.34.46|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 362536 (354K) [application/octet-stream]
Saving to: '/usr/portage/distfiles/scrub-2.6.1.tar.gz.__download__'
 
/usr/portage/distfiles/scrub-2.6.1.tar.gz.__download__             100%[==============================================================================================================================================================>] 354.04K  --.-KB/s    in 0.08s   
 
2019-06-03 16:42:58 (4.24 MB/s) - '/usr/portage/distfiles/scrub-2.6.1.tar.gz.__download__' saved [362536/362536]
 
 * scrub-2.6.1.tar.gz BLAKE2B SHA512 size ;-) ...                                                                                                                                                                                                                  [ ok ]
>>> Unpacking source...
>>> Unpacking scrub-2.6.1.tar.gz to /var/tmp/portage/app-misc/scrub-2.6.1/work
>>> Source unpacked in /var/tmp/portage/app-misc/scrub-2.6.1/work

This should download and unpack the source tarball, without error, as in the example output.

For some exceptionally simple packages like this one, that do not need patching or other more advanced treatment, the ebuild may work just so - with no further adjustments needed.

For best practice, the test suite may be run at this stage - this is particularly true when starting out:

root #ebuild scrub-2.6.1.ebuild clean test install

To actually install the new ebuild on the system, run:

root #ebuild scrub-2.6.1.ebuild clean install merge

Patching upstream source in an ebuild

The source can be patched at installation time, to adapt to Gentoo specificities for example, by setting up patches from an ebuild.

Put the patches in the right directory:

user $cd /var/tmp/portage/app-misc/scrub-2.6.1/work/scrub-2.6.1/

A patch can be created from the unpacked source code as explained in the patches article. Patches should then be listed in an array called PATCHES as explained in the devmanual:

CODE Patches will be applied during src_prepare
PATCHES=(
	"${FILESDIR}"/${P}-foo.patch
	"${FILESDIR}"/${P}-bar.patch
)
 
src_prepare() {
    default
    ...
}

QA testing

Use pkgcheck (dev-util/pkgcheck) to check for QA errors in an ebuild:

user $pkgcheck scan

See also

External resources